doc: correct description of libpq's PQsetnonblocking() mode

[pgsql.git] / doc / src / sgml / libpq.sgml

<!-- doc/src/sgml/libpq.sgml -->

<chapter id="libpq">
 <title><application>libpq</application> &mdash; C Library</title>

 <indexterm zone="libpq">
  <primary>libpq</primary>
 </indexterm>

 <indexterm zone="libpq">
  <primary>C</primary>
 </indexterm>

 <para>
  <application>libpq</application> is the <acronym>C</acronym>
  application programmer's interface to <productname>PostgreSQL</productname>.
  <application>libpq</application> is a set of library functions that allow
  client programs to pass queries to the <productname>PostgreSQL</productname>
  backend server and to receive the results of these queries.
 </para>

 <para>
  <application>libpq</application> is also the underlying engine for several
  other <productname>PostgreSQL</productname> application interfaces, including
  those written for C++, Perl, Python, Tcl and <application>ECPG</application>.
  So some aspects of <application>libpq</application>'s behavior will be
  important to you if you use one of those packages.  In particular,
  <xref linkend="libpq-envars"/>,
  <xref linkend="libpq-pgpass"/> and
  <xref linkend="libpq-ssl"/>
  describe behavior that is visible to the user of any application
  that uses <application>libpq</application>.
 </para>

 <para>
  Some short programs are included at the end of this chapter (<xref linkend="libpq-example"/>) to show how
  to write programs that use <application>libpq</application>.  There are also several
  complete examples of <application>libpq</application> applications in the
  directory <filename>src/test/examples</filename> in the source code distribution.
 </para>

 <para>
  Client programs that use <application>libpq</application> must
  include the header file
  <filename>libpq-fe.h</filename><indexterm><primary>libpq-fe.h</primary></indexterm>
  and must link with the <application>libpq</application> library.
 </para>

 <sect1 id="libpq-connect">
  <title>Database Connection Control Functions</title>

  <para>
   The following functions deal with making a connection to a
   <productname>PostgreSQL</productname> backend server.  An
   application program can have several backend connections open at
   one time.  (One reason to do that is to access more than one
   database.)  Each connection is represented by a
   <structname>PGconn</structname><indexterm><primary>PGconn</primary></indexterm> object, which
   is obtained from the function <xref linkend="libpq-PQconnectdb"/>,
   <xref linkend="libpq-PQconnectdbParams"/>, or
   <xref linkend="libpq-PQsetdbLogin"/>.  Note that these functions will always
   return a non-null object pointer, unless perhaps there is too
   little memory even to allocate the <structname>PGconn</structname> object.
   The <xref linkend="libpq-PQstatus"/> function should be called to check
   the return value for a successful connection before queries are sent
   via the connection object.

   <warning>
    <para>
     If untrusted users have access to a database that has not adopted a
     <link linkend="ddl-schemas-patterns">secure schema usage pattern</link>,
     begin each session by removing publicly-writable schemas from
     <varname>search_path</varname>.  One can set parameter key
     word <literal>options</literal> to
     value <literal>-csearch_path=</literal>.  Alternately, one can
     issue <literal>PQexec(<replaceable>conn</replaceable>, "SELECT
     pg_catalog.set_config('search_path', '', false)")</literal> after
     connecting.  This consideration is not specific
     to <application>libpq</application>; it applies to every interface for
     executing arbitrary SQL commands.
    </para>
   </warning>

   <warning>
    <para>
     On Unix, forking a process with open libpq connections can lead to
     unpredictable results because the parent and child processes share
     the same sockets and operating system resources.  For this reason,
     such usage is not recommended, though doing an <function>exec</function> from
     the child process to load a new executable is safe.
    </para>
   </warning>

   <variablelist>
    <varlistentry id="libpq-PQconnectdbParams">
     <term><function>PQconnectdbParams</function><indexterm><primary>PQconnectdbParams</primary></indexterm></term>
     <listitem>
      <para>
       Makes a new connection to the database server.

<synopsis>
PGconn *PQconnectdbParams(const char * const *keywords,
                          const char * const *values,
                          int expand_dbname);
</synopsis>
      </para>

      <para>
       This function opens a new database connection using the parameters taken
       from two <symbol>NULL</symbol>-terminated arrays. The first,
       <literal>keywords</literal>, is defined as an array of strings, each one
       being a key word. The second, <literal>values</literal>, gives the value
       for each key word. Unlike <xref linkend="libpq-PQsetdbLogin"/> below, the parameter
       set can be extended without changing the function signature, so use of
       this function (or its nonblocking analogs <xref linkend="libpq-PQconnectStartParams"/>
       and <function>PQconnectPoll</function>) is preferred for new application
       programming.
      </para>

      <para>
       The currently recognized parameter key words are listed in
       <xref linkend="libpq-paramkeywords"/>.
      </para>

      <para>
       The passed arrays can be empty to use all default parameters, or can
       contain one or more parameter settings. They must be matched in length.
       Processing will stop at the first <symbol>NULL</symbol> entry
       in the <literal>keywords</literal> array.
       Also, if the <literal>values</literal> entry associated with a
       non-<symbol>NULL</symbol> <literal>keywords</literal> entry is
       <symbol>NULL</symbol> or an empty string, that entry is ignored and
       processing continues with the next pair of array entries.
      </para>

      <para>
       When <literal>expand_dbname</literal> is non-zero, the value for
       the first <parameter>dbname</parameter> key word is checked to see
       if it is a <firstterm>connection string</firstterm>.  If so, it
       is <quote>expanded</quote> into the individual connection
       parameters extracted from the string.  The value is considered to
       be a connection string, rather than just a database name, if it
       contains an equal sign (<literal>=</literal>) or it begins with a
       URI scheme designator.  (More details on connection string formats
       appear in <xref linkend="libpq-connstring"/>.)  Only the first
       occurrence of <parameter>dbname</parameter> is treated in this way;
       any subsequent <parameter>dbname</parameter> parameter is processed
       as a plain database name.
      </para>

      <para>
       In general the parameter arrays are processed from start to end.
       If any key word is repeated, the last value (that is
       not <symbol>NULL</symbol> or empty) is used.  This rule applies in
       particular when a key word found in a connection string conflicts
       with one appearing in the <literal>keywords</literal> array.  Thus,
       the programmer may determine whether array entries can override or
       be overridden by values taken from a connection string.  Array
       entries appearing before an expanded <parameter>dbname</parameter>
       entry can be overridden by fields of the connection string, and in
       turn those fields are overridden by array entries appearing
       after <parameter>dbname</parameter> (but, again, only if those
       entries supply non-empty values).
      </para>

      <para>
       After processing all the array entries and any expanded connection
       string, any connection parameters that remain unset are filled with
       default values.  If an unset parameter's corresponding environment
       variable (see <xref linkend="libpq-envars"/>) is set, its value is
       used.  If the environment variable is not set either, then the
       parameter's built-in default value is used.
      </para>

     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQconnectdb">
     <term><function>PQconnectdb</function><indexterm><primary>PQconnectdb</primary></indexterm></term>
     <listitem>
      <para>
       Makes a new connection to the database server.

<synopsis>
PGconn *PQconnectdb(const char *conninfo);
</synopsis>
      </para>

      <para>
       This function opens a new database connection using the parameters taken
       from the string <literal>conninfo</literal>.
      </para>

      <para>
       The passed string can be empty to use all default parameters, or it can
       contain one or more parameter settings separated by whitespace,
       or it can contain a <acronym>URI</acronym>.
       See <xref linkend="libpq-connstring"/> for details.
     </para>


    </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsetdbLogin">
     <term><function>PQsetdbLogin</function><indexterm><primary>PQsetdbLogin</primary></indexterm></term>
     <listitem>
      <para>
       Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdbLogin(const char *pghost,
                     const char *pgport,
                     const char *pgoptions,
                     const char *pgtty,
                     const char *dbName,
                     const char *login,
                     const char *pwd);
</synopsis>
       </para>

       <para>
        This is the predecessor of <xref linkend="libpq-PQconnectdb"/> with a fixed
        set of parameters.  It has the same functionality except that the
        missing parameters will always take on default values.  Write <symbol>NULL</symbol> or an
        empty string for any one of the fixed parameters that is to be defaulted.
      </para>

      <para>
        If the <parameter>dbName</parameter> contains
        an <symbol>=</symbol> sign or has a valid connection <acronym>URI</acronym> prefix, it
        is taken as a <parameter>conninfo</parameter> string in exactly the same way as
        if it had been passed to <xref linkend="libpq-PQconnectdb"/>, and the remaining
        parameters are then applied as specified for <xref linkend="libpq-PQconnectdbParams"/>.
      </para>

      <para>
        <literal>pgtty</literal> is no longer used and any value passed will
        be ignored.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsetdb">
     <term><function>PQsetdb</function><indexterm><primary>PQsetdb</primary></indexterm></term>
     <listitem>
      <para>
   Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdb(char *pghost,
                char *pgport,
                char *pgoptions,
                char *pgtty,
                char *dbName);
</synopsis>
     </para>

     <para>
      This is a macro that calls <xref linkend="libpq-PQsetdbLogin"/> with null pointers
      for the <parameter>login</parameter> and <parameter>pwd</parameter> parameters.  It is provided
      for backward compatibility with very old programs.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQconnectStartParams">
     <term><function>PQconnectStartParams</function><indexterm><primary>PQconnectStartParams</primary></indexterm></term>
     <term><function>PQconnectStart</function><indexterm><primary>PQconnectStart</primary></indexterm></term>
     <term><function>PQconnectPoll</function><indexterm><primary>PQconnectPoll</primary></indexterm></term>
     <listitem>
      <para>
       <indexterm><primary>nonblocking connection</primary></indexterm>
       Make a connection to the database server in a nonblocking manner.

<synopsis>
PGconn *PQconnectStartParams(const char * const *keywords,
                             const char * const *values,
                             int expand_dbname);

PGconn *PQconnectStart(const char *conninfo);

PostgresPollingStatusType PQconnectPoll(PGconn *conn);
</synopsis>
      </para>

      <para>
       These three functions are used to open a connection to a database server such
       that your application's thread of execution is not blocked on remote I/O
       whilst doing so. The point of this approach is that the waits for I/O to
       complete can occur in the application's main loop, rather than down inside
       <xref linkend="libpq-PQconnectdbParams"/> or <xref linkend="libpq-PQconnectdb"/>, and so the
       application can manage this operation in parallel with other activities.
      </para>

      <para>
       With <xref linkend="libpq-PQconnectStartParams"/>, the database connection is made
       using the parameters taken from the <literal>keywords</literal> and
       <literal>values</literal> arrays, and controlled by <literal>expand_dbname</literal>,
       as described above for <xref linkend="libpq-PQconnectdbParams"/>.
      </para>

      <para>
       With <function>PQconnectStart</function>, the database connection is made
       using the parameters taken from the string <literal>conninfo</literal> as
       described above for <xref linkend="libpq-PQconnectdb"/>.
      </para>

      <para>
       Neither <xref linkend="libpq-PQconnectStartParams"/> nor <function>PQconnectStart</function>
       nor <function>PQconnectPoll</function> will block, so long as a number of
       restrictions are met:
       <itemizedlist>
        <listitem>
         <para>
          The <literal>hostaddr</literal> parameter must be used appropriately
          to prevent DNS queries from being made.  See the documentation of
          this parameter in <xref linkend="libpq-paramkeywords"/> for details.
         </para>
        </listitem>

        <listitem>
         <para>
          If you call <xref linkend="libpq-PQtrace"/>, ensure that the stream object
          into which you trace will not block.
         </para>
        </listitem>

        <listitem>
         <para>
          You must ensure that the socket is in the appropriate state
          before calling <function>PQconnectPoll</function>, as described below.
         </para>
        </listitem>
       </itemizedlist>
      </para>

      <para>
       To begin a nonblocking connection request,
       call <function>PQconnectStart</function>
       or <xref linkend="libpq-PQconnectStartParams"/>.  If the result is null,
       then <application>libpq</application> has been unable to allocate a
       new <structname>PGconn</structname> structure.  Otherwise, a
       valid <structname>PGconn</structname> pointer is returned (though not
       yet representing a valid connection to the database).  Next
       call <literal>PQstatus(conn)</literal>.  If the result
       is <symbol>CONNECTION_BAD</symbol>, the connection attempt has already
       failed, typically because of invalid connection parameters.
      </para>

      <para>
       If <function>PQconnectStart</function>
       or <xref linkend="libpq-PQconnectStartParams"/> succeeds, the next stage
       is to poll <application>libpq</application> so that it can proceed with
       the connection sequence.
       Use <function>PQsocket(conn)</function> to obtain the descriptor of the
       socket underlying the database connection.
       (Caution: do not assume that the socket remains the same
       across <function>PQconnectPoll</function> calls.)
       Loop thus: If <function>PQconnectPoll(conn)</function> last returned
       <symbol>PGRES_POLLING_READING</symbol>, wait until the socket is ready to
       read (as indicated by <function>select()</function>, <function>poll()</function>, or
       similar system function).
       Then call <function>PQconnectPoll(conn)</function> again.
       Conversely, if <function>PQconnectPoll(conn)</function> last returned
       <symbol>PGRES_POLLING_WRITING</symbol>, wait until the socket is ready
       to write, then call <function>PQconnectPoll(conn)</function> again.
       On the first iteration, i.e., if you have yet to call
       <function>PQconnectPoll</function>, behave as if it last returned
       <symbol>PGRES_POLLING_WRITING</symbol>.  Continue this loop until
       <function>PQconnectPoll(conn)</function> returns
       <symbol>PGRES_POLLING_FAILED</symbol>, indicating the connection procedure
       has failed, or <symbol>PGRES_POLLING_OK</symbol>, indicating the connection
       has been successfully made.
      </para>

      <para>
       At any time during connection, the status of the connection can be
       checked by calling <xref linkend="libpq-PQstatus"/>. If this call returns <symbol>CONNECTION_BAD</symbol>, then the
       connection procedure has failed; if the call returns <function>CONNECTION_OK</function>, then the
       connection is ready.  Both of these states are equally detectable
       from the return value of <function>PQconnectPoll</function>, described above. Other states might also occur
       during (and only during) an asynchronous connection procedure. These
       indicate the current stage of the connection procedure and might be useful
       to provide feedback to the user for example. These statuses are:

       <variablelist>
        <varlistentry id="libpq-connection-started">
         <term><symbol>CONNECTION_STARTED</symbol></term>
         <listitem>
          <para>
           Waiting for connection to be made.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-connection-made">
         <term><symbol>CONNECTION_MADE</symbol></term>
         <listitem>
          <para>
           Connection OK; waiting to send.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-connection-awaiting-response">
         <term><symbol>CONNECTION_AWAITING_RESPONSE</symbol></term>
         <listitem>
          <para>
           Waiting for a response from the server.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-connection-auth-ok">
         <term><symbol>CONNECTION_AUTH_OK</symbol></term>
         <listitem>
          <para>
           Received authentication; waiting for backend start-up to finish.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-connection-ssl-startup">
         <term><symbol>CONNECTION_SSL_STARTUP</symbol></term>
         <listitem>
          <para>
           Negotiating SSL encryption.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-connection-setenv">
         <term><symbol>CONNECTION_SETENV</symbol></term>
         <listitem>
          <para>
           Negotiating environment-driven parameter settings.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-connection-check-writable">
         <term><symbol>CONNECTION_CHECK_WRITABLE</symbol></term>
         <listitem>
          <para>
           Checking if connection is able to handle write transactions.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-connection-consume">
         <term><symbol>CONNECTION_CONSUME</symbol></term>
         <listitem>
          <para>
           Consuming any remaining response messages on connection.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>

       Note that, although these constants will remain (in order to maintain
       compatibility), an application should never rely upon these occurring in a
       particular order, or at all, or on the status always being one of these
       documented values. An application might do something like this:
<programlisting>
switch(PQstatus(conn))
{
        case CONNECTION_STARTED:
            feedback = "Connecting...";
            break;

        case CONNECTION_MADE:
            feedback = "Connected to server...";
            break;
.
.
.
        default:
            feedback = "Connecting...";
}
</programlisting>
      </para>

      <para>
       The <literal>connect_timeout</literal> connection parameter is ignored
       when using <function>PQconnectPoll</function>; it is the application's
       responsibility to decide whether an excessive amount of time has elapsed.
       Otherwise, <function>PQconnectStart</function> followed by a
       <function>PQconnectPoll</function> loop is equivalent to
       <xref linkend="libpq-PQconnectdb"/>.
      </para>

      <para>
       Note that when <function>PQconnectStart</function>
       or <xref linkend="libpq-PQconnectStartParams"/> returns a non-null
       pointer, you must call <xref linkend="libpq-PQfinish"/> when you are
       finished with it, in order to dispose of the structure and any
       associated memory blocks.  This must be done even if the connection
       attempt fails or is abandoned.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQconndefaults">
     <term><function>PQconndefaults</function><indexterm><primary>PQconndefaults</primary></indexterm></term>
     <listitem>
      <para>
       Returns the default connection options.
<synopsis>
PQconninfoOption *PQconndefaults(void);

typedef struct
{
    char   *keyword;   /* The keyword of the option */
    char   *envvar;    /* Fallback environment variable name */
    char   *compiled;  /* Fallback compiled in default value */
    char   *val;       /* Option's current value, or NULL */
    char   *label;     /* Label for field in connect dialog */
    char   *dispchar;  /* Indicates how to display this field
                          in a connect dialog. Values are:
                          ""        Display entered value as is
                          "*"       Password field - hide value
                          "D"       Debug option - don't show by default */
    int     dispsize;  /* Field size in characters for dialog */
} PQconninfoOption;
</synopsis>
      </para>

      <para>
       Returns a connection options array.  This can be used to determine
       all possible <xref linkend="libpq-PQconnectdb"/> options and their
       current default values.  The return value points to an array of
       <structname>PQconninfoOption</structname> structures, which ends
       with an entry having a null <structfield>keyword</structfield> pointer.  The
       null pointer is returned if memory could not be allocated. Note that
       the current default values (<structfield>val</structfield> fields)
       will depend on environment variables and other context.  A
       missing or invalid service file will be silently ignored.  Callers
       must treat the connection options data as read-only.
      </para>

      <para>
       After processing the options array, free it by passing it to
       <xref linkend="libpq-PQconninfoFree"/>.  If this is not done, a small amount of memory
       is leaked for each call to <xref linkend="libpq-PQconndefaults"/>.
      </para>

     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQconninfo">
     <term><function>PQconninfo</function><indexterm><primary>PQconninfo</primary></indexterm></term>
     <listitem>
      <para>
       Returns the connection options used by a live connection.
<synopsis>
PQconninfoOption *PQconninfo(PGconn *conn);
</synopsis>
      </para>

      <para>
       Returns a connection options array.  This can be used to determine
       all possible <xref linkend="libpq-PQconnectdb"/> options and the
       values that were used to connect to the server. The return
       value points to an array of <structname>PQconninfoOption</structname>
       structures, which ends with an entry having a null <structfield>keyword</structfield>
       pointer. All notes above for <xref linkend="libpq-PQconndefaults"/> also
       apply to the result of <xref linkend="libpq-PQconninfo"/>.
      </para>

     </listitem>
    </varlistentry>


    <varlistentry id="libpq-PQconninfoParse">
     <term><function>PQconninfoParse</function><indexterm><primary>PQconninfoParse</primary></indexterm></term>
     <listitem>
      <para>
       Returns parsed connection options from the provided connection string.

<synopsis>
PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);
</synopsis>
      </para>

      <para>
       Parses a connection string and returns the resulting options as an
       array; or returns <symbol>NULL</symbol> if there is a problem with the connection
       string.  This function can be used to extract
       the <xref linkend="libpq-PQconnectdb"/> options in the provided
       connection string.  The return value points to an array of
       <structname>PQconninfoOption</structname> structures, which ends
       with an entry having a null <structfield>keyword</structfield> pointer.
      </para>

      <para>
       All legal options will be present in the result array, but the
       <literal>PQconninfoOption</literal> for any option not present
       in the connection string will have <literal>val</literal> set to
       <literal>NULL</literal>; default values are not inserted.
      </para>

      <para>
       If <literal>errmsg</literal> is not <symbol>NULL</symbol>, then <literal>*errmsg</literal> is set
       to <symbol>NULL</symbol> on success, else to a <function>malloc</function>'d error string explaining
       the problem.  (It is also possible for <literal>*errmsg</literal> to be
       set to <symbol>NULL</symbol> and the function to return <symbol>NULL</symbol>;
       this indicates an out-of-memory condition.)
      </para>

      <para>
       After processing the options array, free it by passing it to
       <xref linkend="libpq-PQconninfoFree"/>.  If this is not done, some memory
       is leaked for each call to <xref linkend="libpq-PQconninfoParse"/>.
       Conversely, if an error occurs and <literal>errmsg</literal> is not <symbol>NULL</symbol>,
       be sure to free the error string using <xref linkend="libpq-PQfreemem"/>.
      </para>

   </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfinish">
     <term><function>PQfinish</function><indexterm><primary>PQfinish</primary></indexterm></term>
     <listitem>
      <para>
       Closes  the  connection to the server.  Also frees
       memory used by the <structname>PGconn</structname> object.
<synopsis>
void PQfinish(PGconn *conn);
</synopsis>
      </para>

      <para>
       Note that even if the server connection attempt fails (as
       indicated by <xref linkend="libpq-PQstatus"/>), the application should call <xref linkend="libpq-PQfinish"/>
       to free the memory used by the <structname>PGconn</structname> object.
       The <structname>PGconn</structname> pointer must not be used again after
       <xref linkend="libpq-PQfinish"/> has been called.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQreset">
     <term><function>PQreset</function><indexterm><primary>PQreset</primary></indexterm></term>
     <listitem>
      <para>
       Resets the communication channel to the server.
<synopsis>
void PQreset(PGconn *conn);
</synopsis>
      </para>

      <para>
       This function will close the connection
       to the server and attempt to establish a new
       connection, using all the same
       parameters previously used.  This might be useful for
       error recovery if a working connection is lost.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQresetStart">
     <term><function>PQresetStart</function><indexterm><primary>PQresetStart</primary></indexterm></term>
     <term><function>PQresetPoll</function><indexterm><primary>PQresetPoll</primary></indexterm></term>
     <listitem>
      <para>
       Reset the communication channel to the server, in a nonblocking manner.

<synopsis>
int PQresetStart(PGconn *conn);

PostgresPollingStatusType PQresetPoll(PGconn *conn);
</synopsis>
      </para>

      <para>
       These functions will close the connection to the server and attempt to
       establish a new connection, using all the same
       parameters previously used. This can be useful for error recovery if a
       working connection is lost. They differ from <xref linkend="libpq-PQreset"/> (above) in that they
       act in a nonblocking manner. These functions suffer from the same
       restrictions as <xref linkend="libpq-PQconnectStartParams"/>, <function>PQconnectStart</function>
       and <function>PQconnectPoll</function>.
      </para>

      <para>
       To initiate a connection reset, call
       <xref linkend="libpq-PQresetStart"/>. If it returns 0, the reset has
       failed. If it returns 1, poll the reset using
       <function>PQresetPoll</function> in exactly the same way as you
       would create the connection using <function>PQconnectPoll</function>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQpingParams">
     <term><function>PQpingParams</function><indexterm><primary>PQpingParams</primary></indexterm></term>
     <listitem>
      <para>
       <xref linkend="libpq-PQpingParams"/> reports the status of the
       server.  It accepts connection parameters identical to those of
       <xref linkend="libpq-PQconnectdbParams"/>, described above.  It is not
       necessary to supply correct user name, password, or database name
       values to obtain the server status; however, if incorrect values
       are provided, the server will log a failed connection attempt.

<synopsis>
PGPing PQpingParams(const char * const *keywords,
                    const char * const *values,
                    int expand_dbname);
</synopsis>

       The function returns one of the following values:

       <variablelist>
        <varlistentry id="libpq-PQpingParams-PQPING_OK">
         <term><literal>PQPING_OK</literal></term>
         <listitem>
          <para>
           The server is running and appears to be accepting connections.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-PQpingParams-PQPING_REJECT">
         <term><literal>PQPING_REJECT</literal></term>
         <listitem>
          <para>
           The server is running but is in a state that disallows connections
           (startup, shutdown, or crash recovery).
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-PQpingParams-PQPING_NO_RESPONSE">
         <term><literal>PQPING_NO_RESPONSE</literal></term>
         <listitem>
          <para>
           The server could not be contacted.  This might indicate that the
           server is not running, or that there is something wrong with the
           given connection parameters (for example, wrong port number), or
           that there is a network connectivity problem (for example, a
           firewall blocking the connection request).
          </para>
         </listitem>
        </varlistentry>

        <varlistentry id="libpq-PQpingParams-PQPING_NO_ATTEMPT">
         <term><literal>PQPING_NO_ATTEMPT</literal></term>
         <listitem>
          <para>
           No attempt was made to contact the server, because the supplied
           parameters were obviously incorrect or there was some client-side
           problem (for example, out of memory).
          </para>
         </listitem>
        </varlistentry>
       </variablelist>

      </para>

     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQping">
     <term><function>PQping</function><indexterm><primary>PQping</primary></indexterm></term>
     <listitem>
      <para>
       <xref linkend="libpq-PQping"/> reports the status of the
       server.  It accepts connection parameters identical to those of
       <xref linkend="libpq-PQconnectdb"/>, described above.  It is not
       necessary to supply correct user name, password, or database name
       values to obtain the server status; however, if incorrect values
       are provided, the server will log a failed connection attempt.

<synopsis>
PGPing PQping(const char *conninfo);
</synopsis>
      </para>

      <para>
       The return values are the same as for <xref linkend="libpq-PQpingParams"/>.
      </para>

     </listitem>
    </varlistentry>

    <varlistentry id="libpq-pqsetsslkeypasshook-openssl">
     <term><function>PQsetSSLKeyPassHook_OpenSSL</function><indexterm><primary>PQsetSSLKeyPassHook_OpenSSL</primary></indexterm></term>
     <listitem>
      <para>
       <function>PQsetSSLKeyPassHook_OpenSSL</function> lets an application override
       <application>libpq</application>'s <link linkend="libpq-ssl-clientcert">default
       handling of encrypted client certificate key files</link> using
       <xref linkend="libpq-connect-sslpassword"/> or interactive prompting.

<synopsis>
void PQsetSSLKeyPassHook_OpenSSL(PQsslKeyPassHook_OpenSSL_type hook);
</synopsis>

       The application passes a pointer to a callback function with signature:
<programlisting>
int callback_fn(char *buf, int size, PGconn *conn);
</programlisting>
       which <application>libpq</application> will then call
       <emphasis>instead of</emphasis> its default
       <function>PQdefaultSSLKeyPassHook_OpenSSL</function> handler. The
       callback should determine the password for the key and copy it to
       result-buffer <parameter>buf</parameter> of size
       <parameter>size</parameter>. The string in <parameter>buf</parameter>
       must be null-terminated. The callback must return the length of the
       password stored in <parameter>buf</parameter> excluding the null
       terminator. On failure, the callback should set
       <literal>buf[0] = '\0'</literal> and return 0. See
       <function>PQdefaultSSLKeyPassHook_OpenSSL</function> in
       <application>libpq</application>'s source code for an example.
      </para>

      <para>
       If the user specified an explicit key location,
       its path will be in <literal>conn->sslkey</literal> when the callback
       is invoked. This will be empty if the default key path is being used.
       For keys that are engine specifiers, it is up to engine implementations
       whether they use the <productname>OpenSSL</productname> password
       callback or define their own handling.
      </para>

      <para>
       The app callback may choose to delegate unhandled cases to
       <function>PQdefaultSSLKeyPassHook_OpenSSL</function>,
       or call it first and try something else if it returns 0, or completely override it.
      </para>

      <para>
       The callback <emphasis>must not</emphasis> escape normal flow control with exceptions,
       <function>longjmp(...)</function>, etc. It must return normally.
      </para>

     </listitem>
    </varlistentry>

    <varlistentry id="libpq-pqgetsslkeypasshook-openssl">
     <term><function>PQgetSSLKeyPassHook_OpenSSL</function><indexterm><primary>PQgetSSLKeyPassHook_OpenSSL</primary></indexterm></term>
     <listitem>
      <para>
       <function>PQgetSSLKeyPassHook_OpenSSL</function> returns the current
       client certificate key password hook, or <literal>NULL</literal>
       if none has been set.

<synopsis>
PQsslKeyPassHook_OpenSSL_type PQgetSSLKeyPassHook_OpenSSL(void);
</synopsis>
      </para>

     </listitem>
    </varlistentry>

   </variablelist>
  </para>

  <sect2 id="libpq-connstring">
   <title>Connection Strings</title>

   <indexterm zone="libpq-connstring">
    <primary><literal>conninfo</literal></primary>
   </indexterm>

   <indexterm zone="libpq-connstring">
    <primary><literal>URI</literal></primary>
   </indexterm>

   <para>
    Several <application>libpq</application> functions parse a user-specified string to obtain
    connection parameters.  There are two accepted formats for these strings:
    plain keyword/value strings
    and URIs.  URIs generally follow
    <ulink url="https://tools.ietf.org/html/rfc3986">RFC
    3986</ulink>, except that multi-host connection strings are allowed
    as further described below.
   </para>

   <sect3 id="libpq-connstring-keyword-value">
    <title>Keyword/Value Connection Strings</title>

   <para>
    In the keyword/value format, each parameter setting is in the form
    <replaceable>keyword</replaceable> <literal>=</literal>
    <replaceable>value</replaceable>, with space(s) between settings.
    Spaces around a setting's equal sign are
    optional. To write an empty value, or a value containing spaces, surround it
    with single quotes, for example <literal>keyword = 'a value'</literal>.
    Single quotes and backslashes within
    a value must be escaped with a backslash, i.e., <literal>\'</literal> and
    <literal>\\</literal>.
   </para>

   <para>
    Example:
<programlisting>
host=localhost port=5432 dbname=mydb connect_timeout=10
</programlisting>
   </para>

   <para>
    The recognized parameter key words are listed in <xref
    linkend="libpq-paramkeywords"/>.
   </para>
   </sect3>

   <sect3 id="libpq-connstring-uris">
    <title>Connection URIs</title>

   <para>
   The general form for a connection <acronym>URI</acronym> is:
<synopsis>
postgresql://<optional><replaceable>userspec</replaceable>@</optional><optional><replaceable>hostspec</replaceable></optional><optional>/<replaceable>dbname</replaceable></optional><optional>?<replaceable>paramspec</replaceable></optional>

<phrase>where <replaceable>userspec</replaceable> is:</phrase>

<replaceable>user</replaceable><optional>:<replaceable>password</replaceable></optional>

<phrase>and <replaceable>hostspec</replaceable> is:</phrase>

<optional><replaceable>host</replaceable></optional><optional>:<replaceable>port</replaceable></optional><optional>,...</optional>

<phrase>and <replaceable>paramspec</replaceable> is:</phrase>

<replaceable>name</replaceable>=<replaceable>value</replaceable><optional>&amp;...</optional>
</synopsis>
   </para>

   <para>
    The <acronym>URI</acronym> scheme designator can be either
    <literal>postgresql://</literal> or <literal>postgres://</literal>.  Each
    of the remaining <acronym>URI</acronym> parts is optional.  The
    following examples illustrate valid <acronym>URI</acronym> syntax:
<programlisting>
postgresql://
postgresql://localhost
postgresql://localhost:5433
postgresql://localhost/mydb
postgresql://user@localhost
postgresql://user:secret@localhost
postgresql://other@localhost/otherdb?connect_timeout=10&amp;application_name=myapp
postgresql://host1:123,host2:456/somedb?target_session_attrs=any&amp;application_name=myapp
</programlisting>
    Values that would normally appear in the hierarchical part of
    the <acronym>URI</acronym> can alternatively be given as named
    parameters.  For example:
<programlisting>
postgresql:///mydb?host=localhost&amp;port=5433
</programlisting>
    All named parameters must match key words listed in
    <xref linkend="libpq-paramkeywords"/>, except that for compatibility
    with JDBC connection <acronym>URI</acronym>s, instances
    of <literal>ssl=true</literal> are translated into
    <literal>sslmode=require</literal>.
   </para>

   <para>
    The connection <acronym>URI</acronym> needs to be encoded with <ulink
    url="https://tools.ietf.org/html/rfc3986#section-2.1">percent-encoding</ulink>
    if it includes symbols with special meaning in any of its parts.  Here is
    an example where the equal sign (<literal>=</literal>) is replaced with
    <literal>%3D</literal> and the space character with
    <literal>%20</literal>:
<programlisting>
postgresql://user@localhost:5433/mydb?options=-c%20synchronous_commit%3Doff
</programlisting>
   </para>

   <para>
    The host part may be either a host name or an IP address.  To specify an
    IPv6 address, enclose it in square brackets:
<synopsis>
postgresql://[2001:db8::1234]/database
</synopsis>
   </para>

   <para>
    The host part is interpreted as described for the parameter <xref
    linkend="libpq-connect-host"/>.  In particular, a Unix-domain socket
    connection is chosen if the host part is either empty or looks like an
    absolute path name,
    otherwise a TCP/IP connection is initiated.  Note, however, that the
    slash is a reserved character in the hierarchical part of the URI.  So, to
    specify a non-standard Unix-domain socket directory, either omit the host
    part of the URI and specify the host as a named parameter, or
    percent-encode the path in the host part of the URI:
<programlisting>
postgresql:///dbname?host=/var/lib/postgresql
postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
</programlisting>
   </para>

   <para>
    It is possible to specify multiple host components, each with an optional
    port component, in a single URI.  A URI of the form
    <literal>postgresql://host1:port1,host2:port2,host3:port3/</literal>
    is equivalent to a connection string of the form
    <literal>host=host1,host2,host3 port=port1,port2,port3</literal>.
    As further described below, each
    host will be tried in turn until a connection is successfully established.
   </para>
   </sect3>

   <sect3 id="libpq-multiple-hosts">
     <title>Specifying Multiple Hosts</title>

     <para>
       It is possible to specify multiple hosts to connect to, so that they are
       tried in the given order. In the Keyword/Value format, the <literal>host</literal>,
       <literal>hostaddr</literal>, and <literal>port</literal> options accept comma-separated
       lists of values. The same number of elements must be given in each
       option that is specified, such
       that e.g., the first <literal>hostaddr</literal> corresponds to the first host name,
       the second <literal>hostaddr</literal> corresponds to the second host name, and so
       forth. As an exception, if only one <literal>port</literal> is specified, it
       applies to all the hosts.
     </para>

     <para>
       In the connection URI format, you can list multiple <literal>host:port</literal> pairs
       separated by commas in the <literal>host</literal> component of the URI.
     </para>

     <para>
       In either format, a single host name can translate to multiple network
       addresses. A common example of this is a host that has both an IPv4 and
       an IPv6 address.
     </para>

     <para>
       When multiple hosts are specified, or when a single host name is
       translated to multiple addresses,  all the hosts and addresses will be
       tried in order, until one succeeds. If none of the hosts can be reached,
       the connection fails. If a connection is established successfully, but
       authentication fails, the remaining hosts in the list are not tried.
     </para>

     <para>
       If a password file is used, you can have different passwords for
       different hosts. All the other connection options are the same for every
       host in the list; it is not possible to e.g., specify different
       usernames for different hosts.
     </para>
   </sect3>
  </sect2>

  <sect2 id="libpq-paramkeywords">
   <title>Parameter Key Words</title>

   <para>
    The currently recognized parameter key words are:

    <variablelist>
     <varlistentry id="libpq-connect-host" xreflabel="host">
      <term><literal>host</literal></term>
      <listitem>
       <para>
        Name of host to connect to.<indexterm><primary>host
        name</primary></indexterm> If a host name looks like an absolute path
        name, it specifies Unix-domain communication rather than TCP/IP
        communication; the value is the name of the directory in which the
        socket file is stored.  (On Unix, an absolute path name begins with a
        slash.  On Windows, paths starting with drive letters are also
        recognized.)  If the host name starts with <literal>@</literal>, it is
        taken as a Unix-domain socket in the abstract namespace (currently
        supported on Linux and Windows).
        The default behavior when <literal>host</literal> is not
        specified, or is empty, is to connect to a Unix-domain
        socket<indexterm><primary>Unix domain socket</primary></indexterm> in
        <filename>/tmp</filename> (or whatever socket directory was specified
        when <productname>PostgreSQL</productname> was built).  On Windows,
        the default is to connect to <literal>localhost</literal>.
       </para>
       <para>
        A comma-separated list of host names is also accepted, in which case
        each host name in the list is tried in order; an empty item in the
        list selects the default behavior as explained above. See
        <xref linkend="libpq-multiple-hosts"/> for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-hostaddr" xreflabel="hostaddr">
      <term><literal>hostaddr</literal></term>
      <listitem>
       <para>
        Numeric IP address of host to connect to.  This should be in the
        standard IPv4 address format, e.g., <literal>172.28.40.9</literal>.  If
        your machine supports IPv6, you can also use those addresses.
        TCP/IP communication is
        always used when a nonempty string is specified for this parameter.
        If this parameter is not specified, the value of <literal>host</literal>
        will be looked up to find the corresponding IP address &mdash; or, if
        <literal>host</literal> specifies an IP address, that value will be
        used directly.
       </para>

       <para>
        Using <literal>hostaddr</literal> allows the
        application to avoid a host name look-up, which might be important
        in applications with time constraints. However, a host name is
        required for GSSAPI or SSPI authentication
        methods, as well as for <literal>verify-full</literal> SSL
        certificate verification.  The following rules are used:
        <itemizedlist>
         <listitem>
          <para>
           If <literal>host</literal> is specified
           without <literal>hostaddr</literal>, a host name lookup occurs.
           (When using <function>PQconnectPoll</function>, the lookup occurs
           when <function>PQconnectPoll</function> first considers this host
           name, and it may cause <function>PQconnectPoll</function> to block
           for a significant amount of time.)
          </para>
         </listitem>
         <listitem>
          <para>
           If <literal>hostaddr</literal> is specified without <literal>host</literal>,
           the value for <literal>hostaddr</literal> gives the server network address.
           The connection attempt will fail if the authentication
           method requires a host name.
          </para>
         </listitem>
         <listitem>
          <para>
           If both <literal>host</literal> and <literal>hostaddr</literal> are specified,
           the value for <literal>hostaddr</literal> gives the server network address.
           The value for <literal>host</literal> is ignored unless the
           authentication method requires it, in which case it will be
           used as the host name.
          </para>
         </listitem>
        </itemizedlist>
        Note that authentication is likely to fail if <literal>host</literal>
        is not the name of the server at network address <literal>hostaddr</literal>.
        Also, when both <literal>host</literal> and <literal>hostaddr</literal>
        are specified, <literal>host</literal>
        is used to identify the connection in a password file (see
        <xref linkend="libpq-pgpass"/>).
       </para>

       <para>
        A comma-separated list of <literal>hostaddr</literal> values is also
        accepted, in which case each host in the list is tried in order.
        An empty item in the list causes the corresponding host name to be
        used, or the default host name if that is empty as well. See
        <xref linkend="libpq-multiple-hosts"/> for details.
       </para>
       <para>
        Without either a host name or host address,
        <application>libpq</application> will connect using a local
        Unix-domain socket; or on Windows, it will attempt to connect to
        <literal>localhost</literal>.
       </para>
       </listitem>
      </varlistentry>

      <varlistentry id="libpq-connect-port" xreflabel="port">
       <term><literal>port</literal></term>
       <listitem>
       <para>
        Port number to connect to at the server host, or socket file
        name extension for Unix-domain
        connections.<indexterm><primary>port</primary></indexterm>
        If multiple hosts were given in the <literal>host</literal> or
        <literal>hostaddr</literal> parameters, this parameter may specify a
        comma-separated list of ports of the same length as the host list, or
        it may specify a single port number to be used for all hosts.
        An empty string, or an empty item in a comma-separated list,
        specifies the default port number established
        when <productname>PostgreSQL</productname> was built.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-dbname" xreflabel="dbname">
      <term><literal>dbname</literal></term>
      <listitem>
      <para>
       The database name.  Defaults to be the same as the user name.
       In certain contexts, the value is checked for extended
       formats; see <xref linkend="libpq-connstring"/> for more details on
       those.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-user" xreflabel="user">
      <term><literal>user</literal></term>
      <listitem>
      <para>
       <productname>PostgreSQL</productname> user name to connect as.
       Defaults to be the same as the operating system name of the user
       running the application.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-password" xreflabel="password">
      <term><literal>password</literal></term>
      <listitem>
      <para>
       Password to be used if the server demands password authentication.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-passfile" xreflabel="passfile">
      <term><literal>passfile</literal></term>
      <listitem>
      <para>
       Specifies the name of the file used to store passwords
       (see <xref linkend="libpq-pgpass"/>).
       Defaults to <filename>~/.pgpass</filename>, or
       <filename>%APPDATA%\postgresql\pgpass.conf</filename> on Microsoft Windows.
       (No error is reported if this file does not exist.)
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-require-auth" xreflabel="require_auth">
      <term><literal>require_auth</literal></term>
      <listitem>
      <para>
        Specifies the authentication method that the client requires from the
        server. If the server does not use the required method to authenticate
        the client, or if the authentication handshake is not fully completed by
        the server, the connection will fail. A comma-separated list of methods
        may also be provided, of which the server must use exactly one in order
        for the connection to succeed. By default, any authentication method is
        accepted, and the server is free to skip authentication altogether.
      </para>
      <para>
        Methods may be negated with the addition of a <literal>!</literal>
        prefix, in which case the server must <emphasis>not</emphasis> attempt
        the listed method; any other method is accepted, and the server is free
        not to authenticate the client at all. If a comma-separated list is
        provided, the server may not attempt <emphasis>any</emphasis> of the
        listed negated methods. Negated and non-negated forms may not be
        combined in the same setting.
      </para>
      <para>
        As a final special case, the <literal>none</literal> method requires the
        server not to use an authentication challenge. (It may also be negated,
        to require some form of authentication.)
      </para>
      <para>
        The following methods may be specified:

        <variablelist>
         <varlistentry>
          <term><literal>password</literal></term>
          <listitem>
           <para>
            The server must request plaintext password authentication.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>md5</literal></term>
          <listitem>
           <para>
            The server must request MD5 hashed password authentication.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>gss</literal></term>
          <listitem>
           <para>
            The server must either request a Kerberos handshake via
            <acronym>GSSAPI</acronym> or establish a
            <acronym>GSS</acronym>-encrypted channel (see also
            <xref linkend="libpq-connect-gssencmode" />).
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>sspi</literal></term>
          <listitem>
           <para>
            The server must request Windows <acronym>SSPI</acronym>
            authentication.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>scram-sha-256</literal></term>
          <listitem>
           <para>
            The server must successfully complete a SCRAM-SHA-256 authentication
            exchange with the client.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>none</literal></term>
          <listitem>
           <para>
            The server must not prompt the client for an authentication
            exchange. (This does not prohibit client certificate authentication
            via TLS, nor GSS authentication via its encrypted transport.)
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-channel-binding" xreflabel="channel_binding">
      <term><literal>channel_binding</literal></term>
      <listitem>
      <para>
        This option controls the client's use of channel binding. A setting
        of <literal>require</literal> means that the connection must employ
        channel binding, <literal>prefer</literal> means that the client will
        choose channel binding if available, and <literal>disable</literal>
        prevents the use of channel binding. The default
        is <literal>prefer</literal> if
        <productname>PostgreSQL</productname> is compiled with SSL support;
        otherwise the default is <literal>disable</literal>.
      </para>
      <para>
        Channel binding is a method for the server to authenticate itself to
        the client. It is only supported over SSL connections
        with <productname>PostgreSQL</productname> 11 or later servers using
        the <literal>SCRAM</literal> authentication method.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-connect-timeout" xreflabel="connect_timeout">
      <term><literal>connect_timeout</literal></term>
      <listitem>
      <para>
       Maximum time to wait while connecting, in seconds (write as a decimal integer,
       e.g., <literal>10</literal>).  Zero, negative, or not specified means
       wait indefinitely.  The minimum allowed timeout is 2 seconds, therefore
       a value of <literal>1</literal> is interpreted as <literal>2</literal>.
       This timeout applies separately to each host name or IP address.
       For example, if you specify two hosts and <literal>connect_timeout</literal>
       is 5, each host will time out if no connection is made within 5
       seconds, so the total time spent waiting for a connection might be
       up to 10 seconds.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-client-encoding" xreflabel="client_encoding">
      <term><literal>client_encoding</literal></term>
      <listitem>
      <para>
       This sets the <varname>client_encoding</varname>
       configuration parameter for this connection.  In addition to
       the values accepted by the corresponding server option, you
       can use <literal>auto</literal> to determine the right
       encoding from the current locale in the client
       (<envar>LC_CTYPE</envar> environment variable on Unix
       systems).
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-options" xreflabel="options">
      <term><literal>options</literal></term>
      <listitem>
       <para>
        Specifies command-line options to send to the server at connection
        start.  For example, setting this to <literal>-c geqo=off</literal> sets the
        session's value of the <varname>geqo</varname> parameter to
        <literal>off</literal>.  Spaces within this string are considered to
        separate command-line arguments, unless escaped with a backslash
        (<literal>\</literal>); write <literal>\\</literal> to represent a literal
        backslash.  For a detailed discussion of the available
        options, consult <xref linkend="runtime-config"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-application-name" xreflabel="application_name">
      <term><literal>application_name</literal></term>
      <listitem>
       <para>
        Specifies a value for the <xref linkend="guc-application-name"/>
        configuration parameter.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-fallback-application-name" xreflabel="fallback_application_name">
      <term><literal>fallback_application_name</literal></term>
      <listitem>
       <para>
        Specifies a fallback value for the <xref
        linkend="guc-application-name"/> configuration parameter.
        This value will be used if no value has been given for
        <literal>application_name</literal> via a connection parameter or the
        <envar>PGAPPNAME</envar> environment variable.  Specifying
        a fallback name is useful in generic utility programs that
        wish to set a default application name but allow it to be
        overridden by the user.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-keepalives" xreflabel="keepalives">
      <term><literal>keepalives</literal></term>
      <listitem>
       <para>
        Controls whether client-side TCP keepalives are used. The default
        value is 1, meaning on, but you can change this to 0, meaning off,
        if keepalives are not wanted.  This parameter is ignored for
        connections made via a Unix-domain socket.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-keepalives-idle" xreflabel="keepalives_idle">
      <term><literal>keepalives_idle</literal></term>
      <listitem>
       <para>
        Controls the number of seconds of inactivity after which TCP should
        send a keepalive message to the server.  A value of zero uses the
        system default. This parameter is ignored for connections made via a
        Unix-domain socket, or if keepalives are disabled.
        It is only supported on systems where <symbol>TCP_KEEPIDLE</symbol> or
        an equivalent socket option is available, and on Windows; on other
        systems, it has no effect.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-keepalives-interval" xreflabel="keepalives_interval">
      <term><literal>keepalives_interval</literal></term>
      <listitem>
       <para>
        Controls the number of seconds after which a TCP keepalive message
        that is not acknowledged by the server should be retransmitted.  A
        value of zero uses the system default. This parameter is ignored for
        connections made via a Unix-domain socket, or if keepalives are disabled.
        It is only supported on systems where <symbol>TCP_KEEPINTVL</symbol> or
        an equivalent socket option is available, and on Windows; on other
        systems, it has no effect.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-keepalives-count" xreflabel="keepalives_count">
      <term><literal>keepalives_count</literal></term>
      <listitem>
       <para>
        Controls the number of TCP keepalives that can be lost before the
        client's connection to the server is considered dead.  A value of
        zero uses the system default. This parameter is ignored for
        connections made via a Unix-domain socket, or if keepalives are disabled.
        It is only supported on systems where <symbol>TCP_KEEPCNT</symbol> or
        an equivalent socket option is available; on other systems, it has no
        effect.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-tcp-user-timeout" xreflabel="tcp_user_timeout">
      <term><literal>tcp_user_timeout</literal></term>
      <listitem>
       <para>
        Controls the number of milliseconds that transmitted data may
        remain unacknowledged before a connection is forcibly closed.
        A value of zero uses the system default. This parameter is
        ignored for connections made via a Unix-domain socket.
        It is only supported on systems where <symbol>TCP_USER_TIMEOUT</symbol>
        is available; on other systems, it has no effect.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-replication" xreflabel="replication">
      <term><literal>replication</literal></term>
      <listitem>
      <para>
       This option determines whether the connection should use the
       replication protocol instead of the normal protocol.  This is what
       PostgreSQL replication connections as well as tools such as
       <application>pg_basebackup</application> use internally, but it can
       also be used by third-party applications.  For a description of the
       replication protocol, consult <xref linkend="protocol-replication"/>.
      </para>

      <para>
       The following values, which are case-insensitive, are supported:
       <variablelist>
        <varlistentry>
         <term>
          <literal>true</literal>, <literal>on</literal>,
          <literal>yes</literal>, <literal>1</literal>
         </term>
         <listitem>
          <para>
           The connection goes into physical replication mode.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>database</literal></term>
         <listitem>
          <para>
           The connection goes into logical replication mode, connecting to
           the database specified in the <literal>dbname</literal> parameter.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term>
          <literal>false</literal>, <literal>off</literal>,
          <literal>no</literal>, <literal>0</literal>
         </term>
         <listitem>
          <para>
           The connection is a regular one, which is the default behavior.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>

      <para>
       In physical or logical replication mode, only the simple query protocol
       can be used.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-gssencmode" xreflabel="gssencmode">
      <term><literal>gssencmode</literal></term>
      <listitem>
       <para>
        This option determines whether or with what priority a secure
        <acronym>GSS</acronym> TCP/IP connection will be negotiated with the
        server. There are three modes:

        <variablelist>
         <varlistentry>
          <term><literal>disable</literal></term>
          <listitem>
           <para>
            only try a non-<acronym>GSSAPI</acronym>-encrypted connection
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>prefer</literal> (default)</term>
          <listitem>
           <para>
            if there are <acronym>GSSAPI</acronym> credentials present (i.e.,
            in a credentials cache), first try
            a <acronym>GSSAPI</acronym>-encrypted connection; if that fails or
            there are no credentials, try a
            non-<acronym>GSSAPI</acronym>-encrypted connection.  This is the
            default when <productname>PostgreSQL</productname> has been
            compiled with <acronym>GSSAPI</acronym> support.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>require</literal></term>
          <listitem>
           <para>
            only try a <acronym>GSSAPI</acronym>-encrypted connection
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>

       <para>
        <literal>gssencmode</literal> is ignored for Unix domain socket
        communication.  If <productname>PostgreSQL</productname> is compiled
        without GSSAPI support, using the <literal>require</literal> option
        will cause an error, while <literal>prefer</literal> will be accepted
        but <application>libpq</application> will not actually attempt
        a <acronym>GSSAPI</acronym>-encrypted
        connection.<indexterm><primary>GSSAPI</primary><secondary sortas="libpq">with
        libpq</secondary></indexterm>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslmode" xreflabel="sslmode">
      <term><literal>sslmode</literal></term>
      <listitem>
       <para>
        This option determines whether or with what priority a secure
        <acronym>SSL</acronym> TCP/IP connection will be negotiated with the
        server. There are six modes:

        <variablelist>
         <varlistentry>
          <term><literal>disable</literal></term>
          <listitem>
           <para>
            only try a non-<acronym>SSL</acronym> connection
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>allow</literal></term>
          <listitem>
           <para>
            first try a non-<acronym>SSL</acronym> connection; if that
            fails, try an <acronym>SSL</acronym> connection
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>prefer</literal> (default)</term>
          <listitem>
           <para>
            first try an <acronym>SSL</acronym> connection; if that fails,
            try a non-<acronym>SSL</acronym> connection
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>require</literal></term>
          <listitem>
           <para>
            only try an <acronym>SSL</acronym> connection. If a root CA
            file is present, verify the certificate in the same way as
            if <literal>verify-ca</literal> was specified
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>verify-ca</literal></term>
          <listitem>
           <para>
            only try an <acronym>SSL</acronym> connection, and verify that
            the server certificate is issued by a trusted
            certificate authority (<acronym>CA</acronym>)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>verify-full</literal></term>
          <listitem>
           <para>
            only try an <acronym>SSL</acronym> connection, verify that the
            server certificate is issued by a
            trusted <acronym>CA</acronym> and that the requested server host name
            matches that in the certificate
           </para>
          </listitem>
         </varlistentry>
        </variablelist>

        See <xref linkend="libpq-ssl"/> for a detailed description of how
        these options work.
       </para>

       <para>
        <literal>sslmode</literal> is ignored for Unix domain socket
        communication.
        If <productname>PostgreSQL</productname> is compiled without SSL support,
        using options <literal>require</literal>, <literal>verify-ca</literal>, or
        <literal>verify-full</literal> will cause an error, while
        options <literal>allow</literal> and <literal>prefer</literal> will be
        accepted but <application>libpq</application> will not actually attempt
        an <acronym>SSL</acronym>
        connection.<indexterm><primary>SSL</primary><secondary
        sortas="libpq">with libpq</secondary></indexterm>
       </para>

       <para>
        Note that if <acronym>GSSAPI</acronym> encryption is possible,
        that will be used in preference to <acronym>SSL</acronym>
        encryption, regardless of the value of <literal>sslmode</literal>.
        To force use of <acronym>SSL</acronym> encryption in an
        environment that has working <acronym>GSSAPI</acronym>
        infrastructure (such as a Kerberos server), also
        set <literal>gssencmode</literal> to <literal>disable</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-requiressl" xreflabel="requiressl">
      <term><literal>requiressl</literal></term>
      <listitem>
       <para>
        This option is deprecated in favor of the <literal>sslmode</literal>
        setting.
       </para>

       <para>
        If set to 1, an <acronym>SSL</acronym> connection to the server
        is required (this is equivalent to <literal>sslmode</literal>
        <literal>require</literal>).  <application>libpq</application> will then refuse
        to connect if the server does not accept an
        <acronym>SSL</acronym> connection.  If set to 0 (default),
        <application>libpq</application> will negotiate the connection type with
        the server (equivalent to <literal>sslmode</literal>
        <literal>prefer</literal>).  This option is only available if
        <productname>PostgreSQL</productname> is compiled with SSL support.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslcompression" xreflabel="sslcompression">
      <term><literal>sslcompression</literal></term>
      <listitem>
       <para>
        If set to 1, data sent over SSL connections will be compressed.  If
        set to 0, compression will be disabled.  The default is 0.  This
        parameter is ignored if a connection without SSL is made.
       </para>

       <para>
        SSL compression is nowadays considered insecure and its use is no
        longer recommended.  <productname>OpenSSL</productname> 1.1.0 disables
        compression by default, and many operating system distributions
        disable it in prior versions as well, so setting this parameter to on
        will not have any effect if the server does not accept compression.
        <productname>PostgreSQL</productname> 14 disables compression
        completely in the backend.
       </para>

       <para>
        If security is not a primary concern, compression can improve
        throughput if the network is the bottleneck.  Disabling compression
        can improve response time and throughput if CPU performance is the
        limiting factor.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslcert" xreflabel="sslcert">
      <term><literal>sslcert</literal></term>
      <listitem>
       <para>
        This parameter specifies the file name of the client SSL
        certificate, replacing the default
        <filename>~/.postgresql/postgresql.crt</filename>.
        This parameter is ignored if an SSL connection is not made.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslkey" xreflabel="sslkey">
      <term><literal>sslkey</literal></term>
      <listitem>
       <para>
        This parameter specifies the location for the secret key used for
        the client certificate. It can either specify a file name that will
        be used instead of the default
        <filename>~/.postgresql/postgresql.key</filename>, or it can specify a key
        obtained from an external <quote>engine</quote> (engines are
        <productname>OpenSSL</productname> loadable modules).  An external engine
        specification should consist of a colon-separated engine name and
        an engine-specific key identifier.  This parameter is ignored if an
        SSL connection is not made.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslpassword" xreflabel="sslpassword">
      <term><literal>sslpassword</literal></term>
      <listitem>
       <para>
        This parameter specifies the password for the secret key specified in
        <literal>sslkey</literal>, allowing client certificate private keys
        to be stored in encrypted form on disk even when interactive passphrase
        input is not practical.
       </para>
       <para>
        Specifying this parameter with any non-empty value suppresses the
        <literal>Enter PEM pass phrase:</literal>
        prompt that <productname>OpenSSL</productname> will emit by default
        when an encrypted client certificate key is provided to
        <literal>libpq</literal>.
       </para>
       <para>
        If the key is not encrypted this parameter is ignored. The parameter
        has no effect on keys specified by <productname>OpenSSL</productname>
        engines unless the engine uses the <productname>OpenSSL</productname>
        password callback mechanism for prompts.
       </para>
       <para>
        There is no environment variable equivalent to this option, and no
        facility for looking it up in <filename>.pgpass</filename>. It can be
        used in a service file connection definition. Users with
        more sophisticated uses should consider using <productname>OpenSSL</productname> engines and
        tools like PKCS#11 or USB crypto offload devices.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslcertmode" xreflabel="sslcertmode">
      <term><literal>sslcertmode</literal></term>
      <listitem>
       <para>
        This option determines whether a client certificate may be sent to the
        server, and whether the server is required to request one. There are
        three modes:

        <variablelist>
         <varlistentry>
          <term><literal>disable</literal></term>
          <listitem>
           <para>
            A client certificate is never sent, even if one is available
            (default location or provided via
            <xref linkend="libpq-connect-sslcert" />).
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>allow</literal> (default)</term>
          <listitem>
           <para>
            A certificate may be sent, if the server requests one and the
            client has one to send.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>require</literal></term>
          <listitem>
           <para>
            The server <emphasis>must</emphasis> request a certificate. The
            connection will fail if the client does not send a certificate and
            the server successfully authenticates the client anyway.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>

       <note>
        <para>
         <literal>sslcertmode=require</literal> doesn't add any additional
         security, since there is no guarantee that the server is validating
         the certificate correctly; PostgreSQL servers generally request TLS
         certificates from clients whether they validate them or not. The
         option may be useful when troubleshooting more complicated TLS
         setups.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslrootcert" xreflabel="sslrootcert">
      <term><literal>sslrootcert</literal></term>
      <listitem>
       <para>
        This parameter specifies the name of a file containing SSL
        certificate authority (<acronym>CA</acronym>) certificate(s).
        If the file exists, the server's certificate will be verified
        to be signed by one of these authorities.  The default is
        <filename>~/.postgresql/root.crt</filename>.
       </para>
       <para>
        The special value <literal>system</literal> may be specified instead, in
        which case the system's trusted CA roots will be loaded. The exact
        locations of these root certificates differ by SSL implementation and
        platform. For <productname>OpenSSL</productname> in particular, the
        locations may be further modified by the <envar>SSL_CERT_DIR</envar>
        and <envar>SSL_CERT_FILE</envar> environment variables.
       </para>
       <note>
        <para>
         When using <literal>sslrootcert=system</literal>, the default
         <literal>sslmode</literal> is changed to <literal>verify-full</literal>,
         and any weaker setting will result in an error. In most cases it is
         trivial for anyone to obtain a certificate trusted by the system for a
         hostname they control, rendering <literal>verify-ca</literal> and all
         weaker modes useless.
        </para>
        <para>
         The magic <literal>system</literal> value will take precedence over a
         local certificate file with the same name. If for some reason you find
         yourself in this situation, use an alternative path like
         <literal>sslrootcert=./system</literal> instead.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslcrl" xreflabel="sslcrl">
      <term><literal>sslcrl</literal></term>
      <listitem>
       <para>
        This parameter specifies the file name of the SSL server certificate
        revocation list (CRL).  Certificates listed in this file, if it
        exists, will be rejected while attempting to authenticate the
        server's certificate.  If neither
        <xref linkend="libpq-connect-sslcrl"/> nor
        <xref linkend="libpq-connect-sslcrldir"/> is set, this setting is
        taken as
        <filename>~/.postgresql/root.crl</filename>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslcrldir" xreflabel="sslcrldir">
      <term><literal>sslcrldir</literal></term>
      <listitem>
       <para>
        This parameter specifies the directory name of the SSL server certificate
        revocation list (CRL).  Certificates listed in the files in this
        directory, if it exists, will be rejected while attempting to
        authenticate the server's certificate.
       </para>

       <para>
        The directory needs to be prepared with the
        <productname>OpenSSL</productname> command
        <literal>openssl rehash</literal> or <literal>c_rehash</literal>.  See
        its documentation for details.
       </para>

       <para>
        Both <literal>sslcrl</literal> and <literal>sslcrldir</literal> can be
        specified together.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-sslsni" xreflabel="sslsni">
      <term><literal>sslsni</literal><indexterm><primary>Server Name Indication</primary></indexterm></term>
      <listitem>
       <para>
        If set to 1 (default), libpq sets the TLS extension <quote>Server Name
        Indication</quote> (<acronym>SNI</acronym>) on SSL-enabled connections.
        By setting this parameter to 0, this is turned off.
       </para>

       <para>
        The Server Name Indication can be used by SSL-aware proxies to route
        connections without having to decrypt the SSL stream.  (Note that this
        requires a proxy that is aware of the PostgreSQL protocol handshake,
        not just any SSL proxy.)  However, <acronym>SNI</acronym> makes the
        destination host name appear in cleartext in the network traffic, so
        it might be undesirable in some cases.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-requirepeer" xreflabel="requirepeer">
      <term><literal>requirepeer</literal></term>
      <listitem>
       <para>
        This parameter specifies the operating-system user name of the
        server, for example <literal>requirepeer=postgres</literal>.
        When making a Unix-domain socket connection, if this
        parameter is set, the client checks at the beginning of the
        connection that the server process is running under the specified
        user name; if it is not, the connection is aborted with an error.
        This parameter can be used to provide server authentication similar
        to that available with SSL certificates on TCP/IP connections.
        (Note that if the Unix-domain socket is in
        <filename>/tmp</filename> or another publicly writable location,
        any user could start a server listening there.  Use this parameter
        to ensure that you are connected to a server run by a trusted user.)
        This option is only supported on platforms for which the
        <literal>peer</literal> authentication method is implemented; see
        <xref linkend="auth-peer"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-ssl-min-protocol-version" xreflabel="ssl_min_protocol_version">
      <term><literal>ssl_min_protocol_version</literal></term>
      <listitem>
       <para>
        This parameter specifies the minimum SSL/TLS protocol version to allow
        for the connection. Valid values are <literal>TLSv1</literal>,
        <literal>TLSv1.1</literal>, <literal>TLSv1.2</literal> and
        <literal>TLSv1.3</literal>. The supported protocols depend on the
        version of <productname>OpenSSL</productname> used, older versions
        not supporting the most modern protocol versions. If not specified,
        the default is <literal>TLSv1.2</literal>, which satisfies industry
        best practices as of this writing.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-ssl-max-protocol-version" xreflabel="ssl_max_protocol_version">
      <term><literal>ssl_max_protocol_version</literal></term>
      <listitem>
       <para>
        This parameter specifies the maximum SSL/TLS protocol version to allow
        for the connection. Valid values are <literal>TLSv1</literal>,
        <literal>TLSv1.1</literal>, <literal>TLSv1.2</literal> and
        <literal>TLSv1.3</literal>. The supported protocols depend on the
        version of <productname>OpenSSL</productname> used, older versions
        not supporting the most modern protocol versions. If not set, this
        parameter is ignored and the connection will use the maximum bound
        defined by the backend, if set. Setting the maximum protocol version
        is mainly useful for testing or if some component has issues working
        with a newer protocol.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-krbsrvname" xreflabel="krbsrvname">
      <term><literal>krbsrvname</literal></term>
      <listitem>
       <para>
        Kerberos service name to use when authenticating with GSSAPI.
        This must match the service name specified in the server
        configuration for Kerberos authentication to succeed. (See also
        <xref linkend="gssapi-auth"/>.)
        The default value is normally <literal>postgres</literal>,
        but that can be changed when
        building <productname>PostgreSQL</productname> via
        the <option>--with-krb-srvnam</option> option
        of <application>configure</application>.
        In most environments, this parameter never needs to be changed.
        Some Kerberos implementations might require a different service name,
        such as Microsoft Active Directory which requires the service name
        to be in upper case (<literal>POSTGRES</literal>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-gsslib" xreflabel="gsslib">
      <term><literal>gsslib</literal></term>
      <listitem>
       <para>
        GSS library to use for GSSAPI authentication.
        Currently this is disregarded except on Windows builds that include
        both GSSAPI and SSPI support.  In that case, set
        this to <literal>gssapi</literal> to cause libpq to use the GSSAPI
        library for authentication instead of the default SSPI.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-gssdelegation" xreflabel="gssdelegation">
      <term><literal>gssdelegation</literal></term>
      <listitem>
       <para>
        Forward (delegate) GSS credentials to the server.  The default is
        <literal>0</literal> which means credentials will not be forwarded
        to the server.  Set this to <literal>1</literal> to have credentials
        forwarded when possible.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-service" xreflabel="service">
      <term><literal>service</literal></term>
      <listitem>
       <para>
        Service name to use for additional parameters.  It specifies a service
        name in <filename>pg_service.conf</filename> that holds additional connection parameters.
        This allows applications to specify only a service name so connection parameters
        can be centrally maintained. See <xref linkend="libpq-pgservice"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-target-session-attrs" xreflabel="target_session_attrs">
      <term><literal>target_session_attrs</literal></term>
      <listitem>
       <para>
        This option determines whether the session must have certain
        properties to be acceptable.  It's typically used in combination
        with multiple host names to select the first acceptable alternative
        among several hosts.  There are six modes:

        <variablelist>
         <varlistentry>
          <term><literal>any</literal> (default)</term>
          <listitem>
           <para>
            any successful connection is acceptable
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>read-write</literal></term>
          <listitem>
           <para>
            session must accept read-write transactions by default (that
            is, the server must not be in hot standby mode and
            the <varname>default_transaction_read_only</varname> parameter
            must be <literal>off</literal>)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>read-only</literal></term>
          <listitem>
           <para>
            session must not accept read-write transactions by default (the
            converse)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>primary</literal></term>
          <listitem>
           <para>
            server must not be in hot standby mode
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>standby</literal></term>
          <listitem>
           <para>
            server must be in hot standby mode
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>prefer-standby</literal></term>
          <listitem>
           <para>
            first try to find a standby server, but if none of the listed
            hosts is a standby server, try again in <literal>any</literal>
            mode
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-connect-load-balance-hosts" xreflabel="load_balance_hosts">
      <term><literal>load_balance_hosts</literal></term>
      <listitem>
       <para>
        Controls the order in which the client tries to connect to the available
        hosts and addresses. Once a connection attempt is successful no other
        hosts and addresses will be tried. This parameter is typically used in
        combination with multiple host names or a DNS record that returns
        multiple IPs. This parameter can be used in combination with
        <xref linkend="libpq-connect-target-session-attrs"/>
        to, for example, load balance over standby servers only. Once successfully
        connected, subsequent queries on the returned connection will all be
        sent to the same server. There are currently two modes:
        <variablelist>
         <varlistentry>
          <term><literal>disable</literal> (default)</term>
          <listitem>
           <para>
            No load balancing across hosts is performed.  Hosts are tried in
            the order in which they are provided and addresses are tried in
            the order they are received from DNS or a hosts file.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>random</literal></term>
          <listitem>
           <para>
            Hosts and addresses are tried in random order. This value is mostly
            useful when opening multiple connections at the same time, possibly
            from different machines. This way connections can be load balanced
            across multiple <productname>PostgreSQL</productname> servers.
           </para>
           <para>
            While random load balancing, due to its random nature, will almost
            never result in a completely uniform distribution, it statistically
            gets quite close. One important aspect here is that this algorithm
            uses two levels of random choices: First the hosts
            will be resolved in random order. Then secondly, before resolving
            the next host, all resolved addresses for the current host will be
            tried in random order. This behaviour can skew the amount of
            connections each node gets greatly in certain cases, for instance
            when some hosts resolve to more addresses than others. But such a
            skew can also be used on purpose, e.g. to increase the number of
            connections a larger server gets by providing its hostname multiple
            times in the host string.
           </para>
           <para>
            When using this value it's recommended to also configure a reasonable
            value for <xref linkend="libpq-connect-connect-timeout"/>. Because then,
            if one of the nodes that are used for load balancing is not responding,
            a new node will be tried.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect2>
 </sect1>

 <sect1 id="libpq-status">
  <title>Connection Status Functions</title>

  <para>
   These functions can be used to interrogate the status
   of an existing database connection object.
  </para>

  <tip>
   <para>
    <indexterm><primary>libpq-fe.h</primary></indexterm>
    <indexterm><primary>libpq-int.h</primary></indexterm>
    <application>libpq</application> application programmers should be careful to
    maintain the <structname>PGconn</structname> abstraction.  Use the accessor
    functions described below to get at the contents of <structname>PGconn</structname>.
    Reference to internal <structname>PGconn</structname> fields using
    <filename>libpq-int.h</filename> is not recommended because they are subject to change
    in the future.
   </para>
  </tip>

  <para>
   The following functions return parameter values established at connection.
   These values are fixed for the life of the connection.  If a multi-host
   connection string is used, the values of <xref linkend="libpq-PQhost"/>,
   <xref linkend="libpq-PQport"/>, and <xref linkend="libpq-PQpass"/> can change if a new connection
   is established using the same <structname>PGconn</structname> object.  Other values
   are fixed for the lifetime of the <structname>PGconn</structname> object.

   <variablelist>
    <varlistentry id="libpq-PQdb">
     <term><function>PQdb</function><indexterm><primary>PQdb</primary></indexterm></term>

     <listitem>
      <para>
       Returns the database name of the connection.
<synopsis>
char *PQdb(const PGconn *conn);
</synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQuser">
     <term><function>PQuser</function><indexterm><primary>PQuser</primary></indexterm></term>

     <listitem>
      <para>
       Returns the user name of the connection.
<synopsis>
char *PQuser(const PGconn *conn);
</synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQpass">
     <term><function>PQpass</function><indexterm><primary>PQpass</primary></indexterm></term>

     <listitem>
      <para>
       Returns the password of the connection.
<synopsis>
char *PQpass(const PGconn *conn);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQpass"/> will return either the password specified
       in the connection parameters, or if there was none and the password
       was obtained from the <link linkend="libpq-pgpass">password
       file</link>, it will return that.  In the latter case,
       if multiple hosts were specified in the connection parameters, it is
       not possible to rely on the result of <xref linkend="libpq-PQpass"/> until
       the connection is established.  The status of the connection can be
       checked using the function <xref linkend="libpq-PQstatus"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQhost">
     <term><function>PQhost</function><indexterm><primary>PQhost</primary></indexterm></term>

     <listitem>
      <para>
       Returns the server host name of the active connection.
       This can be a host name, an IP address, or a directory path if the
       connection is via Unix socket.  (The path case can be distinguished
       because it will always be an absolute path, beginning
       with <literal>/</literal>.)
<synopsis>
char *PQhost(const PGconn *conn);
</synopsis>
      </para>

      <para>
       If the connection parameters specified both <literal>host</literal> and
       <literal>hostaddr</literal>, then <xref linkend="libpq-PQhost"/> will
       return the <literal>host</literal> information.  If only
       <literal>hostaddr</literal> was specified, then that is returned.
       If multiple hosts were specified in the connection parameters,
       <xref linkend="libpq-PQhost"/> returns the host actually connected to.
      </para>

      <para>
       <xref linkend="libpq-PQhost"/> returns <symbol>NULL</symbol> if the
       <parameter>conn</parameter> argument is <symbol>NULL</symbol>.
       Otherwise, if there is an error producing the host information (perhaps
       if the connection has not been fully established or there was an
       error), it returns an empty string.
      </para>

      <para>
       If multiple hosts were specified in the connection parameters, it is
       not possible to rely on the result of <xref linkend="libpq-PQhost"/> until
       the connection is established.  The status of the connection can be
       checked using the function <xref linkend="libpq-PQstatus"/>.
      </para>
     </listitem>
    </varlistentry>


    <varlistentry id="libpq-PQhostaddr">
     <term><function>PQhostaddr</function><indexterm><primary>PQhostaddr</primary></indexterm></term>

     <listitem>
      <para>
       Returns the server IP address of the active connection.
       This can be the address that a host name resolved to,
       or an IP address provided through the <literal>hostaddr</literal>
       parameter.
<synopsis>
char *PQhostaddr(const PGconn *conn);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQhostaddr"/> returns <symbol>NULL</symbol> if the
       <parameter>conn</parameter> argument is <symbol>NULL</symbol>.
       Otherwise, if there is an error producing the host information
       (perhaps if the connection has not been fully established or
       there was an error), it returns an empty string.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQport">
     <term><function>PQport</function><indexterm><primary>PQport</primary></indexterm></term>

     <listitem>
      <para>
       Returns the port of the active connection.

<synopsis>
char *PQport(const PGconn *conn);
</synopsis>
      </para>

      <para>
       If multiple ports were specified in the connection parameters,
       <xref linkend="libpq-PQport"/> returns the port actually connected to.
      </para>

      <para>
       <xref linkend="libpq-PQport"/> returns <symbol>NULL</symbol> if the
       <parameter>conn</parameter> argument is <symbol>NULL</symbol>.
       Otherwise, if there is an error producing the port information (perhaps
       if the connection has not been fully established or there was an
       error), it returns an empty string.
      </para>

      <para>
       If multiple ports were specified in the connection parameters, it is
       not possible to rely on the result of <xref linkend="libpq-PQport"/> until
       the connection is established.  The status of the connection can be
       checked using the function <xref linkend="libpq-PQstatus"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQtty">
     <term><function>PQtty</function><indexterm><primary>PQtty</primary></indexterm></term>

     <listitem>
      <para>
       This function no longer does anything, but it remains for backwards
       compatibility.  The function always return an empty string, or
       <symbol>NULL</symbol> if the <parameter>conn</parameter> argument is
       <symbol>NULL</symbol>.

<synopsis>
char *PQtty(const PGconn *conn);
</synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQoptions">
     <term><function>PQoptions</function><indexterm><primary>PQoptions</primary></indexterm></term>

     <listitem>
      <para>
       Returns the command-line options passed in the connection request.
<synopsis>
char *PQoptions(const PGconn *conn);
</synopsis>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   The following functions return status data that can change as operations
   are executed on the <structname>PGconn</structname> object.

   <variablelist>
    <varlistentry id="libpq-PQstatus">
     <term><function>PQstatus</function><indexterm><primary>PQstatus</primary></indexterm></term>

     <listitem>
      <para>
       Returns the status of the connection.
<synopsis>
ConnStatusType PQstatus(const PGconn *conn);
</synopsis>
      </para>

      <para>
       The status can be one of a number of values.  However, only two of
       these are seen outside of an asynchronous connection procedure:
       <literal>CONNECTION_OK</literal> and
       <literal>CONNECTION_BAD</literal>. A good connection to the database
       has the status <literal>CONNECTION_OK</literal>.  A failed
       connection attempt is signaled by status
       <literal>CONNECTION_BAD</literal>.  Ordinarily, an OK status will
       remain so until <xref linkend="libpq-PQfinish"/>, but a communications
       failure might result in the status changing to
       <literal>CONNECTION_BAD</literal> prematurely.  In that case the
       application could try to recover by calling
       <xref linkend="libpq-PQreset"/>.
      </para>

      <para>
       See the entry for <xref linkend="libpq-PQconnectStartParams"/>, <function>PQconnectStart</function>
       and <function>PQconnectPoll</function> with regards to other status codes that
       might be returned.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQtransactionStatus">
     <term><function>PQtransactionStatus</function><indexterm><primary>PQtransactionStatus</primary></indexterm></term>

     <listitem>
      <para>
       Returns the current in-transaction status of the server.

<synopsis>
PGTransactionStatusType PQtransactionStatus(const PGconn *conn);
</synopsis>

       The status can be <literal>PQTRANS_IDLE</literal> (currently idle),
       <literal>PQTRANS_ACTIVE</literal> (a command is in progress),
       <literal>PQTRANS_INTRANS</literal> (idle, in a valid transaction block),
       or <literal>PQTRANS_INERROR</literal> (idle, in a failed transaction block).
       <literal>PQTRANS_UNKNOWN</literal> is reported if the connection is bad.
       <literal>PQTRANS_ACTIVE</literal> is reported only when a query
       has been sent to the server and not yet completed.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQparameterStatus">
     <term><function>PQparameterStatus</function><indexterm><primary>PQparameterStatus</primary></indexterm></term>

     <listitem>
      <para>
       Looks up a current parameter setting of the server.

<synopsis>
const char *PQparameterStatus(const PGconn *conn, const char *paramName);
</synopsis>

       Certain parameter values are reported by the server automatically at
       connection startup or whenever their values change.
       <xref linkend="libpq-PQparameterStatus"/> can be used to interrogate these settings.
       It returns the current value of a parameter if known, or <symbol>NULL</symbol>
       if the parameter is not known.
      </para>

      <para>
       Parameters reported as of the current release include
       <varname>server_version</varname>,
       <varname>server_encoding</varname>,
       <varname>client_encoding</varname>,
       <varname>application_name</varname>,
       <varname>default_transaction_read_only</varname>,
       <varname>in_hot_standby</varname>,
       <varname>is_superuser</varname>,
       <varname>session_authorization</varname>,
       <varname>DateStyle</varname>,
       <varname>IntervalStyle</varname>,
       <varname>TimeZone</varname>,
       <varname>integer_datetimes</varname>, and
       <varname>standard_conforming_strings</varname>.
       (<varname>server_encoding</varname>, <varname>TimeZone</varname>, and
       <varname>integer_datetimes</varname> were not reported by releases before 8.0;
       <varname>standard_conforming_strings</varname> was not reported by releases
       before 8.1;
       <varname>IntervalStyle</varname> was not reported by releases before 8.4;
       <varname>application_name</varname> was not reported by releases before
       9.0;
       <varname>default_transaction_read_only</varname> and
       <varname>in_hot_standby</varname> were not reported by releases before
       14.)
       Note that
       <varname>server_version</varname>,
       <varname>server_encoding</varname> and
       <varname>integer_datetimes</varname>
       cannot change after startup.
      </para>

      <para>
       If no value for <varname>standard_conforming_strings</varname> is reported,
       applications can assume it is <literal>off</literal>, that is, backslashes
       are treated as escapes in string literals.  Also, the presence of
       this parameter can be taken as an indication that the escape string
       syntax (<literal>E'...'</literal>) is accepted.
      </para>

      <para>
       Although the returned pointer is declared <literal>const</literal>, it in fact
       points to mutable storage associated with the <literal>PGconn</literal> structure.
       It is unwise to assume the pointer will remain valid across queries.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQprotocolVersion">
     <term><function>PQprotocolVersion</function><indexterm><primary>PQprotocolVersion</primary></indexterm></term>

     <listitem>
      <para>
       Interrogates the frontend/backend protocol being used.
<synopsis>
int PQprotocolVersion(const PGconn *conn);
</synopsis>
       Applications might wish to use this function to determine whether certain
       features are supported.  Currently, the possible values are 3
       (3.0 protocol), or zero (connection bad).  The protocol version will
       not change after connection startup is complete, but it could
       theoretically change during a connection reset.  The 3.0 protocol is
       supported by <productname>PostgreSQL</productname> server versions 7.4
       and above.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQserverVersion">
     <term><function>PQserverVersion</function><indexterm><primary>PQserverVersion</primary></indexterm></term>

     <listitem>
      <para>
       Returns an integer representing the server version.
<synopsis>
int PQserverVersion(const PGconn *conn);
</synopsis>
      </para>

      <para>
       Applications might use this function to determine the version of the
       database server they are connected to.  The result is formed by
       multiplying the server's major version number by 10000 and adding
       the minor version number.  For example, version 10.1 will be
       returned as 100001, and version 11.0 will be returned as 110000.
       Zero is returned if the connection is bad.
      </para>

      <para>
       Prior to major version 10, <productname>PostgreSQL</productname> used
       three-part version numbers in which the first two parts together
       represented the major version.  For those
       versions, <xref linkend="libpq-PQserverVersion"/> uses two digits for each
       part; for example version 9.1.5 will be returned as 90105, and
       version 9.2.0 will be returned as 90200.
      </para>

      <para>
       Therefore, for purposes of determining feature compatibility,
       applications should divide the result of <xref linkend="libpq-PQserverVersion"/>
       by 100 not 10000 to determine a logical major version number.
       In all release series, only the last two digits differ between
       minor releases (bug-fix releases).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQerrorMessage">
     <term><function>PQerrorMessage</function><indexterm><primary>PQerrorMessage</primary></indexterm></term>

     <listitem>
      <para>
       <indexterm><primary>error message</primary></indexterm> Returns the error message
       most recently generated by an operation on the connection.

<synopsis>
char *PQerrorMessage(const PGconn *conn);
</synopsis>

      </para>

      <para>
       Nearly all <application>libpq</application> functions will set a message for
       <xref linkend="libpq-PQerrorMessage"/> if they fail.  Note that by
       <application>libpq</application> convention, a nonempty
       <xref linkend="libpq-PQerrorMessage"/> result can consist of multiple lines,
       and will include a trailing newline. The caller should not free
       the result directly. It will be freed when the associated
       <structname>PGconn</structname> handle is passed to
       <xref linkend="libpq-PQfinish"/>.  The result string should not be
       expected to remain the same across operations on the
       <literal>PGconn</literal> structure.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsocket">
     <term><function>PQsocket</function><indexterm><primary>PQsocket</primary></indexterm></term>
     <listitem>
      <para>
       Obtains the file descriptor number of the connection socket to
       the server.  A valid descriptor will be greater than or equal
       to 0; a result of -1 indicates that no server connection is
       currently open.  (This will not change during normal operation,
       but could change during connection setup or reset.)

<synopsis>
int PQsocket(const PGconn *conn);
</synopsis>

      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQbackendPID">
     <term><function>PQbackendPID</function><indexterm><primary>PQbackendPID</primary></indexterm></term>
     <listitem>
      <para>
       Returns the process <acronym>ID</acronym> (PID)<indexterm>
        <primary>PID</primary>
        <secondary>determining PID of server process</secondary>
        <tertiary>in libpq</tertiary>
       </indexterm>
       of the backend process handling this connection.

<synopsis>
int PQbackendPID(const PGconn *conn);
</synopsis>
      </para>

      <para>
       The backend <acronym>PID</acronym> is useful for debugging
       purposes and for comparison to <command>NOTIFY</command>
       messages (which include the <acronym>PID</acronym> of the
       notifying backend process).  Note that the
       <acronym>PID</acronym> belongs to a process executing on the
       database server host, not the local host!
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQconnectionNeedsPassword">
     <term><function>PQconnectionNeedsPassword</function><indexterm><primary>PQconnectionNeedsPassword</primary></indexterm></term>
     <listitem>
      <para>
       Returns true (1) if the connection authentication method
       required a password, but none was available.
       Returns false (0) if not.

<synopsis>
int PQconnectionNeedsPassword(const PGconn *conn);
</synopsis>
      </para>

      <para>
       This function can be applied after a failed connection attempt
       to decide whether to prompt the user for a password.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQconnectionUsedPassword">
     <term><function>PQconnectionUsedPassword</function><indexterm><primary>PQconnectionUsedPassword</primary></indexterm></term>
     <listitem>
      <para>
       Returns true (1) if the connection authentication method
       used a password. Returns false (0) if not.

<synopsis>
int PQconnectionUsedPassword(const PGconn *conn);
</synopsis>
      </para>

      <para>
       This function can be applied after either a failed or successful
       connection attempt to detect whether the server demanded a password.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQconnectionUsedGSSAPI">
     <term><function>PQconnectionUsedGSSAPI</function><indexterm><primary>PQconnectionUsedGSSAPI</primary></indexterm></term>
     <listitem>
      <para>
       Returns true (1) if the connection authentication method
       used GSSAPI. Returns false (0) if not.

<synopsis>
int PQconnectionUsedGSSAPI(const PGconn *conn);
</synopsis>
      </para>

      <para>
       This function can be applied to detect whether the connection was
       authenticated with GSSAPI.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
    The following functions return information related to SSL. This information
    usually doesn't change after a connection is established.

    <variablelist>
    <varlistentry id="libpq-PQsslInUse">
     <term><function>PQsslInUse</function><indexterm><primary>PQsslInUse</primary></indexterm></term>
     <listitem>
      <para>
        Returns true (1) if the connection uses SSL, false (0) if not.

<synopsis>
int PQsslInUse(const PGconn *conn);
</synopsis>
      </para>

     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsslAttribute">
     <term><function>PQsslAttribute</function><indexterm><primary>PQsslAttribute</primary></indexterm></term>
     <listitem>
      <para>
        Returns SSL-related information about the connection.

<synopsis>
const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);
</synopsis>
      </para>

      <para>
       The list of available attributes varies depending on the SSL library
       being used and the type of connection.  Returns NULL if the connection
       does not use SSL or the specified attribute name is not defined for the
       library in use.
      </para>

      <para>
       The following attributes are commonly available:
       <variablelist>
        <varlistentry>
         <term><literal>library</literal></term>
          <listitem>
           <para>
            Name of the SSL implementation in use. (Currently, only
            <literal>"OpenSSL"</literal> is implemented)
           </para>
          </listitem>
         </varlistentry>
        <varlistentry>
         <term><literal>protocol</literal></term>
          <listitem>
           <para>
             SSL/TLS version in use. Common values
             are <literal>"TLSv1"</literal>, <literal>"TLSv1.1"</literal>
             and <literal>"TLSv1.2"</literal>, but an implementation may
             return other strings if some other protocol is used.
           </para>
          </listitem>
         </varlistentry>
        <varlistentry>
         <term><literal>key_bits</literal></term>
          <listitem>
           <para>
            Number of key bits used by the encryption algorithm.
           </para>
          </listitem>
         </varlistentry>
        <varlistentry>
         <term><literal>cipher</literal></term>
          <listitem>
           <para>
            A short name of the ciphersuite used, e.g.,
            <literal>"DHE-RSA-DES-CBC3-SHA"</literal>. The names are specific
            to each SSL implementation.
           </para>
          </listitem>
         </varlistentry>
        <varlistentry>
         <term><literal>compression</literal></term>
          <listitem>
           <para>
            Returns "on" if SSL compression is in use, else it returns "off".
           </para>
          </listitem>
         </varlistentry>
       </variablelist>
      </para>

      <para>
       As a special case, the <literal>library</literal> attribute may be
       queried without a connection by passing NULL as
       the <literal>conn</literal> argument.  The result will be the default
       SSL library name, or NULL if <application>libpq</application> was
       compiled without any SSL support.  (Prior
       to <productname>PostgreSQL</productname> version 15, passing NULL as
       the <literal>conn</literal> argument always resulted in NULL.
       Client programs needing to differentiate between the newer and older
       implementations of this case may check the
       <literal>LIBPQ_HAS_SSL_LIBRARY_DETECTION</literal> feature macro.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsslAttributeNames">
     <term><function>PQsslAttributeNames</function><indexterm><primary>PQsslAttributeNames</primary></indexterm></term>
     <listitem>
      <para>
       Returns an array of SSL attribute names that can be used
       in <function>PQsslAttribute()</function>.
       The array is terminated by a NULL pointer.
<synopsis>
const char * const * PQsslAttributeNames(const PGconn *conn);
</synopsis>
      </para>

      <para>
       If <literal>conn</literal> is NULL, the attributes available for the
       default SSL library are returned, or an empty list
       if <application>libpq</application> was compiled without any SSL
       support.  If <literal>conn</literal> is not NULL, the attributes
       available for the SSL library in use for the connection are returned,
       or an empty list if the connection is not encrypted.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsslStruct">
     <term><function>PQsslStruct</function><indexterm><primary>PQsslStruct</primary></indexterm></term>
     <listitem>
      <para>
       Returns a pointer to an SSL-implementation-specific object describing
       the connection.  Returns NULL if the connection is not encrypted
       or the requested type of object is not available from the connection's
       SSL implementation.
<synopsis>
void *PQsslStruct(const PGconn *conn, const char *struct_name);
</synopsis>
      </para>
      <para>
       The struct(s) available depend on the SSL implementation in use.
       For <productname>OpenSSL</productname>, there is one struct,
       available under the name <literal>OpenSSL</literal>,
       and it returns a pointer to
       <productname>OpenSSL</productname>'s <literal>SSL</literal> struct.
       To use this function, code along the following lines could be used:
<programlisting><![CDATA[
#include <libpq-fe.h>
#include <openssl/ssl.h>

...

    SSL *ssl;

    dbconn = PQconnectdb(...);
    ...

    ssl = PQsslStruct(dbconn, "OpenSSL");
    if (ssl)
    {
        /* use OpenSSL functions to access ssl */
    }
]]></programlisting>
      </para>
      <para>
       This structure can be used to verify encryption levels, check server
       certificates, and more. Refer to the <productname>OpenSSL</productname>
       documentation for information about this structure.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQgetssl">
     <term><function>PQgetssl</function><indexterm><primary>PQgetssl</primary></indexterm></term>
     <listitem>
      <para>
       <indexterm><primary>SSL</primary><secondary sortas="libpq">in libpq</secondary></indexterm>
       Returns the SSL structure used in the connection, or NULL
       if SSL is not in use.

<synopsis>
void *PQgetssl(const PGconn *conn);
</synopsis>
      </para>

      <para>
       This function is equivalent to <literal>PQsslStruct(conn, "OpenSSL")</literal>. It should
       not be used in new applications, because the returned struct is
       specific to <productname>OpenSSL</productname> and will not be
       available if another <acronym>SSL</acronym> implementation is used.
       To check if a connection uses SSL, call
       <xref linkend="libpq-PQsslInUse"/> instead, and for more details about the
       connection, use <xref linkend="libpq-PQsslAttribute"/>.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>

 </sect1>

 <sect1 id="libpq-exec">
  <title>Command Execution Functions</title>

  <para>
   Once a connection to a database server has been successfully
   established, the functions described here are used to perform
   SQL queries and commands.
  </para>

  <sect2 id="libpq-exec-main">
   <title>Main Functions</title>

   <para>
    <variablelist>
     <varlistentry id="libpq-PQexec">
      <term><function>PQexec</function><indexterm><primary>PQexec</primary></indexterm></term>

      <listitem>
       <para>
        Submits a command to the server and waits for the result.

<synopsis>
PGresult *PQexec(PGconn *conn, const char *command);
</synopsis>
       </para>

       <para>
        Returns a <structname>PGresult</structname> pointer or possibly a null
        pointer.  A non-null pointer will generally be returned except in
        out-of-memory conditions or serious errors such as inability to send
        the command to the server.  The <xref linkend="libpq-PQresultStatus"/> function
        should be called to check the return value for any errors (including
        the value of a null pointer, in which case it will return
        <symbol>PGRES_FATAL_ERROR</symbol>).  Use
        <xref linkend="libpq-PQerrorMessage"/> to get more information about such
        errors.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    The command string can include multiple SQL commands
    (separated by semicolons).  Multiple queries sent in a single
    <xref linkend="libpq-PQexec"/> call are processed in a single transaction, unless
    there are explicit <command>BEGIN</command>/<command>COMMIT</command>
    commands included in the query string to divide it into multiple
    transactions.  (See <xref linkend="protocol-flow-multi-statement"/>
    for more details about how the server handles multi-query strings.)
    Note however that the returned
    <structname>PGresult</structname> structure describes only the result
    of the last command executed from the string.  Should one of the
    commands fail, processing of the string stops with it and the returned
    <structname>PGresult</structname> describes the error condition.
   </para>

   <para>
    <variablelist>
     <varlistentry id="libpq-PQexecParams">
      <term><function>PQexecParams</function><indexterm><primary>PQexecParams</primary></indexterm></term>

      <listitem>
       <para>
        Submits a command to the server and waits for the result,
        with the ability to pass parameters separately from the SQL
        command text.

<synopsis>
PGresult *PQexecParams(PGconn *conn,
                       const char *command,
                       int nParams,
                       const Oid *paramTypes,
                       const char * const *paramValues,
                       const int *paramLengths,
                       const int *paramFormats,
                       int resultFormat);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQexecParams"/> is like <xref linkend="libpq-PQexec"/>, but offers additional
        functionality: parameter values can be specified separately from the command
        string proper, and query results can be requested in either text or binary
        format.
       </para>

       <para>
        The function arguments are:

        <variablelist>
         <varlistentry>
          <term><parameter>conn</parameter></term>

          <listitem>
           <para>
            The connection object to send the command through.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>command</parameter></term>
          <listitem>
           <para>
            The SQL command string to be executed. If parameters are used,
            they are referred to in the command string as <literal>$1</literal>,
            <literal>$2</literal>, etc.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>nParams</parameter></term>
          <listitem>
           <para>
            The number of parameters supplied; it is the length of the arrays
            <parameter>paramTypes[]</parameter>, <parameter>paramValues[]</parameter>,
            <parameter>paramLengths[]</parameter>, and <parameter>paramFormats[]</parameter>. (The
            array pointers can be <symbol>NULL</symbol> when <parameter>nParams</parameter>
            is zero.)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramTypes[]</parameter></term>
          <listitem>
           <para>
            Specifies, by OID, the data types to be assigned to the
            parameter symbols.  If <parameter>paramTypes</parameter> is
            <symbol>NULL</symbol>, or any particular element in the array
            is zero, the server infers a data type for the parameter symbol
            in the same way it would do for an untyped literal string.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramValues[]</parameter></term>
          <listitem>
           <para>
            Specifies the actual values of the parameters.  A null pointer
            in this array means the corresponding parameter is null;
            otherwise the pointer points to a zero-terminated text string
            (for text format) or binary data in the format expected by the
            server (for binary format).
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramLengths[]</parameter></term>
          <listitem>
           <para>
            Specifies the actual data lengths of binary-format parameters.
            It is ignored for null parameters and text-format parameters.
            The array pointer can be null when there are no binary parameters.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramFormats[]</parameter></term>
          <listitem>
           <para>
            Specifies whether parameters are text (put a zero in the
            array entry for the corresponding parameter) or binary (put
            a one in the array entry for the corresponding parameter).
            If the array pointer is null then all parameters are presumed
            to be text strings.
           </para>
           <para>
            Values passed in binary format require knowledge of
            the internal representation expected by the backend.
            For example, integers must be passed in network byte
            order.  Passing <type>numeric</type> values requires
            knowledge of the server storage format, as implemented
            in
            <filename>src/backend/utils/adt/numeric.c::numeric_send()</filename> and
            <filename>src/backend/utils/adt/numeric.c::numeric_recv()</filename>.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>resultFormat</parameter></term>
          <listitem>
           <para>
            Specify zero to obtain results in text format, or one to obtain
            results in binary format.  (There is not currently a provision
            to obtain different result columns in different formats,
            although that is possible in the underlying protocol.)
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The primary advantage of <xref linkend="libpq-PQexecParams"/> over
    <xref linkend="libpq-PQexec"/> is that parameter values can be separated from the
    command string, thus avoiding the need for tedious and error-prone
    quoting and escaping.
   </para>

   <para>
    Unlike <xref linkend="libpq-PQexec"/>, <xref linkend="libpq-PQexecParams"/> allows at most
    one SQL command in the given string.  (There can be semicolons in it,
    but not more than one nonempty command.)  This is a limitation of the
    underlying protocol, but has some usefulness as an extra defense against
    SQL-injection attacks.
   </para>

   <tip>
    <para>
     Specifying parameter types via OIDs is tedious, particularly if you prefer
     not to hard-wire particular OID values into your program.  However, you can
     avoid doing so even in cases where the server by itself cannot determine the
     type of the parameter, or chooses a different type than you want.  In the
     SQL command text, attach an explicit cast to the parameter symbol to show what
     data type you will send.  For example:
<programlisting>
SELECT * FROM mytable WHERE x = $1::bigint;
</programlisting>
     This forces parameter <literal>$1</literal> to be treated as <type>bigint</type>, whereas
     by default it would be assigned the same type as <literal>x</literal>.  Forcing the
     parameter type decision, either this way or by specifying a numeric type OID,
     is strongly recommended when sending parameter values in binary format, because
     binary format has less redundancy than text format and so there is less chance
     that the server will detect a type mismatch mistake for you.
    </para>
   </tip>

   <para>
    <variablelist>
     <varlistentry id="libpq-PQprepare">
      <term><function>PQprepare</function><indexterm><primary>PQprepare</primary></indexterm></term>

      <listitem>
       <para>
        Submits a request to create a prepared statement with the
        given parameters, and waits for completion.
<synopsis>
PGresult *PQprepare(PGconn *conn,
                    const char *stmtName,
                    const char *query,
                    int nParams,
                    const Oid *paramTypes);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQprepare"/> creates a prepared statement for later
        execution with <xref linkend="libpq-PQexecPrepared"/>.  This feature allows
        commands to be executed repeatedly without being parsed and
        planned each time;  see <xref linkend="sql-prepare"/> for details.
       </para>

       <para>
        The function creates a prepared statement named
        <parameter>stmtName</parameter> from the <parameter>query</parameter> string, which
        must contain a single SQL command.  <parameter>stmtName</parameter> can be
        <literal>""</literal> to create an unnamed statement, in which case any
        pre-existing unnamed statement is automatically replaced; otherwise
        it is an error if the statement name is already defined in the
        current session.  If any parameters are used, they are referred
        to in the query as <literal>$1</literal>, <literal>$2</literal>, etc.
        <parameter>nParams</parameter> is the number of parameters for which types
        are pre-specified in the array <parameter>paramTypes[]</parameter>.  (The
        array pointer can be <symbol>NULL</symbol> when
        <parameter>nParams</parameter> is zero.) <parameter>paramTypes[]</parameter>
        specifies, by OID, the data types to be assigned to the parameter
        symbols.  If <parameter>paramTypes</parameter> is <symbol>NULL</symbol>,
        or any particular element in the array is zero, the server assigns
        a data type to the parameter symbol in the same way it would do
        for an untyped literal string.  Also, the query can use parameter
        symbols with numbers higher than <parameter>nParams</parameter>; data types
        will be inferred for these symbols as well.  (See
        <xref linkend="libpq-PQdescribePrepared"/> for a means to find out
        what data types were inferred.)
       </para>

       <para>
        As with <xref linkend="libpq-PQexec"/>, the result is normally a
        <structname>PGresult</structname> object whose contents indicate
        server-side success or failure.  A null result indicates
        out-of-memory or inability to send the command at all.  Use
        <xref linkend="libpq-PQerrorMessage"/> to get more information about
        such errors.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    Prepared statements for use with <xref linkend="libpq-PQexecPrepared"/> can also
    be created by executing SQL <xref linkend="sql-prepare"/>
    statements.
   </para>

   <para>
    <variablelist>
     <varlistentry id="libpq-PQexecPrepared">
      <term><function>PQexecPrepared</function><indexterm><primary>PQexecPrepared</primary></indexterm></term>

      <listitem>
       <para>
        Sends a request to execute a prepared statement with given
        parameters, and waits for the result.
<synopsis>
PGresult *PQexecPrepared(PGconn *conn,
                         const char *stmtName,
                         int nParams,
                         const char * const *paramValues,
                         const int *paramLengths,
                         const int *paramFormats,
                         int resultFormat);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQexecPrepared"/> is like <xref linkend="libpq-PQexecParams"/>,
        but the command to be executed is specified by naming a
        previously-prepared statement, instead of giving a query string.
        This feature allows commands that will be used repeatedly to be
        parsed and planned just once, rather than each time they are
        executed.  The statement must have been prepared previously in
        the current session.
       </para>

       <para>
        The parameters are identical to <xref linkend="libpq-PQexecParams"/>, except that the
        name of a prepared statement is given instead of a query string, and the
        <parameter>paramTypes[]</parameter> parameter is not present (it is not needed since
        the prepared statement's parameter types were determined when it was created).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQdescribePrepared">
      <term><function>PQdescribePrepared</function><indexterm><primary>PQdescribePrepared</primary></indexterm></term>

      <listitem>
       <para>
        Submits a request to obtain information about the specified
        prepared statement, and waits for completion.
<synopsis>
PGresult *PQdescribePrepared(PGconn *conn, const char *stmtName);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQdescribePrepared"/> allows an application to obtain
        information about a previously prepared statement.
       </para>

       <para>
        <parameter>stmtName</parameter> can be <literal>""</literal> or <symbol>NULL</symbol> to reference
        the unnamed statement, otherwise it must be the name of an existing
        prepared statement.  On success, a <structname>PGresult</structname> with
        status <literal>PGRES_COMMAND_OK</literal> is returned.  The
        functions <xref linkend="libpq-PQnparams"/> and
        <xref linkend="libpq-PQparamtype"/> can be applied to this
        <structname>PGresult</structname> to obtain information about the parameters
        of the prepared statement, and the functions
        <xref linkend="libpq-PQnfields"/>, <xref linkend="libpq-PQfname"/>,
        <xref linkend="libpq-PQftype"/>, etc. provide information about the
        result columns (if any) of the statement.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQdescribePortal">
      <term><function>PQdescribePortal</function><indexterm><primary>PQdescribePortal</primary></indexterm></term>

      <listitem>
       <para>
        Submits a request to obtain information about the specified
        portal, and waits for completion.
<synopsis>
PGresult *PQdescribePortal(PGconn *conn, const char *portalName);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQdescribePortal"/> allows an application to obtain
        information about a previously created portal.
        (<application>libpq</application> does not provide any direct access to
        portals, but you can use this function to inspect the properties
        of a cursor created with a <command>DECLARE CURSOR</command> SQL command.)
       </para>

       <para>
        <parameter>portalName</parameter> can be <literal>""</literal> or <symbol>NULL</symbol> to reference
        the unnamed portal, otherwise it must be the name of an existing
        portal.  On success, a <structname>PGresult</structname> with status
        <literal>PGRES_COMMAND_OK</literal> is returned.  The functions
        <xref linkend="libpq-PQnfields"/>, <xref linkend="libpq-PQfname"/>,
        <xref linkend="libpq-PQftype"/>, etc. can be applied to the
        <structname>PGresult</structname> to obtain information about the result
        columns (if any) of the portal.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQclosePrepared">
      <term><function>PQclosePrepared</function><indexterm><primary>PQclosePrepared</primary></indexterm></term>

      <listitem>
       <para>
        Submits a request to close the specified prepared statement, and waits
        for completion.
<synopsis>
PGresult *PQclosePrepared(PGconn *conn, const char *stmtName);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQclosePrepared"/> allows an application to close
        a previously prepared statement. Closing a statement releases all
        of its associated resources on the server and allows its name to be
        reused.
       </para>

       <para>
        <parameter>stmtName</parameter> can be <literal>""</literal> or
        <symbol>NULL</symbol> to reference the unnamed statement. It is fine
        if no statement exists with this name, in that case the operation is a
        no-op. On success, a <structname>PGresult</structname> with
        status <literal>PGRES_COMMAND_OK</literal> is returned.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQclosePortal">
      <term><function>PQclosePortal</function><indexterm><primary>PQclosePortal</primary></indexterm></term>

      <listitem>
       <para>
        Submits a request to close the specified portal, and waits for
        completion.
<synopsis>
PGresult *PQclosePortal(PGconn *conn, const char *portalName);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQclosePortal"/> allows an application to trigger
        a close of a previously created portal. Closing a portal releases all
        of its associated resources on the server and allows its name to be
        reused. (<application>libpq</application> does not provide any
        direct access to portals, but you can use this function to close a
        cursor created with a <command>DECLARE CURSOR</command> SQL command.)
       </para>

       <para>
        <parameter>portalName</parameter> can be <literal>""</literal> or
        <symbol>NULL</symbol> to reference the unnamed portal. It is fine
        if no portal exists with this name, in that case the operation is a
        no-op. On success, a <structname>PGresult</structname> with status
        <literal>PGRES_COMMAND_OK</literal> is returned.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The <structname>PGresult</structname><indexterm><primary>PGresult</primary></indexterm>
    structure encapsulates the result returned by the server.
    <application>libpq</application> application programmers should be
    careful to maintain the <structname>PGresult</structname> abstraction.
    Use the accessor functions below to get at the contents of
    <structname>PGresult</structname>.  Avoid directly referencing the
    fields of the <structname>PGresult</structname> structure because they
    are subject to change in the future.

    <variablelist>
     <varlistentry id="libpq-PQresultStatus">
      <term><function>PQresultStatus</function><indexterm><primary>PQresultStatus</primary></indexterm></term>

      <listitem>
       <para>
        Returns the result status of the command.
<synopsis>
ExecStatusType PQresultStatus(const PGresult *res);
</synopsis>
       </para>

       <para>
        <xref linkend="libpq-PQresultStatus"/> can return one of the following values:

        <variablelist>
         <varlistentry id="libpq-pgres-empty-query">
          <term><literal>PGRES_EMPTY_QUERY</literal></term>
          <listitem>
           <para>
            The string sent to the server was empty.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-command-ok">
          <term><literal>PGRES_COMMAND_OK</literal></term>
          <listitem>
           <para>
            Successful completion of a command returning no data.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-tuples-ok">
          <term><literal>PGRES_TUPLES_OK</literal></term>
          <listitem>
           <para>
            Successful completion of a command returning data (such as
            a <command>SELECT</command> or <command>SHOW</command>).
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-copy-out">
          <term><literal>PGRES_COPY_OUT</literal></term>
          <listitem>
           <para>
            Copy Out (from server) data transfer started.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-copy-in">
          <term><literal>PGRES_COPY_IN</literal></term>
          <listitem>
           <para>
            Copy In (to server) data transfer started.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-bad-response">
          <term><literal>PGRES_BAD_RESPONSE</literal></term>
          <listitem>
           <para>
            The server's response was not understood.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-nonfatal-error">
          <term><literal>PGRES_NONFATAL_ERROR</literal></term>
          <listitem>
           <para>
            A nonfatal error (a notice or warning) occurred.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-fatal-error">
          <term><literal>PGRES_FATAL_ERROR</literal></term>
          <listitem>
           <para>
            A fatal error occurred.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-copy-both">
          <term><literal>PGRES_COPY_BOTH</literal></term>
          <listitem>
           <para>
            Copy In/Out (to and from server) data transfer started.  This
            feature is currently used only for streaming replication,
            so this status should not occur in ordinary applications.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-single-tuple">
          <term><literal>PGRES_SINGLE_TUPLE</literal></term>
          <listitem>
           <para>
            The <structname>PGresult</structname> contains a single result tuple
            from the current command.  This status occurs only when
            single-row mode has been selected for the query
            (see <xref linkend="libpq-single-row-mode"/>).
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-pipeline-sync">
          <term><literal>PGRES_PIPELINE_SYNC</literal></term>
          <listitem>
           <para>
            The <structname>PGresult</structname> represents a
            synchronization point in pipeline mode, requested by
            <xref linkend="libpq-PQpipelineSync"/>.
            This status occurs only when pipeline mode has been selected.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pgres-pipeline-aborted">
          <term><literal>PGRES_PIPELINE_ABORTED</literal></term>
          <listitem>
           <para>
            The <structname>PGresult</structname> represents a pipeline that has
            received an error from the server.  <function>PQgetResult</function>
            must be called repeatedly, and each time it will return this status code
            until the end of the current pipeline, at which point it will return
            <literal>PGRES_PIPELINE_SYNC</literal> and normal processing can
            resume.
           </para>
          </listitem>
         </varlistentry>

        </variablelist>

        If the result status is <literal>PGRES_TUPLES_OK</literal> or
        <literal>PGRES_SINGLE_TUPLE</literal>, then
        the functions described below can be used to retrieve the rows
        returned by the query.  Note that a <command>SELECT</command>
        command that happens to retrieve zero rows still shows
        <literal>PGRES_TUPLES_OK</literal>.
        <literal>PGRES_COMMAND_OK</literal> is for commands that can never
        return rows (<command>INSERT</command> or <command>UPDATE</command>
        without a <literal>RETURNING</literal> clause,
        etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> might
        indicate a bug in the client software.
       </para>

       <para>
        A result of status <symbol>PGRES_NONFATAL_ERROR</symbol> will
        never be returned directly by <xref linkend="libpq-PQexec"/> or other
        query execution functions; results of this kind are instead passed
        to the notice processor (see <xref
        linkend="libpq-notice-processing"/>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQresStatus">
      <term><function>PQresStatus</function><indexterm><primary>PQresStatus</primary></indexterm></term>

      <listitem>
       <para>
        Converts the enumerated type returned by
        <xref linkend="libpq-PQresultStatus"/> into a string constant describing the
        status code. The caller should not free the result.

<synopsis>
char *PQresStatus(ExecStatusType status);
</synopsis>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQresultErrorMessage">
      <term><function>PQresultErrorMessage</function><indexterm><primary>PQresultErrorMessage</primary></indexterm></term>

      <listitem>
       <para>
        Returns the error message associated with the command, or an empty string
        if there was no error.
<synopsis>
char *PQresultErrorMessage(const PGresult *res);
</synopsis>
        If there was an error, the returned string will include a trailing
        newline.  The caller should not free the result directly. It will
        be freed when the associated <structname>PGresult</structname> handle is
        passed to <xref linkend="libpq-PQclear"/>.
       </para>

       <para>
        Immediately following a <xref linkend="libpq-PQexec"/> or
        <xref linkend="libpq-PQgetResult"/> call,
        <xref linkend="libpq-PQerrorMessage"/> (on the connection) will return
        the same string as <xref linkend="libpq-PQresultErrorMessage"/> (on
        the result).  However, a <structname>PGresult</structname> will
        retain its error message until destroyed, whereas the connection's
        error message will change when subsequent operations are done.
        Use <xref linkend="libpq-PQresultErrorMessage"/> when you want to
        know the status associated with a particular
        <structname>PGresult</structname>; use
        <xref linkend="libpq-PQerrorMessage"/> when you want to know the
        status from the latest operation on the connection.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQresultVerboseErrorMessage">
      <term><function>PQresultVerboseErrorMessage</function><indexterm><primary>PQresultVerboseErrorMessage</primary></indexterm></term>

      <listitem>
       <para>
        Returns a reformatted version of the error message associated with
        a <structname>PGresult</structname> object.
<synopsis>
char *PQresultVerboseErrorMessage(const PGresult *res,
                                  PGVerbosity verbosity,
                                  PGContextVisibility show_context);
</synopsis>
        In some situations a client might wish to obtain a more detailed
        version of a previously-reported error.
        <xref linkend="libpq-PQresultVerboseErrorMessage"/> addresses this need
        by computing the message that would have been produced
        by <xref linkend="libpq-PQresultErrorMessage"/> if the specified
        verbosity settings had been in effect for the connection when the
        given <structname>PGresult</structname> was generated.  If
        the <structname>PGresult</structname> is not an error result,
        <quote>PGresult is not an error result</quote> is reported instead.
        The returned string includes a trailing newline.
       </para>

       <para>
        Unlike most other functions for extracting data from
        a <structname>PGresult</structname>, the result of this function is a freshly
        allocated string.  The caller must free it
        using <function>PQfreemem()</function> when the string is no longer needed.
       </para>

       <para>
        A NULL return is possible if there is insufficient memory.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQresultErrorField">
      <term><function>PQresultErrorField</function><indexterm><primary>PQresultErrorField</primary></indexterm></term>
      <listitem>
       <para>
        Returns an individual field of an error report.
<synopsis>
char *PQresultErrorField(const PGresult *res, int fieldcode);
</synopsis>
        <parameter>fieldcode</parameter> is an error field identifier; see the symbols
        listed below.  <symbol>NULL</symbol> is returned if the
        <structname>PGresult</structname> is not an error or warning result,
        or does not include the specified field.  Field values will normally
        not include a trailing newline. The caller should not free the
        result directly. It will be freed when the
        associated <structname>PGresult</structname> handle is passed to
        <xref linkend="libpq-PQclear"/>.
       </para>

       <para>
        The following field codes are available:
        <variablelist>
         <varlistentry id="libpq-pg-diag-severity">
          <term><symbol>PG_DIAG_SEVERITY</symbol></term>
          <listitem>
           <para>
            The severity; the field contents are <literal>ERROR</literal>,
            <literal>FATAL</literal>, or <literal>PANIC</literal> (in an error message),
            or <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
            <literal>INFO</literal>, or <literal>LOG</literal> (in a notice message), or
            a localized translation of one of these.  Always present.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-PG-diag-severity-nonlocalized">
          <term><symbol>PG_DIAG_SEVERITY_NONLOCALIZED</symbol></term>
          <listitem>
           <para>
            The severity; the field contents are <literal>ERROR</literal>,
            <literal>FATAL</literal>, or <literal>PANIC</literal> (in an error message),
            or <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
            <literal>INFO</literal>, or <literal>LOG</literal> (in a notice message).
            This is identical to the <symbol>PG_DIAG_SEVERITY</symbol> field except
            that the contents are never localized.  This is present only in
            reports generated by <productname>PostgreSQL</productname> versions 9.6
            and later.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-sqlstate">
          <term><symbol>PG_DIAG_SQLSTATE</symbol><indexterm
          ><primary>error codes</primary><secondary>libpq</secondary></indexterm></term>
          <listitem>
           <para>
            The SQLSTATE code for the error. The SQLSTATE code identifies
            the type of error that has occurred; it can be used by
            front-end applications to perform specific operations (such
            as error handling) in response to a particular database error.
            For a list of the possible SQLSTATE codes, see <xref
            linkend="errcodes-appendix"/>. This field is not localizable,
            and is always present.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-message-primary">
          <term><symbol>PG_DIAG_MESSAGE_PRIMARY</symbol></term>
          <listitem>
           <para>
            The primary human-readable error message (typically one line).
            Always present.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-message-detail">
          <term><symbol>PG_DIAG_MESSAGE_DETAIL</symbol></term>
          <listitem>
           <para>
            Detail: an optional secondary error message carrying more
            detail about the problem.  Might run to multiple lines.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-message-hint">
          <term><symbol>PG_DIAG_MESSAGE_HINT</symbol></term>
          <listitem>
           <para>
            Hint: an optional suggestion what to do about the problem.
            This is intended to differ from detail in that it offers advice
            (potentially inappropriate) rather than hard facts.  Might
            run to multiple lines.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-statement-position">
          <term><symbol>PG_DIAG_STATEMENT_POSITION</symbol></term>
          <listitem>
           <para>
            A string containing a decimal integer indicating an error cursor
            position as an index into the original statement string.  The
            first character has index 1, and positions are measured in
            characters not bytes.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-internal-position">
          <term><symbol>PG_DIAG_INTERNAL_POSITION</symbol></term>
          <listitem>
           <para>
            This is defined the same as the
            <symbol>PG_DIAG_STATEMENT_POSITION</symbol> field, but it is used
            when the cursor position refers to an internally generated
            command rather than the one submitted by the client.  The
            <symbol>PG_DIAG_INTERNAL_QUERY</symbol> field will always appear when
            this field appears.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-internal-query">
          <term><symbol>PG_DIAG_INTERNAL_QUERY</symbol></term>
          <listitem>
           <para>
            The text of a failed internally-generated command.  This could
            be, for example, an SQL query issued by a PL/pgSQL function.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-context">
          <term><symbol>PG_DIAG_CONTEXT</symbol></term>
          <listitem>
           <para>
            An indication of the context in which the error occurred.
            Presently this includes a call stack traceback of active
            procedural language functions and internally-generated queries.
            The trace is one entry per line, most recent first.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-schema-name">
          <term><symbol>PG_DIAG_SCHEMA_NAME</symbol></term>
          <listitem>
           <para>
            If the error was associated with a specific database object,
            the name of the schema containing that object, if any.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-table-name">
          <term><symbol>PG_DIAG_TABLE_NAME</symbol></term>
          <listitem>
           <para>
            If the error was associated with a specific table, the name of the
            table.  (Refer to the schema name field for the name of the
            table's schema.)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-column-name">
          <term><symbol>PG_DIAG_COLUMN_NAME</symbol></term>
          <listitem>
           <para>
            If the error was associated with a specific table column, the name
            of the column.  (Refer to the schema and table name fields to
            identify the table.)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-datatype-name">
          <term><symbol>PG_DIAG_DATATYPE_NAME</symbol></term>
          <listitem>
           <para>
            If the error was associated with a specific data type, the name of
            the data type.  (Refer to the schema name field for the name of
            the data type's schema.)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-constraint-name">
          <term><symbol>PG_DIAG_CONSTRAINT_NAME</symbol></term>
          <listitem>
           <para>
            If the error was associated with a specific constraint, the name
            of the constraint.  Refer to fields listed above for the
            associated table or domain.  (For this purpose, indexes are
            treated as constraints, even if they weren't created with
            constraint syntax.)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-source-file">
          <term><symbol>PG_DIAG_SOURCE_FILE</symbol></term>
          <listitem>
           <para>
            The file name of the source-code location where the error was
            reported.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-source-line">
          <term><symbol>PG_DIAG_SOURCE_LINE</symbol></term>
          <listitem>
           <para>
            The line number of the source-code location where the error
            was reported.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry id="libpq-pg-diag-source-function">
          <term><symbol>PG_DIAG_SOURCE_FUNCTION</symbol></term>
          <listitem>
           <para>
            The name of the source-code function reporting the error.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>

       <note>
        <para>
         The fields for schema name, table name, column name, data type name,
         and constraint name are supplied only for a limited number of error
         types; see <xref linkend="errcodes-appendix"/>.  Do not assume that
         the presence of any of these fields guarantees the presence of
         another field.  Core error sources observe the interrelationships
         noted above, but user-defined functions may use these fields in other
         ways.  In the same vein, do not assume that these fields denote
         contemporary objects in the current database.
        </para>
       </note>

       <para>
        The client is responsible for formatting displayed information to meet
        its needs; in particular it should break long lines as needed.
        Newline characters appearing in the error message fields should be
        treated as paragraph breaks, not line breaks.
       </para>

       <para>
        Errors generated internally by <application>libpq</application> will
        have severity and primary message, but typically no other fields.
       </para>

       <para>
        Note that error fields are only available from
        <structname>PGresult</structname> objects, not
        <structname>PGconn</structname> objects; there is no
        <function>PQerrorField</function> function.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="libpq-PQclear">
      <term><function>PQclear</function><indexterm><primary>PQclear</primary></indexterm></term>
      <listitem>
       <para>
        Frees  the  storage  associated with a
        <structname>PGresult</structname>.  Every command result should be
        freed via <xref linkend="libpq-PQclear"/> when it  is  no  longer
        needed.

<synopsis>
void PQclear(PGresult *res);
</synopsis>

        If the argument is a <symbol>NULL</symbol> pointer, no operation is
        performed.
       </para>

       <para>
        You can keep a <structname>PGresult</structname> object around for
        as long as you need it; it does not go away when you issue a new
        command, nor even if you close the connection.  To get rid of it,
        you must call <xref linkend="libpq-PQclear"/>.  Failure to do this
        will result in memory leaks in your application.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect2>

  <sect2 id="libpq-exec-select-info">
   <title>Retrieving Query Result Information</title>

   <para>
    These functions are used to extract information from a
    <structname>PGresult</structname> object that represents a successful
    query result (that is, one that has status
    <literal>PGRES_TUPLES_OK</literal> or <literal>PGRES_SINGLE_TUPLE</literal>).
    They can also be used to extract
    information from a successful Describe operation: a Describe's result
    has all the same column information that actual execution of the query
    would provide, but it has zero rows.  For objects with other status values,
    these functions will act as though the result has zero rows and zero columns.
   </para>

   <variablelist>
    <varlistentry id="libpq-PQntuples">
     <term><function>PQntuples</function><indexterm><primary>PQntuples</primary></indexterm></term>

     <listitem>
      <para>
       Returns the number of rows (tuples) in the query result.
       (Note that <structname>PGresult</structname> objects are limited to no more
       than <literal>INT_MAX</literal> rows, so an <type>int</type> result is
       sufficient.)

<synopsis>
int PQntuples(const PGresult *res);
</synopsis>

      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQnfields">
     <term><function>PQnfields</function><indexterm><primary>PQnfields</primary></indexterm></term>

     <listitem>
      <para>
       Returns the number of columns (fields) in each row of the query
       result.

<synopsis>
int PQnfields(const PGresult *res);
</synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfname">
     <term><function>PQfname</function><indexterm><primary>PQfname</primary></indexterm></term>

     <listitem>
      <para>
       Returns the column name associated with the given column number.
       Column numbers start at 0. The caller should not free the result
       directly. It will be freed when the associated
       <structname>PGresult</structname> handle is passed to
       <xref linkend="libpq-PQclear"/>.
<synopsis>
char *PQfname(const PGresult *res,
              int column_number);
</synopsis>
      </para>

      <para>
       <symbol>NULL</symbol> is returned if the column number is out of range.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfnumber">
     <term><function>PQfnumber</function><indexterm><primary>PQfnumber</primary></indexterm></term>

     <listitem>
      <para>
       Returns the column number associated with the given column name.
<synopsis>
int PQfnumber(const PGresult *res,
              const char *column_name);
</synopsis>
      </para>

      <para>
       -1 is returned if the given name does not match any column.
      </para>

      <para>
       The given name is treated like an identifier in an SQL command,
       that is, it is downcased unless double-quoted.  For example, given
       a query result generated from the SQL command:
<programlisting>
SELECT 1 AS FOO, 2 AS "BAR";
</programlisting>
       we would have the results:
<programlisting>
PQfname(res, 0)              <lineannotation>foo</lineannotation>
PQfname(res, 1)              <lineannotation>BAR</lineannotation>
PQfnumber(res, "FOO")        <lineannotation>0</lineannotation>
PQfnumber(res, "foo")        <lineannotation>0</lineannotation>
PQfnumber(res, "BAR")        <lineannotation>-1</lineannotation>
PQfnumber(res, "\"BAR\"")    <lineannotation>1</lineannotation>
</programlisting>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQftable">
     <term><function>PQftable</function><indexterm><primary>PQftable</primary></indexterm></term>

     <listitem>
      <para>
       Returns the OID of the table from which the given column was
       fetched.  Column numbers start at 0.
<synopsis>
Oid PQftable(const PGresult *res,
             int column_number);
</synopsis>
      </para>

      <para>
       <literal>InvalidOid</literal> is returned if the column number is out of range,
       or if the specified column is not a simple reference to a table column.
       You can query the system table <literal>pg_class</literal> to determine
       exactly which table is referenced.
      </para>

      <para>
       The type <type>Oid</type> and the constant
       <literal>InvalidOid</literal> will be defined when you include
       the <application>libpq</application> header file. They will both
       be some integer type.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQftablecol">
     <term><function>PQftablecol</function><indexterm><primary>PQftablecol</primary></indexterm></term>

     <listitem>
      <para>
       Returns the column number (within its table) of the column making
       up the specified query result column.  Query-result column numbers
       start at 0, but table columns have nonzero numbers.
<synopsis>
int PQftablecol(const PGresult *res,
                int column_number);
</synopsis>
      </para>

      <para>
       Zero is returned if the column number is out of range, or if the
       specified column is not a simple reference to a table column.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfformat">
     <term><function>PQfformat</function><indexterm><primary>PQfformat</primary></indexterm></term>

     <listitem>
      <para>
       Returns the format code indicating the format of the given
       column.  Column numbers start at 0.
<synopsis>
int PQfformat(const PGresult *res,
              int column_number);
</synopsis>
      </para>

      <para>
       Format code zero indicates textual data representation, while format
       code one indicates binary representation.  (Other codes are reserved
       for future definition.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQftype">
     <term><function>PQftype</function><indexterm><primary>PQftype</primary></indexterm></term>

     <listitem>
      <para>
       Returns the data type associated with the given  column number.
       The  integer  returned is the internal OID number of the type.
       Column numbers start at 0.
<synopsis>
Oid PQftype(const PGresult *res,
            int column_number);
</synopsis>
      </para>

      <para>
       You can query the system table <literal>pg_type</literal> to
       obtain the names and properties of the various data types. The
       <acronym>OID</acronym>s of the built-in data types are defined
       in the file <filename>catalog/pg_type_d.h</filename>
       in the <productname>PostgreSQL</productname>
       installation's <filename>include</filename> directory.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfmod">
     <term><function>PQfmod</function><indexterm><primary>PQfmod</primary></indexterm></term>

     <listitem>
      <para>
       Returns  the type modifier of the column associated with the
       given column number.  Column numbers start at 0.
<synopsis>
int PQfmod(const PGresult *res,
           int column_number);
</synopsis>
      </para>

      <para>
       The interpretation of modifier values is type-specific; they
       typically indicate precision or size limits.  The value -1 is
       used to indicate <quote>no information available</quote>.  Most data
       types do not use modifiers, in which case the value is always
       -1.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfsize">
     <term><function>PQfsize</function><indexterm><primary>PQfsize</primary></indexterm></term>

     <listitem>
      <para>
       Returns  the  size  in bytes of the column associated with the
       given column number.  Column numbers start at 0.
<synopsis>
int PQfsize(const PGresult *res,
            int column_number);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQfsize"/> returns the space allocated for this column
       in a database row, in other words the size of the server's
       internal representation of the data type.  (Accordingly, it is
       not really very useful to clients.) A negative value indicates
       the data type is variable-length.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQbinaryTuples">
     <term><function>PQbinaryTuples</function><indexterm><primary>PQbinaryTuples</primary></indexterm></term>

     <listitem>
      <para>
       Returns 1 if the <structname>PGresult</structname> contains binary data
       and 0 if it contains text data.
<synopsis>
int PQbinaryTuples(const PGresult *res);
</synopsis>
      </para>

      <para>
       This function is deprecated (except for its use in connection with
       <command>COPY</command>), because it is possible for a single
       <structname>PGresult</structname> to contain text data in some columns and
       binary data in others.  <xref linkend="libpq-PQfformat"/> is preferred.
       <xref linkend="libpq-PQbinaryTuples"/> returns 1 only if all columns of the
       result are binary (format 1).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQgetvalue">
     <term><function>PQgetvalue</function><indexterm><primary>PQgetvalue</primary></indexterm></term>

     <listitem>
      <para>
       Returns a single field value of one row of a
       <structname>PGresult</structname>.  Row and column numbers start
       at 0.  The caller should not free the result directly.  It will
       be freed when the associated <structname>PGresult</structname> handle is
       passed to <xref linkend="libpq-PQclear"/>.
<synopsis>
char *PQgetvalue(const PGresult *res,
                 int row_number,
                 int column_number);
</synopsis>
      </para>

      <para>
       For data in text format, the value returned by
       <xref linkend="libpq-PQgetvalue"/> is a null-terminated character
       string  representation of the field value.  For data in binary
       format, the value is in the binary representation determined by
       the data type's <function>typsend</function> and <function>typreceive</function>
       functions.  (The value is actually followed by a zero byte in
       this case too, but that is not ordinarily useful, since the
       value is likely to contain embedded nulls.)
      </para>

      <para>
       An empty string is returned if the field value is null.  See
       <xref linkend="libpq-PQgetisnull"/> to distinguish null values from
       empty-string values.
      </para>

      <para>
       The pointer returned  by  <xref linkend="libpq-PQgetvalue"/> points
       to storage that is part of the <structname>PGresult</structname>
       structure.  One should not modify the data it points to, and one
       must explicitly copy the data into other storage if it is to be
       used past the lifetime of the  <structname>PGresult</structname>
       structure itself.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQgetisnull">
     <term><function>PQgetisnull</function><indexterm
     ><primary>PQgetisnull</primary></indexterm><indexterm
     ><primary>null value</primary><secondary sortas="libpq">in libpq</secondary></indexterm></term>

     <listitem>
      <para>
       Tests a field for a null value.  Row and column numbers start
       at 0.
<synopsis>
int PQgetisnull(const PGresult *res,
                int row_number,
                int column_number);
</synopsis>
      </para>

      <para>
       This function returns  1 if the field is null and 0 if it
       contains a non-null value.  (Note that
       <xref linkend="libpq-PQgetvalue"/> will return an empty string,
       not a null pointer, for a null field.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQgetlength">
     <term><function>PQgetlength</function><indexterm><primary>PQgetlength</primary></indexterm></term>

     <listitem>
      <para>
       Returns the actual length of a field value in bytes.  Row and
       column numbers start at 0.
<synopsis>
int PQgetlength(const PGresult *res,
                int row_number,
                int column_number);
</synopsis>
      </para>

      <para>
       This is the actual data length for the particular data value,
       that is, the size of the object pointed to by
       <xref linkend="libpq-PQgetvalue"/>.  For text data format this is
       the same as <function>strlen()</function>.  For binary format this is
       essential information.  Note that one should <emphasis>not</emphasis>
       rely on <xref linkend="libpq-PQfsize"/> to obtain the actual data
       length.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQnparams">
     <term><function>PQnparams</function><indexterm><primary>PQnparams</primary></indexterm></term>

     <listitem>
      <para>
       Returns the number of parameters of a prepared statement.
<synopsis>
int PQnparams(const PGresult *res);
</synopsis>
      </para>

      <para>
       This function is only useful when inspecting the result of
       <xref linkend="libpq-PQdescribePrepared"/>.  For other types of results it
       will return zero.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQparamtype">
     <term><function>PQparamtype</function><indexterm><primary>PQparamtype</primary></indexterm></term>

     <listitem>
      <para>
       Returns the data type of the indicated statement parameter.
       Parameter numbers start at 0.
<synopsis>
Oid PQparamtype(const PGresult *res, int param_number);
</synopsis>
      </para>

      <para>
       This function is only useful when inspecting the result of
       <xref linkend="libpq-PQdescribePrepared"/>.  For other types of results it
       will return zero.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQprint">
     <term><function>PQprint</function><indexterm><primary>PQprint</primary></indexterm></term>

     <listitem>
      <para>
       Prints out all the rows and,  optionally,  the column names  to
       the specified output stream.
<synopsis>
void PQprint(FILE *fout,      /* output stream */
             const PGresult *res,
             const PQprintOpt *po);
typedef struct
{
    pqbool  header;      /* print output field headings and row count */
    pqbool  align;       /* fill align the fields */
    pqbool  standard;    /* old brain dead format */
    pqbool  html3;       /* output HTML tables */
    pqbool  expanded;    /* expand tables */
    pqbool  pager;       /* use pager for output if needed */
    char    *fieldSep;   /* field separator */
    char    *tableOpt;   /* attributes for HTML table element */
    char    *caption;    /* HTML table caption */
    char    **fieldName; /* null-terminated array of replacement field names */
} PQprintOpt;
</synopsis>
      </para>

      <para>
       This function was formerly used by <application>psql</application>
       to print query results, but this is no longer the case.  Note
       that it assumes all the data is in text format.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-exec-nonselect">
   <title>Retrieving Other Result Information</title>

   <para>
    These functions are used to extract other information from
    <structname>PGresult</structname> objects.
   </para>

   <variablelist>
    <varlistentry id="libpq-PQcmdStatus">
     <term><function>PQcmdStatus</function><indexterm><primary>PQcmdStatus</primary></indexterm></term>

     <listitem>
      <para>
       Returns the command status tag from the SQL command that generated
       the <structname>PGresult</structname>.
<synopsis>
char *PQcmdStatus(PGresult *res);
</synopsis>
      </para>

      <para>
       Commonly this is just the name of the command, but it might include
       additional data such as the number of rows processed. The caller
       should not free the result directly. It will be freed when the
       associated <structname>PGresult</structname> handle is passed to
       <xref linkend="libpq-PQclear"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQcmdTuples">
     <term><function>PQcmdTuples</function><indexterm><primary>PQcmdTuples</primary></indexterm></term>

     <listitem>
      <para>
       Returns the number of rows affected by the SQL command.
<synopsis>
char *PQcmdTuples(PGresult *res);
</synopsis>
      </para>

      <para>
       This function returns a string containing the number of rows
       affected by the <acronym>SQL</acronym> statement that generated the
       <structname>PGresult</structname>. This function can only be used following
       the execution of a <command>SELECT</command>, <command>CREATE TABLE AS</command>,
       <command>INSERT</command>, <command>UPDATE</command>, <command>DELETE</command>,
       <command>MERGE</command>, <command>MOVE</command>, <command>FETCH</command>,
       or <command>COPY</command> statement, or an <command>EXECUTE</command> of a
       prepared query that contains an <command>INSERT</command>,
       <command>UPDATE</command>, <command>DELETE</command>,
       or <command>MERGE</command> statement.
       If the command that generated the <structname>PGresult</structname> was anything
       else, <xref linkend="libpq-PQcmdTuples"/> returns an empty string. The caller
       should not free the return value directly. It will be freed when
       the associated <structname>PGresult</structname> handle is passed to
       <xref linkend="libpq-PQclear"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQoidValue">
     <term><function>PQoidValue</function><indexterm><primary>PQoidValue</primary></indexterm></term>

     <listitem>
      <para>
       Returns the OID<indexterm><primary>OID</primary><secondary>in libpq</secondary></indexterm>
       of the inserted row, if the <acronym>SQL</acronym> command was an
       <command>INSERT</command> that inserted exactly one row into a table that
       has OIDs, or a <command>EXECUTE</command> of a prepared query containing
       a suitable <command>INSERT</command> statement.  Otherwise, this function
       returns <literal>InvalidOid</literal>. This function will also
       return <literal>InvalidOid</literal> if the table affected by the
       <command>INSERT</command> statement does not contain OIDs.
<synopsis>
Oid PQoidValue(const PGresult *res);
</synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQoidStatus">
     <term><function>PQoidStatus</function><indexterm><primary>PQoidStatus</primary></indexterm></term>

     <listitem>
      <para>
       This function is deprecated in favor of
       <xref linkend="libpq-PQoidValue"/> and is not thread-safe.
       It returns a string with the OID of the inserted row, while
       <xref linkend="libpq-PQoidValue"/> returns the OID value.
<synopsis>
char *PQoidStatus(const PGresult *res);
</synopsis>
      </para>

     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

  <sect2 id="libpq-exec-escape-string">
   <title>Escaping Strings for Inclusion in SQL Commands</title>

   <indexterm zone="libpq-exec-escape-string">
    <primary>escaping strings</primary>
    <secondary>in libpq</secondary>
   </indexterm>

   <variablelist>
    <varlistentry id="libpq-PQescapeLiteral">
     <term><function>PQescapeLiteral</function><indexterm><primary>PQescapeLiteral</primary></indexterm></term>

     <listitem>
     <para>
<synopsis>
char *PQescapeLiteral(PGconn *conn, const char *str, size_t length);
</synopsis>
     </para>

     <para>
      <xref linkend="libpq-PQescapeLiteral"/> escapes a string for
      use within an SQL command.  This is useful when inserting data
      values as literal constants in SQL commands.  Certain characters
      (such as quotes and backslashes) must be escaped to prevent them
      from being interpreted specially by the SQL parser.
      <xref linkend="libpq-PQescapeLiteral"/> performs this operation.
     </para>

     <para>
      <xref linkend="libpq-PQescapeLiteral"/> returns an escaped version of the
      <parameter>str</parameter> parameter in memory allocated with
      <function>malloc()</function>.  This memory should be freed using
      <function>PQfreemem()</function> when the result is no longer needed.
      A terminating zero byte is not required, and should not be
      counted in <parameter>length</parameter>.  (If a terminating zero byte is found
      before <parameter>length</parameter> bytes are processed,
      <xref linkend="libpq-PQescapeLiteral"/> stops at the zero; the behavior is
      thus rather like <function>strncpy</function>.) The
      return string has all special characters replaced so that they can
      be properly processed by the <productname>PostgreSQL</productname>
      string literal parser.  A terminating zero byte is also added.  The
      single quotes that must surround <productname>PostgreSQL</productname>
      string literals are included in the result string.
     </para>

     <para>
      On error, <xref linkend="libpq-PQescapeLiteral"/> returns <symbol>NULL</symbol> and a suitable
      message is stored in the <parameter>conn</parameter> object.
     </para>

     <tip>
      <para>
       It is especially important to do proper escaping when handling
       strings that were received from an untrustworthy source.
       Otherwise there is a security risk: you are vulnerable to
       <quote>SQL injection</quote> attacks wherein unwanted SQL commands are
       fed to your database.
      </para>
     </tip>

     <para>
      Note that it is neither necessary nor correct to do escaping when a data
      value is passed as a separate parameter in <xref linkend="libpq-PQexecParams"/> or
      its sibling routines.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQescapeIdentifier">
     <term><function>PQescapeIdentifier</function><indexterm><primary>PQescapeIdentifier</primary></indexterm></term>

     <listitem>
     <para>
<synopsis>
char *PQescapeIdentifier(PGconn *conn, const char *str, size_t length);
</synopsis>
     </para>

     <para>
      <xref linkend="libpq-PQescapeIdentifier"/> escapes a string for
      use as an SQL identifier, such as a table, column, or function name.
      This is useful when a user-supplied identifier might contain
      special characters that would otherwise not be interpreted as part
      of the identifier by the SQL parser, or when the identifier might
      contain upper case characters whose case should be preserved.
     </para>

     <para>
      <xref linkend="libpq-PQescapeIdentifier"/> returns a version of the
      <parameter>str</parameter> parameter escaped as an SQL identifier
      in memory allocated with <function>malloc()</function>.  This memory must be
      freed using <function>PQfreemem()</function> when the result is no longer
      needed.  A terminating zero byte is not required, and should not be
      counted in <parameter>length</parameter>.  (If a terminating zero byte is found
      before <parameter>length</parameter> bytes are processed,
      <xref linkend="libpq-PQescapeIdentifier"/> stops at the zero; the behavior is
      thus rather like <function>strncpy</function>.) The
      return string has all special characters replaced so that it
      will be properly processed as an SQL identifier.  A terminating zero byte
      is also added.  The return string will also be surrounded by double
      quotes.
     </para>

     <para>
      On error, <xref linkend="libpq-PQescapeIdentifier"/> returns <symbol>NULL</symbol> and a suitable
      message is stored in the <parameter>conn</parameter> object.
     </para>

     <tip>
      <para>
       As with string literals, to prevent SQL injection attacks,
       SQL identifiers must be escaped when they are received from an
       untrustworthy source.
      </para>
     </tip>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQescapeStringConn">
     <term><function>PQescapeStringConn</function><indexterm><primary>PQescapeStringConn</primary></indexterm></term>

     <listitem>
     <para>
<synopsis>
size_t PQescapeStringConn(PGconn *conn,
                          char *to, const char *from, size_t length,
                          int *error);
</synopsis>
     </para>

     <para>
      <xref linkend="libpq-PQescapeStringConn"/> escapes string literals, much like
      <xref linkend="libpq-PQescapeLiteral"/>.  Unlike <xref linkend="libpq-PQescapeLiteral"/>,
      the caller is responsible for providing an appropriately sized buffer.
      Furthermore, <xref linkend="libpq-PQescapeStringConn"/> does not generate the
      single quotes that must surround <productname>PostgreSQL</productname> string
      literals; they should be provided in the SQL command that the
      result is inserted into.  The parameter <parameter>from</parameter> points to
      the first character of the string that is to be escaped, and the
      <parameter>length</parameter> parameter gives the number of bytes in this
      string.  A terminating zero byte is not required, and should not be
      counted in <parameter>length</parameter>.  (If a terminating zero byte is found
      before <parameter>length</parameter> bytes are processed,
      <xref linkend="libpq-PQescapeStringConn"/> stops at the zero; the behavior is
      thus rather like <function>strncpy</function>.) <parameter>to</parameter> shall point
      to a buffer that is able to hold at least one more byte than twice
      the value of <parameter>length</parameter>, otherwise the behavior is undefined.
      Behavior is likewise undefined if the <parameter>to</parameter> and
      <parameter>from</parameter> strings overlap.
     </para>

     <para>
      If the <parameter>error</parameter> parameter is not <symbol>NULL</symbol>, then
      <literal>*error</literal> is set to zero on success, nonzero on error.
      Presently the only possible error conditions involve invalid multibyte
      encoding in the source string.  The output string is still generated
      on error, but it can be expected that the server will reject it as
      malformed.  On error, a suitable message is stored in the
      <parameter>conn</parameter> object, whether or not <parameter>error</parameter> is <symbol>NULL</symbol>.
     </para>

     <para>
      <xref linkend="libpq-PQescapeStringConn"/> returns the number of bytes written
      to <parameter>to</parameter>, not including the terminating zero byte.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQescapeString">
     <term><function>PQescapeString</function><indexterm><primary>PQescapeString</primary></indexterm></term>

     <listitem>
     <para>
       <xref linkend="libpq-PQescapeString"/> is an older, deprecated version of
       <xref linkend="libpq-PQescapeStringConn"/>.
<synopsis>
size_t PQescapeString (char *to, const char *from, size_t length);
</synopsis>
     </para>

     <para>
      The only difference from <xref linkend="libpq-PQescapeStringConn"/> is that
      <xref linkend="libpq-PQescapeString"/> does not take <structname>PGconn</structname>
      or <parameter>error</parameter> parameters.
      Because of this, it cannot adjust its behavior depending on the
      connection properties (such as character encoding) and therefore
      <emphasis>it might give the wrong results</emphasis>.  Also, it has no way
      to report error conditions.
     </para>

     <para>
      <xref linkend="libpq-PQescapeString"/> can be used safely in
      client programs that work with only one <productname>PostgreSQL</productname>
      connection at a time (in this case it can find out what it needs to
      know <quote>behind the scenes</quote>).  In other contexts it is a security
      hazard and should be avoided in favor of
      <xref linkend="libpq-PQescapeStringConn"/>.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQescapeByteaConn">
     <term><function>PQescapeByteaConn</function><indexterm><primary>PQescapeByteaConn</primary></indexterm></term>

     <listitem>
     <para>
       Escapes binary data for use within an SQL command with the type
       <type>bytea</type>.  As with <xref linkend="libpq-PQescapeStringConn"/>,
       this is only used when inserting data directly into an SQL command string.
<synopsis>
unsigned char *PQescapeByteaConn(PGconn *conn,
                                 const unsigned char *from,
                                 size_t from_length,
                                 size_t *to_length);
</synopsis>
      </para>

      <para>
       Certain byte values must be escaped when used as part of a
       <type>bytea</type> literal in an <acronym>SQL</acronym> statement.
       <xref linkend="libpq-PQescapeByteaConn"/> escapes bytes using
       either hex encoding or backslash escaping.  See <xref
       linkend="datatype-binary"/> for more information.
      </para>

      <para>
       The <parameter>from</parameter> parameter points to the first
       byte of the string that is to be escaped, and the
       <parameter>from_length</parameter> parameter gives the number of
       bytes in this binary string.  (A terminating zero byte is
       neither necessary nor counted.)  The <parameter>to_length</parameter>
       parameter points to a variable that will hold the resultant
       escaped string length. This result string length includes the terminating
       zero byte of the result.
      </para>

      <para>
       <xref linkend="libpq-PQescapeByteaConn"/> returns an escaped version of the
       <parameter>from</parameter> parameter binary string in memory
       allocated with <function>malloc()</function>.  This memory should be freed using
       <function>PQfreemem()</function> when the result is no longer needed.  The
       return string has all special characters replaced so that they can
       be properly processed by the <productname>PostgreSQL</productname>
       string literal parser, and the <type>bytea</type> input function. A
       terminating zero byte is also added.  The single quotes that must
       surround <productname>PostgreSQL</productname> string literals are
       not part of the result string.
      </para>

      <para>
       On error, a null pointer is returned, and a suitable error message
       is stored in the <parameter>conn</parameter> object.  Currently, the only
       possible error is insufficient memory for the result string.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQescapeBytea">
     <term><function>PQescapeBytea</function><indexterm><primary>PQescapeBytea</primary></indexterm></term>

     <listitem>
      <para>
       <xref linkend="libpq-PQescapeBytea"/> is an older, deprecated version of
       <xref linkend="libpq-PQescapeByteaConn"/>.
<synopsis>
unsigned char *PQescapeBytea(const unsigned char *from,
                             size_t from_length,
                             size_t *to_length);
</synopsis>
      </para>

      <para>
       The only difference from <xref linkend="libpq-PQescapeByteaConn"/> is that
       <xref linkend="libpq-PQescapeBytea"/> does not take a <structname>PGconn</structname>
       parameter.  Because of this, <xref linkend="libpq-PQescapeBytea"/> can
       only be used safely in client programs that use a single
       <productname>PostgreSQL</productname> connection at a time (in this case
       it can find out what it needs to know <quote>behind the
       scenes</quote>).  It <emphasis>might give the wrong results</emphasis> if
       used in programs that use multiple database connections (use
       <xref linkend="libpq-PQescapeByteaConn"/> in such cases).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQunescapeBytea">
     <term><function>PQunescapeBytea</function><indexterm><primary>PQunescapeBytea</primary></indexterm></term>

     <listitem>
      <para>
       Converts a string representation of binary data into binary data
       &mdash; the reverse of <xref linkend="libpq-PQescapeBytea"/>.  This
       is needed when retrieving <type>bytea</type> data in text format,
       but not when retrieving it in binary format.

<synopsis>
unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
</synopsis>
      </para>

      <para>
       The <parameter>from</parameter> parameter points to a string
       such as might be returned by <xref linkend="libpq-PQgetvalue"/> when applied
       to a <type>bytea</type> column. <xref linkend="libpq-PQunescapeBytea"/>
       converts this string representation into its binary representation.
       It returns a pointer to a buffer allocated with
       <function>malloc()</function>, or <symbol>NULL</symbol> on error, and puts the size of
       the buffer in <parameter>to_length</parameter>. The result must be
       freed using <xref linkend="libpq-PQfreemem"/> when it is no longer needed.
      </para>

      <para>
       This conversion is not exactly the inverse of
       <xref linkend="libpq-PQescapeBytea"/>, because the string is not expected
       to be <quote>escaped</quote> when received from <xref linkend="libpq-PQgetvalue"/>.
       In particular this means there is no need for string quoting considerations,
       and so no need for a <structname>PGconn</structname> parameter.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

 </sect1>

 <sect1 id="libpq-async">
  <title>Asynchronous Command Processing</title>

  <indexterm zone="libpq-async">
   <primary>nonblocking connection</primary>
  </indexterm>

  <para>
   The <xref linkend="libpq-PQexec"/> function is adequate for submitting
   commands in normal, synchronous applications.  It has a few
   deficiencies, however, that can be of importance to some users:

   <itemizedlist>
    <listitem>
     <para>
      <xref linkend="libpq-PQexec"/> waits for the command to be completed.
      The application might have other work to do (such as maintaining a
      user interface), in which case it won't want to block waiting for
      the response.
     </para>
    </listitem>

    <listitem>
     <para>
      Since the execution of the client application is suspended while it
      waits for the result, it is hard for the application to decide that
      it would like to try to cancel the ongoing command.  (It can be done
      from a signal handler, but not otherwise.)
     </para>
    </listitem>

    <listitem>
     <para>
      <xref linkend="libpq-PQexec"/> can return only one
      <structname>PGresult</structname> structure.  If the submitted command
      string contains multiple <acronym>SQL</acronym> commands, all but
      the last <structname>PGresult</structname> are discarded by
      <xref linkend="libpq-PQexec"/>.
     </para>
    </listitem>

    <listitem>
     <para>
      <xref linkend="libpq-PQexec"/> always collects the command's entire result,
      buffering it in a single <structname>PGresult</structname>.  While
      this simplifies error-handling logic for the application, it can be
      impractical for results containing many rows.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   Applications that do not like these limitations can instead use the
   underlying functions that <xref linkend="libpq-PQexec"/> is built from:
   <xref linkend="libpq-PQsendQuery"/> and <xref linkend="libpq-PQgetResult"/>.
   There are also
   <xref linkend="libpq-PQsendQueryParams"/>,
   <xref linkend="libpq-PQsendPrepare"/>,
   <xref linkend="libpq-PQsendQueryPrepared"/>,
   <xref linkend="libpq-PQsendDescribePrepared"/>,
   <xref linkend="libpq-PQsendDescribePortal"/>,
   <xref linkend="libpq-PQsendClosePrepared"/>, and
   <xref linkend="libpq-PQsendClosePortal"/>,
   which can be used with <xref linkend="libpq-PQgetResult"/> to duplicate
   the functionality of
   <xref linkend="libpq-PQexecParams"/>,
   <xref linkend="libpq-PQprepare"/>,
   <xref linkend="libpq-PQexecPrepared"/>,
   <xref linkend="libpq-PQdescribePrepared"/>,
   <xref linkend="libpq-PQdescribePortal"/>
   <xref linkend="libpq-PQclosePrepared"/>, and
   <xref linkend="libpq-PQclosePortal"/>
   respectively.

   <variablelist>
    <varlistentry id="libpq-PQsendQuery">
     <term><function>PQsendQuery</function><indexterm><primary>PQsendQuery</primary></indexterm></term>

     <listitem>
      <para>
       Submits a command to the server without waiting for the result(s).
       1 is returned if the command was successfully dispatched and 0 if
       not (in which case, use <xref linkend="libpq-PQerrorMessage"/> to get more
       information about the failure).
<synopsis>
int PQsendQuery(PGconn *conn, const char *command);
</synopsis>

       After successfully calling <xref linkend="libpq-PQsendQuery"/>, call
       <xref linkend="libpq-PQgetResult"/> one or more times to obtain the
       results.  <xref linkend="libpq-PQsendQuery"/> cannot be called again
       (on the same connection) until <xref linkend="libpq-PQgetResult"/>
       has returned a null pointer, indicating that the command is done.
      </para>

      <para>
       In pipeline mode, this function is disallowed.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendQueryParams">
     <term><function>PQsendQueryParams</function><indexterm><primary>PQsendQueryParams</primary></indexterm></term>

     <listitem>
      <para>
       Submits a command and separate parameters to the server without
       waiting for the result(s).
<synopsis>
int PQsendQueryParams(PGconn *conn,
                      const char *command,
                      int nParams,
                      const Oid *paramTypes,
                      const char * const *paramValues,
                      const int *paramLengths,
                      const int *paramFormats,
                      int resultFormat);
</synopsis>

       This is equivalent to <xref linkend="libpq-PQsendQuery"/> except that
       query parameters can be specified separately from the query string.
       The function's parameters are handled identically to
       <xref linkend="libpq-PQexecParams"/>.  Like
       <xref linkend="libpq-PQexecParams"/>, it allows only one command in the
       query string.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendPrepare">
     <term><function>PQsendPrepare</function><indexterm><primary>PQsendPrepare</primary></indexterm></term>

     <listitem>
      <para>
       Sends a request to create a prepared statement with the given
       parameters, without waiting for completion.
<synopsis>
int PQsendPrepare(PGconn *conn,
                  const char *stmtName,
                  const char *query,
                  int nParams,
                  const Oid *paramTypes);
</synopsis>

       This is an asynchronous version of <xref linkend="libpq-PQprepare"/>: it
       returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <xref linkend="libpq-PQgetResult"/> to
       determine whether the server successfully created the prepared
       statement.  The function's parameters are handled identically to
       <xref linkend="libpq-PQprepare"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendQueryPrepared">
     <term><function>PQsendQueryPrepared</function><indexterm><primary>PQsendQueryPrepared</primary></indexterm></term>

     <listitem>
      <para>
       Sends a request to execute a prepared statement with given
       parameters, without waiting for the result(s).
<synopsis>
int PQsendQueryPrepared(PGconn *conn,
                        const char *stmtName,
                        int nParams,
                        const char * const *paramValues,
                        const int *paramLengths,
                        const int *paramFormats,
                        int resultFormat);
</synopsis>

       This is similar to <xref linkend="libpq-PQsendQueryParams"/>, but
       the command to be executed is specified by naming a
       previously-prepared statement, instead of giving a query string.
       The function's parameters are handled identically to
       <xref linkend="libpq-PQexecPrepared"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendDescribePrepared">
     <term><function>PQsendDescribePrepared</function><indexterm><primary>PQsendDescribePrepared</primary></indexterm></term>

     <listitem>
      <para>
       Submits a request to obtain information about the specified
       prepared statement, without waiting for completion.
<synopsis>
int PQsendDescribePrepared(PGconn *conn, const char *stmtName);
</synopsis>

       This is an asynchronous version of <xref linkend="libpq-PQdescribePrepared"/>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <xref linkend="libpq-PQgetResult"/> to
       obtain the results.  The function's parameters are handled
       identically to <xref linkend="libpq-PQdescribePrepared"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendDescribePortal">
     <term><function>PQsendDescribePortal</function><indexterm><primary>PQsendDescribePortal</primary></indexterm></term>

     <listitem>
      <para>
       Submits a request to obtain information about the specified
       portal, without waiting for completion.
<synopsis>
int PQsendDescribePortal(PGconn *conn, const char *portalName);
</synopsis>

       This is an asynchronous version of <xref linkend="libpq-PQdescribePortal"/>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <xref linkend="libpq-PQgetResult"/> to
       obtain the results.  The function's parameters are handled
       identically to <xref linkend="libpq-PQdescribePortal"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendClosePrepared">
     <term><function>PQsendClosePrepared</function><indexterm><primary>PQsendClosePrepared</primary></indexterm></term>

     <listitem>
      <para>
       Submits a request to close the specified prepared statement, without
       waiting for completion.
<synopsis>
int PQsendClosePrepared(PGconn *conn, const char *stmtName);
</synopsis>

       This is an asynchronous version of <xref linkend="libpq-PQclosePrepared"/>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <xref linkend="libpq-PQgetResult"/> to
       obtain the results.  The function's parameters are handled
       identically to <xref linkend="libpq-PQclosePrepared"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendClosePortal">
     <term><function>PQsendClosePortal</function><indexterm><primary>PQsendClosePortal</primary></indexterm></term>

     <listitem>
      <para>
       Submits a request to close specified portal, without waiting for
       completion.
<synopsis>
int PQsendClosePortal(PGconn *conn, const char *portalName);
</synopsis>

       This is an asynchronous version of <xref linkend="libpq-PQclosePortal"/>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <xref linkend="libpq-PQgetResult"/> to
       obtain the results.  The function's parameters are handled
       identically to <xref linkend="libpq-PQclosePortal"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQgetResult">
     <term><function>PQgetResult</function><indexterm><primary>PQgetResult</primary></indexterm></term>

     <listitem>
      <para>
       Waits for the next result from a prior
       <xref linkend="libpq-PQsendQuery"/>,
       <xref linkend="libpq-PQsendQueryParams"/>,
       <xref linkend="libpq-PQsendPrepare"/>,
       <xref linkend="libpq-PQsendQueryPrepared"/>,
       <xref linkend="libpq-PQsendDescribePrepared"/>,
       <xref linkend="libpq-PQsendDescribePortal"/>,
       <xref linkend="libpq-PQsendClosePrepared"/>,
       <xref linkend="libpq-PQsendClosePortal"/>, or
       <xref linkend="libpq-PQpipelineSync"/>
       call, and returns it.
       A null pointer is returned when the command is complete and there
       will be no more results.
<synopsis>
PGresult *PQgetResult(PGconn *conn);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQgetResult"/> must be called repeatedly until
       it returns a null pointer, indicating that the command is done.
       (If called when no command is active,
       <xref linkend="libpq-PQgetResult"/> will just return a null pointer
       at once.) Each non-null result from
       <xref linkend="libpq-PQgetResult"/> should be processed using the
       same <structname>PGresult</structname> accessor functions previously
       described.  Don't forget to free each result object with
       <xref linkend="libpq-PQclear"/> when done with it.  Note that
       <xref linkend="libpq-PQgetResult"/> will block only if a command is
       active and the necessary response data has not yet been read by
       <xref linkend="libpq-PQconsumeInput"/>.
      </para>

      <para>
       In pipeline mode, <function>PQgetResult</function> will return normally
       unless an error occurs; for any subsequent query sent after the one
       that caused the error until (and excluding) the next synchronization point,
       a special result of type <literal>PGRES_PIPELINE_ABORTED</literal> will
       be returned, and a null pointer will be returned after it.
       When the pipeline synchronization point is reached, a result of type
       <literal>PGRES_PIPELINE_SYNC</literal> will be returned.
       The result of the next query after the synchronization point follows
       immediately (that is, no null pointer is returned after
       the synchronization point.)
      </para>

      <note>
       <para>
        Even when <xref linkend="libpq-PQresultStatus"/> indicates a fatal
        error, <xref linkend="libpq-PQgetResult"/> should be called until it
        returns a null pointer, to allow <application>libpq</application> to
        process the error information completely.
       </para>
      </note>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   Using <xref linkend="libpq-PQsendQuery"/> and
   <xref linkend="libpq-PQgetResult"/> solves one of
   <xref linkend="libpq-PQexec"/>'s problems:  If a command string contains
   multiple <acronym>SQL</acronym> commands, the results of those commands
   can be obtained individually.  (This allows a simple form of overlapped
   processing, by the way: the client can be handling the results of one
   command while the server is still working on later queries in the same
   command string.)
  </para>

  <para>
   Another frequently-desired feature that can be obtained with
   <xref linkend="libpq-PQsendQuery"/> and <xref linkend="libpq-PQgetResult"/>
   is retrieving large query results a row at a time.  This is discussed
   in <xref linkend="libpq-single-row-mode"/>.
  </para>

  <para>
   By itself, calling <xref linkend="libpq-PQgetResult"/>
   will still cause the client to block until the server completes the
   next <acronym>SQL</acronym> command.  This can be avoided by proper
   use of two more functions:

   <variablelist>
    <varlistentry id="libpq-PQconsumeInput">
     <term><function>PQconsumeInput</function><indexterm><primary>PQconsumeInput</primary></indexterm>
     </term>

     <listitem>
      <para>
       If input is available from the server, consume it.
<synopsis>
int PQconsumeInput(PGconn *conn);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQconsumeInput"/> normally returns 1 indicating
       <quote>no error</quote>, but returns 0 if there was some kind of
       trouble (in which case <xref linkend="libpq-PQerrorMessage"/> can be
       consulted).  Note that the result does not say whether any input
       data was actually collected. After calling
       <xref linkend="libpq-PQconsumeInput"/>, the application can check
       <xref linkend="libpq-PQisBusy"/> and/or
       <function>PQnotifies</function> to see if their state has changed.
      </para>

      <para>
       <xref linkend="libpq-PQconsumeInput"/> can be called even if the
       application is not prepared to deal with a result or notification
       just yet.  The function will read available data and save it in
       a buffer, thereby causing a <function>select()</function>
       read-ready indication to go away.  The application can thus use
       <xref linkend="libpq-PQconsumeInput"/> to clear the
       <function>select()</function> condition immediately, and then
       examine the results at leisure.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQisBusy">
     <term><function>PQisBusy</function><indexterm><primary>PQisBusy</primary></indexterm></term>

     <listitem>
      <para>
       Returns 1 if a command is busy, that is,
       <xref linkend="libpq-PQgetResult"/> would block waiting for input.
       A 0 return indicates that <xref linkend="libpq-PQgetResult"/> can be
       called with assurance of not blocking.
<synopsis>
int PQisBusy(PGconn *conn);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQisBusy"/> will not itself attempt to read data
       from the server; therefore <xref linkend="libpq-PQconsumeInput"/>
       must be invoked first, or the busy state will never end.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   A typical application using these functions will have a main loop that
   uses <function>select()</function> or <function>poll()</function> to wait for
   all the conditions that it must respond to.  One of the conditions
   will be input available from the server, which in terms of
   <function>select()</function> means readable data on the file
   descriptor identified by <xref linkend="libpq-PQsocket"/>.  When the main
   loop detects input ready, it should call
   <xref linkend="libpq-PQconsumeInput"/> to read the input.  It can then
   call <xref linkend="libpq-PQisBusy"/>, followed by
   <xref linkend="libpq-PQgetResult"/> if <xref linkend="libpq-PQisBusy"/>
   returns false (0).  It can also call <function>PQnotifies</function>
   to detect <command>NOTIFY</command> messages (see <xref
   linkend="libpq-notify"/>).
  </para>

  <para>
   A client that uses
   <xref linkend="libpq-PQsendQuery"/>/<xref linkend="libpq-PQgetResult"/>
   can also attempt to cancel a command that is still being processed
   by the server; see <xref linkend="libpq-cancel"/>.  But regardless of
   the return value of <xref linkend="libpq-PQcancel"/>, the application
   must continue with the normal result-reading sequence using
   <xref linkend="libpq-PQgetResult"/>.  A successful cancellation will
   simply cause the command to terminate sooner than it would have
   otherwise.
  </para>

  <para>
   By using the functions described above, it is possible to avoid
   blocking while waiting for input from the database server.  However,
   it is still possible that the application will block waiting to send
   output to the server.  This is relatively uncommon but can happen if
   very long SQL commands or data values are sent.  (It is much more
   probable if the application sends data via <command>COPY IN</command>,
   however.)  To prevent this possibility and achieve completely
   nonblocking database operation, the following additional functions
   can be used.

   <variablelist>
    <varlistentry id="libpq-PQsetnonblocking">
     <term><function>PQsetnonblocking</function><indexterm><primary>PQsetnonblocking</primary></indexterm></term>

     <listitem>
      <para>
       Sets the nonblocking status of the connection.
<synopsis>
int PQsetnonblocking(PGconn *conn, int arg);
</synopsis>
      </para>

      <para>
       Sets the state of the connection to nonblocking if
       <parameter>arg</parameter> is 1, or blocking if
       <parameter>arg</parameter> is 0.  Returns 0 if OK, -1 if error.
      </para>

      <para>
       In the nonblocking state, successful calls to
       <xref linkend="libpq-PQsendQuery"/>, <xref linkend="libpq-PQputline"/>,
       <xref linkend="libpq-PQputnbytes"/>, <xref linkend="libpq-PQputCopyData"/>,
       and <xref linkend="libpq-PQendcopy"/> will not block;  their changes
       are stored in the local output buffer until they are flushed.
       Unsuccessful calls will return an error and must be retried.
      </para>

      <para>
       Note that <xref linkend="libpq-PQexec"/> does not honor nonblocking
       mode; if it is called, it will act in blocking fashion anyway.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQisnonblocking">
     <term><function>PQisnonblocking</function><indexterm><primary>PQisnonblocking</primary></indexterm></term>

     <listitem>
      <para>
       Returns the blocking status of the database connection.
<synopsis>
int PQisnonblocking(const PGconn *conn);
</synopsis>
      </para>

      <para>
       Returns 1 if the connection is set to nonblocking mode and 0 if
       blocking.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQflush">
     <term><function>PQflush</function><indexterm><primary>PQflush</primary></indexterm></term>

      <listitem>
       <para>
       Attempts to flush any queued output data to the server.  Returns
       0 if successful (or if the send queue is empty), -1 if it failed
       for some reason, or 1 if it was unable to send all the data in
       the send queue yet (this case can only occur if the connection
       is nonblocking).
<synopsis>
int PQflush(PGconn *conn);
</synopsis>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   After sending any command or data on a nonblocking connection, call
   <xref linkend="libpq-PQflush"/>.  If it returns 1, wait for the socket
   to become read- or write-ready.  If it becomes write-ready, call
   <xref linkend="libpq-PQflush"/> again.  If it becomes read-ready, call
   <xref linkend="libpq-PQconsumeInput"/>, then call
   <xref linkend="libpq-PQflush"/> again.  Repeat until
   <xref linkend="libpq-PQflush"/> returns 0.  (It is necessary to check for
   read-ready and drain the input with <xref linkend="libpq-PQconsumeInput"/>,
   because the server can block trying to send us data, e.g., NOTICE
   messages, and won't read our data until we read its.)  Once
   <xref linkend="libpq-PQflush"/> returns 0, wait for the socket to be
   read-ready and then read the response as described above.
  </para>

 </sect1>

 <sect1 id="libpq-pipeline-mode">
  <title>Pipeline Mode</title>

  <indexterm zone="libpq-pipeline-mode">
   <primary>libpq</primary>
   <secondary>pipeline mode</secondary>
  </indexterm>

  <indexterm zone="libpq-pipeline-mode">
   <primary>pipelining</primary>
   <secondary>in libpq</secondary>
  </indexterm>

  <indexterm zone="libpq-pipeline-mode">
   <primary>batch mode</primary>
   <secondary>in libpq</secondary>
  </indexterm>

  <para>
   <application>libpq</application> pipeline mode allows applications to
   send a query without having to read the result of the previously
   sent query.  Taking advantage of the pipeline mode, a client will wait
   less for the server, since multiple queries/results can be
   sent/received in a single network transaction.
  </para>

  <para>
   While pipeline mode provides a significant performance boost, writing
   clients using the pipeline mode is more complex because it involves
   managing a queue of pending queries and finding which result
   corresponds to which query in the queue.
  </para>

  <para>
   Pipeline mode also generally consumes more memory on both the client and server,
   though careful and aggressive management of the send/receive queue can mitigate
   this.  This applies whether or not the connection is in blocking or non-blocking
   mode.
  </para>

  <para>
   While <application>libpq</application>'s pipeline API was introduced in
   <productname>PostgreSQL</productname> 14, it is a client-side feature
   which doesn't require special server support and works on any server
   that supports the v3 extended query protocol.  For more information see
   <xref linkend="protocol-flow-pipelining"/>.
  </para>

  <sect2 id="libpq-pipeline-using">
   <title>Using Pipeline Mode</title>

   <para>
    To issue pipelines, the application must switch the connection
    into pipeline mode,
    which is done with <xref linkend="libpq-PQenterPipelineMode"/>.
    <xref linkend="libpq-PQpipelineStatus"/> can be used
    to test whether pipeline mode is active.
    In pipeline mode, only <link linkend="libpq-async">asynchronous operations</link>
    that utilize the extended query protocol
    are permitted, command strings containing multiple SQL commands are
    disallowed, and so is <literal>COPY</literal>.
    Using synchronous command execution functions
    such as <function>PQfn</function>,
    <function>PQexec</function>,
    <function>PQexecParams</function>,
    <function>PQprepare</function>,
    <function>PQexecPrepared</function>,
    <function>PQdescribePrepared</function>,
    <function>PQdescribePortal</function>,
    <function>PQclosePrepared</function>,
    <function>PQclosePortal</function>,
    is an error condition.
    <function>PQsendQuery</function> is
    also disallowed, because it uses the simple query protocol.
    Once all dispatched commands have had their results processed, and
    the end pipeline result has been consumed, the application may return
    to non-pipelined mode with <xref linkend="libpq-PQexitPipelineMode"/>.
   </para>

   <note>
    <para>
     It is best to use pipeline mode with <application>libpq</application> in
     <link linkend="libpq-PQsetnonblocking">non-blocking mode</link>. If used
     in blocking mode it is possible for a client/server deadlock to occur.
      <footnote>
       <para>
        The client will block trying to send queries to the server, but the
        server will block trying to send results to the client from queries
        it has already processed. This only occurs when the client sends
        enough queries to fill both its output buffer and the server's receive
        buffer before it switches to processing input from the server,
        but it's hard to predict exactly when that will happen.
       </para>
      </footnote>
    </para>
   </note>

   <sect3 id="libpq-pipeline-sending">
    <title>Issuing Queries</title>

    <para>
     After entering pipeline mode, the application dispatches requests using
     <xref linkend="libpq-PQsendQueryParams"/>
     or its prepared-query sibling
     <xref linkend="libpq-PQsendQueryPrepared"/>.
     These requests are queued on the client-side until flushed to the server;
     this occurs when <xref linkend="libpq-PQpipelineSync"/> is used to
     establish a synchronization point in the pipeline,
     or when <xref linkend="libpq-PQflush"/> is called.
     The functions <xref linkend="libpq-PQsendPrepare"/>,
     <xref linkend="libpq-PQsendDescribePrepared"/>,
     <xref linkend="libpq-PQsendDescribePortal"/>,
     <xref linkend="libpq-PQsendClosePrepared"/>, and
     <xref linkend="libpq-PQsendClosePortal"/> also work in pipeline mode.
     Result processing is described below.
    </para>

    <para>
     The server executes statements, and returns results, in the order the
     client sends them.  The server will begin executing the commands in the
     pipeline immediately, not waiting for the end of the pipeline.
     Note that results are buffered on the server side; the server flushes
     that buffer when a synchronization point is established with
     <function>PQpipelineSync</function>, or when
     <function>PQsendFlushRequest</function> is called.
     If any statement encounters an error, the server aborts the current
     transaction and does not execute any subsequent command in the queue
     until the next synchronization point;
     a <literal>PGRES_PIPELINE_ABORTED</literal> result is produced for
     each such command.
     (This remains true even if the commands in the pipeline would rollback
     the transaction.)
     Query processing resumes after the synchronization point.
    </para>

    <para>
     It's fine for one operation to depend on the results of a
     prior one; for example, one query may define a table that the next
     query in the same pipeline uses. Similarly, an application may
     create a named prepared statement and execute it with later
     statements in the same pipeline.
    </para>
   </sect3>

   <sect3 id="libpq-pipeline-results">
    <title>Processing Results</title>

    <para>
     To process the result of one query in a pipeline, the application calls
     <function>PQgetResult</function> repeatedly and handles each result
     until <function>PQgetResult</function> returns null.
     The result from the next query in the pipeline may then be retrieved using
     <function>PQgetResult</function> again and the cycle repeated.
     The application handles individual statement results as normal.
     When the results of all the queries in the pipeline have been
     returned, <function>PQgetResult</function> returns a result
     containing the status value <literal>PGRES_PIPELINE_SYNC</literal>
    </para>

    <para>
     The client may choose to defer result processing until the complete
     pipeline has been sent, or interleave that with sending further
     queries in the pipeline; see <xref linkend="libpq-pipeline-interleave"/>.
    </para>

    <para>
     To enter single-row mode, call <function>PQsetSingleRowMode</function>
     before retrieving results with <function>PQgetResult</function>.
     This mode selection is effective only for the query currently
     being processed. For more information on the use of
     <function>PQsetSingleRowMode</function>,
     refer to <xref linkend="libpq-single-row-mode"/>.
    </para>

    <para>
     <function>PQgetResult</function> behaves the same as for normal
     asynchronous processing except that it may contain the new
     <type>PGresult</type> types <literal>PGRES_PIPELINE_SYNC</literal>
     and <literal>PGRES_PIPELINE_ABORTED</literal>.
     <literal>PGRES_PIPELINE_SYNC</literal> is reported exactly once for each
     <function>PQpipelineSync</function> at the corresponding point
     in the pipeline.
     <literal>PGRES_PIPELINE_ABORTED</literal> is emitted in place of a normal
     query result for the first error and all subsequent results
     until the next <literal>PGRES_PIPELINE_SYNC</literal>;
     see <xref linkend="libpq-pipeline-errors"/>.
    </para>

    <para>
     <function>PQisBusy</function>, <function>PQconsumeInput</function>, etc
     operate as normal when processing pipeline results.  In particular,
     a call to <function>PQisBusy</function> in the middle of a pipeline
     returns 0 if the results for all the queries issued so far have been
     consumed.
    </para>

    <para>
     <application>libpq</application> does not provide any information to the
     application about the query currently being processed (except that
     <function>PQgetResult</function> returns null to indicate that we start
     returning the results of next query). The application must keep track
     of the order in which it sent queries, to associate them with their
     corresponding results.
     Applications will typically use a state machine or a FIFO queue for this.
    </para>

   </sect3>

   <sect3 id="libpq-pipeline-errors">
    <title>Error Handling</title>

    <para>
     From the client's perspective, after <function>PQresultStatus</function>
     returns <literal>PGRES_FATAL_ERROR</literal>,
     the pipeline is flagged as aborted.
     <function>PQresultStatus</function> will report a
     <literal>PGRES_PIPELINE_ABORTED</literal> result for each remaining queued
     operation in an aborted pipeline. The result for
     <function>PQpipelineSync</function> is reported as
     <literal>PGRES_PIPELINE_SYNC</literal> to signal the end of the aborted pipeline
     and resumption of normal result processing.
    </para>

    <para>
     The client <emphasis>must</emphasis> process results with
     <function>PQgetResult</function> during error recovery.
    </para>

    <para>
     If the pipeline used an implicit transaction, then operations that have
     already executed are rolled back and operations that were queued to follow
     the failed operation are skipped entirely. The same behavior holds if the
     pipeline starts and commits a single explicit transaction (i.e. the first
     statement is <literal>BEGIN</literal> and the last is
     <literal>COMMIT</literal>) except that the session remains in an aborted
     transaction state at the end of the pipeline. If a pipeline contains
     <emphasis>multiple explicit transactions</emphasis>, all transactions that
     committed prior to the error remain committed, the currently in-progress
     transaction is aborted, and all subsequent operations are skipped completely,
     including subsequent transactions.  If a pipeline synchronization point
     occurs with an explicit transaction block in aborted state, the next pipeline
     will become aborted immediately unless the next command puts the transaction
     in normal mode with <command>ROLLBACK</command>.
    </para>

    <note>
     <para>
      The client must not assume that work is committed when it
      <emphasis>sends</emphasis> a <literal>COMMIT</literal> &mdash; only when the
      corresponding result is received to confirm the commit is complete.
      Because errors arrive asynchronously, the application needs to be able to
      restart from the last <emphasis>received</emphasis> committed change and
      resend work done after that point if something goes wrong.
     </para>
    </note>
   </sect3>

   <sect3 id="libpq-pipeline-interleave">
    <title>Interleaving Result Processing and Query Dispatch</title>

    <para>
     To avoid deadlocks on large pipelines the client should be structured
     around a non-blocking event loop using operating system facilities
     such as <function>select</function>, <function>poll</function>,
     <function>WaitForMultipleObjectEx</function>, etc.
    </para>

    <para>
     The client application should generally maintain a queue of work
     remaining to be dispatched and a queue of work that has been dispatched
     but not yet had its results processed. When the socket is writable
     it should dispatch more work. When the socket is readable it should
     read results and process them, matching them up to the next entry in
     its corresponding results queue.  Based on available memory, results from the
     socket should be read frequently: there's no need to wait until the
     pipeline end to read the results.  Pipelines should be scoped to logical
     units of work, usually (but not necessarily) one transaction per pipeline.
     There's no need to exit pipeline mode and re-enter it between pipelines,
     or to wait for one pipeline to finish before sending the next.
    </para>

    <para>
     An example using <function>select()</function> and a simple state
     machine to track sent and received work is in
     <filename>src/test/modules/libpq_pipeline/libpq_pipeline.c</filename>
     in the PostgreSQL source distribution.
    </para>
   </sect3>
  </sect2>

  <sect2 id="libpq-pipeline-functions">
   <title>Functions Associated with Pipeline Mode</title>

   <variablelist>

    <varlistentry id="libpq-PQpipelineStatus">
     <term><function>PQpipelineStatus</function><indexterm><primary>PQpipelineStatus</primary></indexterm></term>

     <listitem>
      <para>
      Returns the current pipeline mode status of the
      <application>libpq</application> connection.
<synopsis>
PGpipelineStatus PQpipelineStatus(const PGconn *conn);
</synopsis>
      </para>

      <para>
       <function>PQpipelineStatus</function> can return one of the following values:

       <variablelist>
        <varlistentry>
         <term>
          <literal>PQ_PIPELINE_ON</literal>
         </term>
         <listitem>
          <para>
           The <application>libpq</application> connection is in
           pipeline mode.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term>
          <literal>PQ_PIPELINE_OFF</literal>
         </term>
         <listitem>
          <para>
           The <application>libpq</application> connection is
           <emphasis>not</emphasis> in pipeline mode.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term>
          <literal>PQ_PIPELINE_ABORTED</literal>
         </term>
         <listitem>
          <para>
           The <application>libpq</application> connection is in pipeline
           mode and an error occurred while processing the current pipeline.
           The aborted flag is cleared when <function>PQgetResult</function>
           returns a result of type <literal>PGRES_PIPELINE_SYNC</literal>.
          </para>
         </listitem>
        </varlistentry>

       </variablelist>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQenterPipelineMode">
     <term><function>PQenterPipelineMode</function><indexterm><primary>PQenterPipelineMode</primary></indexterm></term>

     <listitem>
      <para>
      Causes a connection to enter pipeline mode if it is currently idle or
      already in pipeline mode.

<synopsis>
int PQenterPipelineMode(PGconn *conn);
</synopsis>

      </para>
      <para>
       Returns 1 for success.
       Returns 0 and has no effect if the connection is not currently
       idle, i.e., it has a result ready, or it is waiting for more
       input from the server, etc.
       This function does not actually send anything to the server,
       it just changes the <application>libpq</application> connection
       state.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQexitPipelineMode">
     <term><function>PQexitPipelineMode</function><indexterm><primary>PQexitPipelineMode</primary></indexterm></term>

     <listitem>
      <para>
       Causes a connection to exit pipeline mode if it is currently in pipeline mode
       with an empty queue and no pending results.
<synopsis>
int PQexitPipelineMode(PGconn *conn);
</synopsis>
      </para>
      <para>
       Returns 1 for success.  Returns 1 and takes no action if not in
       pipeline mode. If the current statement isn't finished processing,
       or <function>PQgetResult</function> has not been called to collect
       results from all previously sent query, returns 0 (in which case,
       use <xref linkend="libpq-PQerrorMessage"/> to get more information
       about the failure).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQpipelineSync">
     <term><function>PQpipelineSync</function><indexterm><primary>PQpipelineSync</primary></indexterm></term>

     <listitem>
      <para>
       Marks a synchronization point in a pipeline by sending a
       <link linkend="protocol-flow-ext-query">sync message</link>
       and flushing the send buffer. This serves as
       the delimiter of an implicit transaction and an error recovery
       point; see <xref linkend="libpq-pipeline-errors"/>.

<synopsis>
int PQpipelineSync(PGconn *conn);
</synopsis>
      </para>
      <para>
       Returns 1 for success. Returns 0 if the connection is not in
       pipeline mode or sending a
       <link linkend="protocol-flow-ext-query">sync message</link>
       failed.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsendFlushRequest">
     <term><function>PQsendFlushRequest</function><indexterm><primary>PQsendFlushRequest</primary></indexterm></term>

      <listitem>
       <para>
        Sends a request for the server to flush its output buffer.
<synopsis>
int PQsendFlushRequest(PGconn *conn);
</synopsis>
       </para>

       <para>
        Returns 1 for success.  Returns 0 on any failure.
       </para>
       <para>
        The server flushes its output buffer automatically as a result of
        <function>PQpipelineSync</function> being called, or
        on any request when not in pipeline mode; this function is useful
        to cause the server to flush its output buffer in pipeline mode
        without establishing a synchronization point.
        Note that the request is not itself flushed to the server automatically;
        use <function>PQflush</function> if necessary.
       </para>
      </listitem>
     </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-pipeline-tips">
   <title>When to Use Pipeline Mode</title>

   <para>
    Much like asynchronous query mode, there is no meaningful performance
    overhead when using pipeline mode. It increases client application complexity,
    and extra caution is required to prevent client/server deadlocks, but
    pipeline mode can offer considerable performance improvements, in exchange for
    increased memory usage from leaving state around longer.
   </para>

   <para>
    Pipeline mode is most useful when the server is distant, i.e., network latency
    (<quote>ping time</quote>) is high, and also when many small operations
    are being performed in rapid succession.  There is usually less benefit
    in using pipelined commands when each query takes many multiples of the client/server
    round-trip time to execute.  A 100-statement operation run on a server
    300 ms round-trip-time away would take 30 seconds in network latency alone
    without pipelining; with pipelining it may spend as little as 0.3 s waiting for
    results from the server.
   </para>

   <para>
    Use pipelined commands when your application does lots of small
    <literal>INSERT</literal>, <literal>UPDATE</literal> and
    <literal>DELETE</literal> operations that can't easily be transformed
    into operations on sets, or into a <literal>COPY</literal> operation.
   </para>

   <para>
    Pipeline mode is not useful when information from one operation is required by
    the client to produce the next operation. In such cases, the client
    would have to introduce a synchronization point and wait for a full client/server
    round-trip to get the results it needs. However, it's often possible to
    adjust the client design to exchange the required information server-side.
    Read-modify-write cycles are especially good candidates; for example:
<programlisting>
BEGIN;
SELECT x FROM mytable WHERE id = 42 FOR UPDATE;
-- result: x=2
-- client adds 1 to x:
UPDATE mytable SET x = 3 WHERE id = 42;
COMMIT;
</programlisting>
    could be much more efficiently done with:
<programlisting>
UPDATE mytable SET x = x + 1 WHERE id = 42;
</programlisting>
   </para>

   <para>
    Pipelining is less useful, and more complex, when a single pipeline contains
    multiple transactions (see <xref linkend="libpq-pipeline-errors"/>).
   </para>
  </sect2>
 </sect1>

 <sect1 id="libpq-single-row-mode">
  <title>Retrieving Query Results Row-by-Row</title>

  <indexterm zone="libpq-single-row-mode">
   <primary>libpq</primary>
   <secondary>single-row mode</secondary>
  </indexterm>

  <para>
   Ordinarily, <application>libpq</application> collects an SQL command's
   entire result and returns it to the application as a single
   <structname>PGresult</structname>.  This can be unworkable for commands
   that return a large number of rows.  For such cases, applications can use
   <xref linkend="libpq-PQsendQuery"/> and <xref linkend="libpq-PQgetResult"/> in
   <firstterm>single-row mode</firstterm>.  In this mode, the result row(s) are
   returned to the application one at a time, as they are received from the
   server.
  </para>

  <para>
   To enter single-row mode, call <xref linkend="libpq-PQsetSingleRowMode"/>
   immediately after a successful call of <xref linkend="libpq-PQsendQuery"/>
   (or a sibling function).  This mode selection is effective only for the
   currently executing query.  Then call <xref linkend="libpq-PQgetResult"/>
   repeatedly, until it returns null, as documented in <xref
   linkend="libpq-async"/>.  If the query returns any rows, they are returned
   as individual <structname>PGresult</structname> objects, which look like
   normal query results except for having status code
   <literal>PGRES_SINGLE_TUPLE</literal> instead of
   <literal>PGRES_TUPLES_OK</literal>.  After the last row, or immediately if
   the query returns zero rows, a zero-row object with status
   <literal>PGRES_TUPLES_OK</literal> is returned; this is the signal that no
   more rows will arrive.  (But note that it is still necessary to continue
   calling <xref linkend="libpq-PQgetResult"/> until it returns null.)  All of
   these <structname>PGresult</structname> objects will contain the same row
   description data (column names, types, etc.) that an ordinary
   <structname>PGresult</structname> object for the query would have.
   Each object should be freed with <xref linkend="libpq-PQclear"/> as usual.
  </para>

  <para>
   When using pipeline mode, single-row mode needs to be activated for each
   query in the pipeline before retrieving results for that query
   with <function>PQgetResult</function>.
   See <xref linkend="libpq-pipeline-mode"/> for more information.
  </para>

  <para>
   <variablelist>
    <varlistentry id="libpq-PQsetSingleRowMode">
     <term><function>PQsetSingleRowMode</function><indexterm><primary>PQsetSingleRowMode</primary></indexterm></term>

     <listitem>
      <para>
       Select single-row mode for the currently-executing query.

<synopsis>
int PQsetSingleRowMode(PGconn *conn);
</synopsis>
      </para>

      <para>
       This function can only be called immediately after
       <xref linkend="libpq-PQsendQuery"/> or one of its sibling functions,
       before any other operation on the connection such as
       <xref linkend="libpq-PQconsumeInput"/> or
       <xref linkend="libpq-PQgetResult"/>.  If called at the correct time,
       the function activates single-row mode for the current query and
       returns 1.  Otherwise the mode stays unchanged and the function
       returns 0.  In any case, the mode reverts to normal after
       completion of the current query.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <caution>
   <para>
    While processing a query, the server may return some rows and then
    encounter an error, causing the query to be aborted.  Ordinarily,
    <application>libpq</application> discards any such rows and reports only the
    error.  But in single-row mode, those rows will have already been
    returned to the application.  Hence, the application will see some
    <literal>PGRES_SINGLE_TUPLE</literal> <structname>PGresult</structname>
    objects followed by a <literal>PGRES_FATAL_ERROR</literal> object.  For
    proper transactional behavior, the application must be designed to
    discard or undo whatever has been done with the previously-processed
    rows, if the query ultimately fails.
   </para>
  </caution>

 </sect1>

 <sect1 id="libpq-cancel">
  <title>Canceling Queries in Progress</title>

  <indexterm zone="libpq-cancel">
   <primary>canceling</primary>
   <secondary>SQL command</secondary>
  </indexterm>

  <para>
   A client application can request cancellation of a command that is
   still being processed by the server, using the functions described in
   this section.

   <variablelist>
    <varlistentry id="libpq-PQgetCancel">
     <term><function>PQgetCancel</function><indexterm><primary>PQgetCancel</primary></indexterm></term>

     <listitem>
      <para>
       Creates a data structure containing the information needed to cancel
       a command issued through a particular database connection.
<synopsis>
PGcancel *PQgetCancel(PGconn *conn);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQgetCancel"/> creates a
       <structname>PGcancel</structname><indexterm><primary>PGcancel</primary></indexterm> object
       given a <structname>PGconn</structname> connection object.  It will return
       <symbol>NULL</symbol> if the given <parameter>conn</parameter> is <symbol>NULL</symbol> or an invalid
       connection.  The <structname>PGcancel</structname> object is an opaque
       structure that is not meant to be accessed directly by the
       application; it can only be passed to <xref linkend="libpq-PQcancel"/>
       or <xref linkend="libpq-PQfreeCancel"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfreeCancel">
     <term><function>PQfreeCancel</function><indexterm><primary>PQfreeCancel</primary></indexterm></term>

     <listitem>
      <para>
       Frees a data structure created by <xref linkend="libpq-PQgetCancel"/>.
<synopsis>
void PQfreeCancel(PGcancel *cancel);
</synopsis>
      </para>

      <para>
       <xref linkend="libpq-PQfreeCancel"/> frees a data object previously created
       by <xref linkend="libpq-PQgetCancel"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQcancel">
     <term><function>PQcancel</function><indexterm><primary>PQcancel</primary></indexterm></term>

     <listitem>
      <para>
       Requests that the server abandon processing of the current command.
<synopsis>
int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);
</synopsis>
      </para>

      <para>
       The return value is 1 if the cancel request was successfully
       dispatched and 0 if not.  If not, <parameter>errbuf</parameter> is filled
       with an explanatory error message.  <parameter>errbuf</parameter>
       must be a char array of size <parameter>errbufsize</parameter> (the
       recommended size is 256 bytes).
      </para>

      <para>
       Successful dispatch is no guarantee that the request will have
       any effect, however.  If the cancellation is effective, the current
       command will terminate early and return an error result.  If the
       cancellation fails (say, because the server was already done
       processing the command), then there will be no visible result at
       all.
      </para>

      <para>
       <xref linkend="libpq-PQcancel"/> can safely be invoked from a signal
       handler, if the <parameter>errbuf</parameter> is a local variable in the
       signal handler.  The <structname>PGcancel</structname> object is read-only
       as far as <xref linkend="libpq-PQcancel"/> is concerned, so it can
       also be invoked from a thread that is separate from the one
       manipulating the <structname>PGconn</structname> object.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

   <variablelist>
    <varlistentry id="libpq-PQrequestCancel">
     <term><function>PQrequestCancel</function><indexterm><primary>PQrequestCancel</primary></indexterm></term>

     <listitem>
      <para>
       <xref linkend="libpq-PQrequestCancel"/> is a deprecated variant of
       <xref linkend="libpq-PQcancel"/>.
<synopsis>
int PQrequestCancel(PGconn *conn);
</synopsis>
      </para>

      <para>
       Requests that the server abandon processing of the current
       command.  It operates directly on the
       <structname>PGconn</structname> object, and in case of failure stores the
       error message in the <structname>PGconn</structname> object (whence it can
       be retrieved by <xref linkend="libpq-PQerrorMessage"/>).  Although
       the functionality is the same, this approach is not safe within
       multiple-thread programs or signal handlers, since it is possible
       that overwriting the <structname>PGconn</structname>'s error message will
       mess up the operation currently in progress on the connection.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

 </sect1>

 <sect1 id="libpq-fastpath">
  <title>The Fast-Path Interface</title>

  <indexterm zone="libpq-fastpath">
   <primary>fast path</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> provides a fast-path interface
   to send simple function calls to the server.
  </para>

  <tip>
   <para>
    This interface is somewhat obsolete, as one can achieve similar
    performance and greater functionality by setting up a prepared
    statement to define the function call.  Then, executing the statement
    with binary transmission of parameters and results substitutes for a
    fast-path function call.
   </para>
  </tip>

  <para>
   The function <function id="libpq-PQfn">PQfn</function><indexterm><primary>PQfn</primary></indexterm>
   requests execution of a server function via the fast-path interface:
<synopsis>
PGresult *PQfn(PGconn *conn,
               int fnid,
               int *result_buf,
               int *result_len,
               int result_is_int,
               const PQArgBlock *args,
               int nargs);

typedef struct
{
    int len;
    int isint;
    union
    {
        int *ptr;
        int integer;
    } u;
} PQArgBlock;
</synopsis>
  </para>

  <para>
   The <parameter>fnid</parameter> argument is the OID of the function to be
   executed.  <parameter>args</parameter> and <parameter>nargs</parameter> define the
   parameters to be passed to the function; they must match the declared
   function argument list.  When the <parameter>isint</parameter> field of a
   parameter structure is true, the <parameter>u.integer</parameter> value is sent
   to the server as an integer of the indicated length (this must be
   2 or 4 bytes); proper byte-swapping occurs.  When <parameter>isint</parameter>
   is false, the indicated number of bytes at <parameter>*u.ptr</parameter> are
   sent with no processing; the data must be in the format expected by
   the server for binary transmission of the function's argument data
   type.  (The declaration of <parameter>u.ptr</parameter> as being of
   type <type>int *</type> is historical; it would be better to consider
   it <type>void *</type>.)
   <parameter>result_buf</parameter> points to the buffer in which to place
   the function's return value.  The caller must have allocated sufficient
   space to store the return value.  (There is no check!) The actual result
   length in bytes will be returned in the integer pointed to by
   <parameter>result_len</parameter>.  If a 2- or 4-byte integer result
   is expected, set <parameter>result_is_int</parameter> to 1, otherwise
   set it to 0.  Setting <parameter>result_is_int</parameter> to 1 causes
   <application>libpq</application> to byte-swap the value if necessary, so that it
   is delivered as a proper <type>int</type> value for the client machine;
   note that a 4-byte integer is delivered into <parameter>*result_buf</parameter>
   for either allowed result size.
   When <parameter>result_is_int</parameter> is 0, the binary-format byte string
   sent by the server is returned unmodified. (In this case it's better
   to consider <parameter>result_buf</parameter> as being of
   type <type>void *</type>.)
  </para>

  <para>
   <function>PQfn</function> always returns a valid
   <structname>PGresult</structname> pointer, with
   status <literal>PGRES_COMMAND_OK</literal> for success
   or <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
   The result status should be
   checked before the result is used.   The caller is responsible for
   freeing  the  <structname>PGresult</structname>  with
   <xref linkend="libpq-PQclear"/> when it is no longer needed.
  </para>

  <para>
   To pass a NULL argument to the function, set
   the <parameter>len</parameter> field of that parameter structure
   to <literal>-1</literal>; the <parameter>isint</parameter>
   and <parameter>u</parameter> fields are then irrelevant.
  </para>

  <para>
   If the function returns NULL, <parameter>*result_len</parameter> is set
   to <literal>-1</literal>, and <parameter>*result_buf</parameter> is not
   modified.
  </para>

  <para>
   Note that it is not possible to handle set-valued results when using
   this interface.  Also, the function must be a plain function, not an
   aggregate, window function, or procedure.
  </para>

 </sect1>

 <sect1 id="libpq-notify">
  <title>Asynchronous Notification</title>

  <indexterm zone="libpq-notify">
   <primary>NOTIFY</primary>
   <secondary>in libpq</secondary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> offers asynchronous notification
   via the <command>LISTEN</command> and <command>NOTIFY</command>
   commands.  A client session registers its interest in a particular
   notification channel with the <command>LISTEN</command> command (and
   can stop listening with the <command>UNLISTEN</command> command).  All
   sessions listening on a particular channel will be notified
   asynchronously when a <command>NOTIFY</command> command with that
   channel name is executed by any session. A <quote>payload</quote> string can
   be passed to communicate additional data to the listeners.
  </para>

  <para>
   <application>libpq</application> applications submit
   <command>LISTEN</command>, <command>UNLISTEN</command>,
   and <command>NOTIFY</command> commands as
   ordinary SQL commands.  The arrival of <command>NOTIFY</command>
   messages can subsequently be detected by calling
   <function id="libpq-PQnotifies">PQnotifies</function>.<indexterm><primary>PQnotifies</primary></indexterm>
  </para>

  <para>
   The function <function>PQnotifies</function> returns the next notification
   from a list of unhandled notification messages received from the server.
   It returns a null pointer if there are no pending notifications.  Once a
   notification is returned from <function>PQnotifies</function>, it is considered
   handled and will be removed from the list of notifications.

<synopsis>
PGnotify *PQnotifies(PGconn *conn);

typedef struct pgNotify
{
    char *relname;              /* notification channel name */
    int  be_pid;                /* process ID of notifying server process */
    char *extra;                /* notification payload string */
} PGnotify;
</synopsis>

   After processing a <structname>PGnotify</structname> object returned
   by <function>PQnotifies</function>, be sure to free it with
   <xref linkend="libpq-PQfreemem"/>.  It is sufficient to free the
   <structname>PGnotify</structname> pointer; the
   <structfield>relname</structfield> and <structfield>extra</structfield>
   fields do not represent separate allocations.  (The names of these fields
   are historical; in particular, channel names need not have anything to
   do with relation names.)
  </para>

  <para>
   <xref linkend="libpq-example-2"/> gives a sample program that illustrates
   the use of asynchronous notification.
  </para>

  <para>
   <function>PQnotifies</function> does not actually read data from the
   server; it just returns messages previously absorbed by another
   <application>libpq</application> function.  In ancient releases of
   <application>libpq</application>, the only way to ensure timely receipt
   of <command>NOTIFY</command> messages was to constantly submit commands, even
   empty ones, and then check <function>PQnotifies</function> after each
   <xref linkend="libpq-PQexec"/>.  While this still works, it is deprecated
   as a waste of processing power.
  </para>

  <para>
   A better way to check for <command>NOTIFY</command> messages when you have no
   useful commands to execute is to call
   <xref linkend="libpq-PQconsumeInput"/>, then check
   <function>PQnotifies</function>.  You can use
   <function>select()</function> to wait for data to arrive from the
   server, thereby using no <acronym>CPU</acronym> power unless there is
   something to do.  (See <xref linkend="libpq-PQsocket"/> to obtain the file
   descriptor number to use with <function>select()</function>.) Note that
   this will work OK whether you submit commands with
   <xref linkend="libpq-PQsendQuery"/>/<xref linkend="libpq-PQgetResult"/> or
   simply use <xref linkend="libpq-PQexec"/>.  You should, however, remember
   to check <function>PQnotifies</function> after each
   <xref linkend="libpq-PQgetResult"/> or <xref linkend="libpq-PQexec"/>, to
   see if any notifications came in during the processing of the command.
  </para>

 </sect1>

 <sect1 id="libpq-copy">
  <title>Functions Associated with the <command>COPY</command> Command</title>

  <indexterm zone="libpq-copy">
   <primary>COPY</primary>
   <secondary>with libpq</secondary>
  </indexterm>

  <para>
   The <command>COPY</command> command in
   <productname>PostgreSQL</productname> has options to read from or write
   to the network connection used by <application>libpq</application>.
   The functions described in this section allow applications to take
   advantage of this capability by supplying or consuming copied data.
  </para>

  <para>
   The overall process is that the application first issues the SQL
   <command>COPY</command> command via <xref linkend="libpq-PQexec"/> or one
   of the equivalent functions.  The response to this (if there is no
   error in the command) will be a <structname>PGresult</structname> object bearing
   a status code of <literal>PGRES_COPY_OUT</literal> or
   <literal>PGRES_COPY_IN</literal> (depending on the specified copy
   direction).  The application should then use the functions of this
   section to receive or transmit data rows.  When the data transfer is
   complete, another <structname>PGresult</structname> object is returned to indicate
   success or failure of the transfer.  Its status will be
   <literal>PGRES_COMMAND_OK</literal> for success or
   <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
   At this point further SQL commands can be issued via
   <xref linkend="libpq-PQexec"/>.  (It is not possible to execute other SQL
   commands using the same connection while the <command>COPY</command>
   operation is in progress.)
  </para>

  <para>
   If a <command>COPY</command> command is issued via
   <xref linkend="libpq-PQexec"/> in a string that could contain additional
   commands, the application must continue fetching results via
   <xref linkend="libpq-PQgetResult"/> after completing the <command>COPY</command>
   sequence.  Only when <xref linkend="libpq-PQgetResult"/> returns
   <symbol>NULL</symbol> is it certain that the <xref linkend="libpq-PQexec"/>
   command string is done and it is safe to issue more commands.
  </para>

  <para>
   The functions of this section should be executed only after obtaining
   a result status of <literal>PGRES_COPY_OUT</literal> or
   <literal>PGRES_COPY_IN</literal> from <xref linkend="libpq-PQexec"/> or
   <xref linkend="libpq-PQgetResult"/>.
  </para>

  <para>
   A <structname>PGresult</structname> object bearing one of these status values
   carries some additional data about the <command>COPY</command> operation
   that is starting.  This additional data is available using functions
   that are also used in connection with query results:

   <variablelist>
    <varlistentry id="libpq-PQnfields-1">
     <term><function>PQnfields</function><indexterm
     ><primary>PQnfields</primary><secondary>with COPY</secondary></indexterm></term>

     <listitem>
      <para>
       Returns the number of columns (fields) to be copied.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQbinaryTuples-1">
     <term><function>PQbinaryTuples</function><indexterm
     ><primary>PQbinaryTuples</primary><secondary>with COPY</secondary></indexterm></term>

     <listitem>
      <para>
       0 indicates the overall copy format is textual (rows separated by
       newlines, columns separated by separator characters, etc.).  1
       indicates the overall copy format is binary.  See <xref
       linkend="sql-copy"/> for more information.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQfformat-1">
     <term><function>PQfformat</function><indexterm
     ><primary>PQfformat</primary><secondary>with COPY</secondary></indexterm></term>

     <listitem>
      <para>
       Returns the format code (0 for text, 1 for binary) associated with
       each column of the copy operation.  The per-column format codes
       will always be zero when the overall copy format is textual, but
       the binary format can support both text and binary columns.
       (However, as of the current implementation of <command>COPY</command>,
       only binary columns appear in a binary copy; so the per-column
       formats always match the overall format at present.)
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <sect2 id="libpq-copy-send">
   <title>Functions for Sending <command>COPY</command> Data</title>

   <para>
    These functions are used to send data during <literal>COPY FROM
    STDIN</literal>.  They will fail if called when the connection is not in
    <literal>COPY_IN</literal> state.
   </para>

   <variablelist>
    <varlistentry id="libpq-PQputCopyData">
     <term><function>PQputCopyData</function><indexterm><primary>PQputCopyData</primary></indexterm></term>

     <listitem>
      <para>
       Sends data to the server during <literal>COPY_IN</literal> state.
<synopsis>
int PQputCopyData(PGconn *conn,
                  const char *buffer,
                  int nbytes);
</synopsis>
      </para>

      <para>
       Transmits the <command>COPY</command> data in the specified
       <parameter>buffer</parameter>, of length <parameter>nbytes</parameter>, to the server.
       The result is 1 if the data was queued, zero if it was not queued
       because of full buffers (this will only happen in nonblocking mode),
       or -1 if an error occurred.
       (Use <xref linkend="libpq-PQerrorMessage"/> to retrieve details if
       the return value is -1.  If the value is zero, wait for write-ready
       and try again.)
      </para>

      <para>
       The application can divide the <command>COPY</command> data stream
       into buffer loads of any convenient size.  Buffer-load boundaries
       have no semantic significance when sending.  The contents of the
       data stream must match the data format expected by the
       <command>COPY</command> command; see <xref linkend="sql-copy"/> for details.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQputCopyEnd">
     <term><function>PQputCopyEnd</function><indexterm><primary>PQputCopyEnd</primary></indexterm></term>

     <listitem>
      <para>
       Sends end-of-data indication to the server during <literal>COPY_IN</literal> state.
<synopsis>
int PQputCopyEnd(PGconn *conn,
                 const char *errormsg);
</synopsis>
      </para>

      <para>
       Ends the <literal>COPY_IN</literal> operation successfully if
       <parameter>errormsg</parameter> is <symbol>NULL</symbol>.  If
       <parameter>errormsg</parameter> is not <symbol>NULL</symbol> then the
       <command>COPY</command> is forced to fail, with the string pointed to by
       <parameter>errormsg</parameter> used as the error message.  (One should not
       assume that this exact error message will come back from the server,
       however, as the server might have already failed the
       <command>COPY</command> for its own reasons.)
      </para>

      <para>
       The result is 1 if the termination message was sent; or in
       nonblocking mode, this may only indicate that the termination
       message was successfully queued.  (In nonblocking mode, to be
       certain that the data has been sent, you should next wait for
       write-ready and call <xref linkend="libpq-PQflush"/>, repeating until it
       returns zero.)  Zero indicates that the function could not queue
       the termination message because of full buffers; this will only
       happen in nonblocking mode.  (In this case, wait for
       write-ready and try the <xref linkend="libpq-PQputCopyEnd"/> call
       again.)  If a hard error occurs, -1 is returned; you can use
       <xref linkend="libpq-PQerrorMessage"/> to retrieve details.
      </para>

      <para>
       After successfully calling <xref linkend="libpq-PQputCopyEnd"/>, call
       <xref linkend="libpq-PQgetResult"/> to obtain the final result status of the
       <command>COPY</command> command.  One can wait for this result to be
       available in the usual way.  Then return to normal operation.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

  <sect2 id="libpq-copy-receive">
   <title>Functions for Receiving <command>COPY</command> Data</title>

   <para>
    These functions are used to receive data during <literal>COPY TO
    STDOUT</literal>.  They will fail if called when the connection is not in
    <literal>COPY_OUT</literal> state.
   </para>

   <variablelist>
    <varlistentry id="libpq-PQgetCopyData">
     <term><function>PQgetCopyData</function><indexterm><primary>PQgetCopyData</primary></indexterm></term>

     <listitem>
      <para>
       Receives data from the server during <literal>COPY_OUT</literal> state.
<synopsis>
int PQgetCopyData(PGconn *conn,
                  char **buffer,
                  int async);
</synopsis>
      </para>

      <para>
       Attempts to obtain another row of data from the server during a
       <command>COPY</command>.  Data is always returned one data row at
       a time; if only a partial row is available, it is not returned.
       Successful return of a data row involves allocating a chunk of
       memory to hold the data.  The <parameter>buffer</parameter> parameter must
       be non-<symbol>NULL</symbol>.  <parameter>*buffer</parameter> is set to
       point to the allocated memory, or to <symbol>NULL</symbol> in cases
       where no buffer is returned.  A non-<symbol>NULL</symbol> result
       buffer should be freed using <xref linkend="libpq-PQfreemem"/> when no longer
       needed.
      </para>

      <para>
       When a row is successfully returned, the return value is the number
       of data bytes in the row (this will always be greater than zero).
       The returned string is always null-terminated, though this is
       probably only useful for textual <command>COPY</command>.  A result
       of zero indicates that the <command>COPY</command> is still in
       progress, but no row is yet available (this is only possible when
       <parameter>async</parameter> is true).  A result of -1 indicates that the
       <command>COPY</command> is done.  A result of -2 indicates that an
       error occurred (consult <xref linkend="libpq-PQerrorMessage"/> for the reason).
      </para>

      <para>
       When <parameter>async</parameter> is true (not zero),
       <xref linkend="libpq-PQgetCopyData"/> will not block waiting for input; it
       will return zero if the <command>COPY</command> is still in progress
       but no complete row is available.  (In this case wait for read-ready
       and then call <xref linkend="libpq-PQconsumeInput"/> before calling
       <xref linkend="libpq-PQgetCopyData"/> again.)  When <parameter>async</parameter> is
       false (zero), <xref linkend="libpq-PQgetCopyData"/> will block until data is
       available or the operation completes.
      </para>

      <para>
       After <xref linkend="libpq-PQgetCopyData"/> returns -1, call
       <xref linkend="libpq-PQgetResult"/> to obtain the final result status of the
       <command>COPY</command> command.  One can wait for this result to be
       available in the usual way.  Then return to normal operation.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

  <sect2 id="libpq-copy-deprecated">
   <title>Obsolete Functions for <command>COPY</command></title>

   <para>
    These functions represent older methods of handling <command>COPY</command>.
    Although they still work, they are deprecated due to poor error handling,
    inconvenient methods of detecting end-of-data, and lack of support for binary
    or nonblocking transfers.
   </para>

   <variablelist>
    <varlistentry id="libpq-PQgetline">
     <term><function>PQgetline</function><indexterm><primary>PQgetline</primary></indexterm></term>

     <listitem>
      <para>
       Reads  a  newline-terminated  line  of  characters (transmitted
       by the server) into a buffer string of size <parameter>length</parameter>.
<synopsis>
int PQgetline(PGconn *conn,
              char *buffer,
              int length);
</synopsis>
      </para>

      <para>
       This function copies up to <parameter>length</parameter>-1 characters into
       the buffer and converts the terminating newline into a zero byte.
       <xref linkend="libpq-PQgetline"/> returns <symbol>EOF</symbol> at the
       end of input, 0 if the entire line has been read, and 1 if the
       buffer is full but the terminating newline has not yet been read.
       </para>
       <para>
       Note that the application must check to see if a new line consists
       of  the  two characters  <literal>\.</literal>, which  indicates
       that the server has finished sending the results  of  the
       <command>COPY</command> command.  If  the  application might receive
       lines that are more than <parameter>length</parameter>-1  characters  long,
       care is needed to be sure it recognizes the <literal>\.</literal>
       line correctly (and does not, for example, mistake the end of a
       long data line for a terminator line).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQgetlineAsync">
     <term><function>PQgetlineAsync</function><indexterm><primary>PQgetlineAsync</primary></indexterm></term>

     <listitem>
      <para>
       Reads a row of <command>COPY</command> data (transmitted  by the
       server) into a buffer without blocking.
<synopsis>
int PQgetlineAsync(PGconn *conn,
                   char *buffer,
                   int bufsize);
</synopsis>
      </para>

      <para>
       This function is similar to <xref linkend="libpq-PQgetline"/>, but it can be used
       by applications
       that must read <command>COPY</command> data asynchronously, that is, without blocking.
       Having issued the <command>COPY</command> command and gotten a <literal>PGRES_COPY_OUT</literal>
       response, the
       application should call <xref linkend="libpq-PQconsumeInput"/> and
       <xref linkend="libpq-PQgetlineAsync"/> until the
       end-of-data signal is detected.
       </para>
       <para>
       Unlike <xref linkend="libpq-PQgetline"/>, this function takes
       responsibility for detecting end-of-data.
      </para>

      <para>
       On each call, <xref linkend="libpq-PQgetlineAsync"/> will return data if a
       complete data row is available in <application>libpq</application>'s input buffer.
       Otherwise, no data is returned until the rest of the row arrives.
       The function returns -1 if the end-of-copy-data marker has been recognized,
       or 0 if no data is available, or a positive number giving the number of
       bytes of data returned.  If -1 is returned, the caller must next call
       <xref linkend="libpq-PQendcopy"/>, and then return to normal processing.
      </para>

      <para>
       The data returned will not extend beyond a data-row boundary.  If possible
       a whole row will be returned at one time.  But if the buffer offered by
       the caller is too small to hold a row sent by the server, then a partial
       data row will be returned.  With textual data this can be detected by testing
       whether the last returned byte is <literal>\n</literal> or not.  (In a binary
       <command>COPY</command>, actual parsing of the <command>COPY</command> data format will be needed to make the
       equivalent determination.)
       The returned string is not null-terminated.  (If you want to add a
       terminating null, be sure to pass a <parameter>bufsize</parameter> one smaller
       than the room actually available.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQputline">
     <term><function>PQputline</function><indexterm><primary>PQputline</primary></indexterm></term>

     <listitem>
      <para>
       Sends  a  null-terminated  string  to  the server.  Returns 0 if
       OK and <symbol>EOF</symbol> if unable to send the string.
<synopsis>
int PQputline(PGconn *conn,
              const char *string);
</synopsis>
      </para>

      <para>
       The <command>COPY</command> data stream sent by a series of calls
       to <xref linkend="libpq-PQputline"/> has the same format as that
       returned by <xref linkend="libpq-PQgetlineAsync"/>, except that
       applications are not obliged to send exactly one data row per
       <xref linkend="libpq-PQputline"/> call; it is okay to send a partial
       line or multiple lines per call.
      </para>

      <note>
       <para>
        Before <productname>PostgreSQL</productname> protocol 3.0, it was necessary
        for the application to explicitly send the two characters
        <literal>\.</literal> as a final line to indicate to the server that it had
        finished sending <command>COPY</command> data.  While this still works, it is deprecated and the
        special meaning of <literal>\.</literal> can be expected to be removed in a
        future release.  It is sufficient to call <xref linkend="libpq-PQendcopy"/> after
        having sent the actual data.
       </para>
      </note>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQputnbytes">
     <term><function>PQputnbytes</function><indexterm><primary>PQputnbytes</primary></indexterm></term>

     <listitem>
      <para>
       Sends  a  non-null-terminated  string  to  the server.  Returns
       0 if OK and <symbol>EOF</symbol> if unable to send the string.
<synopsis>
int PQputnbytes(PGconn *conn,
                const char *buffer,
                int nbytes);
</synopsis>
      </para>

      <para>
       This is exactly like <xref linkend="libpq-PQputline"/>, except that the data
       buffer need not be null-terminated since the number of bytes to send is
       specified directly.  Use this procedure when sending binary data.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQendcopy">
     <term><function>PQendcopy</function><indexterm><primary>PQendcopy</primary></indexterm></term>

     <listitem>
      <para>
       Synchronizes with the server.
<synopsis>
int PQendcopy(PGconn *conn);
</synopsis>
       This function waits until the  server  has  finished  the copying.
       It should either be issued when the  last  string  has  been sent
       to  the  server using <xref linkend="libpq-PQputline"/> or when the
       last string has been  received  from  the  server using
       <function>PQgetline</function>.  It must be issued or the server
       will get <quote>out of sync</quote> with  the client.   Upon return
       from this function, the server is ready to receive the next SQL
       command.  The return value is 0  on  successful  completion,
       nonzero otherwise.  (Use <xref linkend="libpq-PQerrorMessage"/> to
       retrieve details if the return value is nonzero.)
      </para>

      <para>
       When using <xref linkend="libpq-PQgetResult"/>, the application should
       respond to a <literal>PGRES_COPY_OUT</literal> result by executing
       <xref linkend="libpq-PQgetline"/> repeatedly, followed by
       <xref linkend="libpq-PQendcopy"/> after the terminator line is seen.
       It should then return to the <xref linkend="libpq-PQgetResult"/> loop
       until <xref linkend="libpq-PQgetResult"/> returns a null pointer.
       Similarly a <literal>PGRES_COPY_IN</literal> result is processed
       by a series of <xref linkend="libpq-PQputline"/> calls followed by
       <xref linkend="libpq-PQendcopy"/>, then return to the
       <xref linkend="libpq-PQgetResult"/> loop.  This arrangement will
       ensure that a <command>COPY</command> command embedded in a series
       of <acronym>SQL</acronym> commands will be executed correctly.
      </para>

      <para>
       Older applications are likely to submit a <command>COPY</command>
       via <xref linkend="libpq-PQexec"/> and assume that the transaction
       is done after <xref linkend="libpq-PQendcopy"/>.  This will work
       correctly only if the <command>COPY</command> is the only
       <acronym>SQL</acronym> command in the command string.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

 </sect1>

 <sect1 id="libpq-control">
  <title>Control Functions</title>

  <para>
   These functions control miscellaneous details of <application>libpq</application>'s
   behavior.
  </para>

  <variablelist>
   <varlistentry id="libpq-PQclientEncoding">
    <term><function>PQclientEncoding</function><indexterm><primary>PQclientEncoding</primary></indexterm></term>

    <listitem>
     <para>
      Returns the client encoding.
<synopsis>
int PQclientEncoding(const PGconn *<replaceable>conn</replaceable>);
</synopsis>

      Note that it returns the encoding ID, not a symbolic string
      such as <literal>EUC_JP</literal>. If unsuccessful, it returns -1.
      To convert an encoding ID to an encoding name, you
      can use:

<synopsis>
char *pg_encoding_to_char(int <replaceable>encoding_id</replaceable>);
</synopsis>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQsetClientEncoding">
    <term><function>PQsetClientEncoding</function><indexterm><primary>PQsetClientEncoding</primary></indexterm></term>

    <listitem>
     <para>
      Sets the client encoding.
<synopsis>
int PQsetClientEncoding(PGconn *<replaceable>conn</replaceable>, const char *<replaceable>encoding</replaceable>);
</synopsis>

      <replaceable>conn</replaceable> is a connection to the server,
      and <replaceable>encoding</replaceable> is the encoding you want to
      use. If the function successfully sets the encoding, it returns 0,
      otherwise -1. The current encoding for this connection can be
      determined by using <xref linkend="libpq-PQclientEncoding"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQsetErrorVerbosity">
    <term><function>PQsetErrorVerbosity</function><indexterm><primary>PQsetErrorVerbosity</primary></indexterm></term>

    <listitem>
     <para>
      Determines the verbosity of messages returned by
      <xref linkend="libpq-PQerrorMessage"/> and <xref linkend="libpq-PQresultErrorMessage"/>.
<synopsis>
typedef enum
{
    PQERRORS_TERSE,
    PQERRORS_DEFAULT,
    PQERRORS_VERBOSE,
    PQERRORS_SQLSTATE
} PGVerbosity;

PGVerbosity PQsetErrorVerbosity(PGconn *conn, PGVerbosity verbosity);
</synopsis>

      <xref linkend="libpq-PQsetErrorVerbosity"/> sets the verbosity mode,
      returning the connection's previous setting.
      In <firstterm>TERSE</firstterm> mode, returned messages include
      severity, primary text, and position only; this will normally fit on a
      single line.  The <firstterm>DEFAULT</firstterm> mode produces messages
      that include the above plus any detail, hint, or context fields (these
      might span multiple lines).  The <firstterm>VERBOSE</firstterm> mode
      includes all available fields.  The <firstterm>SQLSTATE</firstterm>
      mode includes only the error severity and the <symbol>SQLSTATE</symbol>
      error code, if one is available (if not, the output is like
      <firstterm>TERSE</firstterm> mode).
     </para>

     <para>
      Changing the verbosity setting does not affect the messages available
      from already-existing <structname>PGresult</structname> objects, only
      subsequently-created ones.
      (But see <xref linkend="libpq-PQresultVerboseErrorMessage"/> if you
      want to print a previous error with a different verbosity.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQsetErrorContextVisibility">
    <term><function>PQsetErrorContextVisibility</function><indexterm><primary>PQsetErrorContextVisibility</primary></indexterm></term>

    <listitem>
     <para>
      Determines the handling of <literal>CONTEXT</literal> fields in messages
      returned by <xref linkend="libpq-PQerrorMessage"/>
      and <xref linkend="libpq-PQresultErrorMessage"/>.
<synopsis>
typedef enum
{
    PQSHOW_CONTEXT_NEVER,
    PQSHOW_CONTEXT_ERRORS,
    PQSHOW_CONTEXT_ALWAYS
} PGContextVisibility;

PGContextVisibility PQsetErrorContextVisibility(PGconn *conn, PGContextVisibility show_context);
</synopsis>

      <xref linkend="libpq-PQsetErrorContextVisibility"/> sets the context display mode,
      returning the connection's previous setting.  This mode controls
      whether the <literal>CONTEXT</literal> field is included in messages.
      The <firstterm>NEVER</firstterm> mode
      never includes <literal>CONTEXT</literal>, while <firstterm>ALWAYS</firstterm> always
      includes it if available.  In <firstterm>ERRORS</firstterm> mode (the
      default), <literal>CONTEXT</literal> fields are included only in error
      messages, not in notices and warnings.
      (However, if the verbosity setting is <firstterm>TERSE</firstterm>
      or <firstterm>SQLSTATE</firstterm>, <literal>CONTEXT</literal> fields
      are omitted regardless of the context display mode.)
     </para>

     <para>
      Changing this mode does not
      affect the messages available from
      already-existing <structname>PGresult</structname> objects, only
      subsequently-created ones.
      (But see <xref linkend="libpq-PQresultVerboseErrorMessage"/> if you
      want to print a previous error with a different display mode.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQtrace">
    <term><function>PQtrace</function><indexterm><primary>PQtrace</primary></indexterm></term>

    <listitem>
     <para>
      Enables tracing of the client/server communication to a debugging file
      stream.
<synopsis>
void PQtrace(PGconn *conn, FILE *stream);
</synopsis>
     </para>

     <para>
      Each line consists of: an optional timestamp, a direction indicator
      (<literal>F</literal> for messages from client to server
      or <literal>B</literal> for messages from server to client),
      message length, message type, and message contents.
      Non-message contents fields (timestamp, direction, length and message type)
      are separated by a tab. Message contents are separated by a space.
      Protocol strings are enclosed in double quotes, while strings used as data
      values are enclosed in single quotes.  Non-printable chars are printed as
      hexadecimal escapes.
      Further message-type-specific detail can be found in
      <xref linkend="protocol-message-formats"/>.
     </para>

     <note>
      <para>
       On Windows, if the <application>libpq</application> library and an application are
       compiled with different flags, this function call will crash the
       application because the internal representation of the <literal>FILE</literal>
       pointers differ.  Specifically, multithreaded/single-threaded,
       release/debug, and static/dynamic flags should be the same for the
       library and all applications using that library.
      </para>
     </note>

    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQsetTraceFlags">
    <term><function>PQsetTraceFlags</function><indexterm><primary>PQsetTraceFlags</primary></indexterm></term>

    <listitem>
     <para>
      Controls the tracing behavior of client/server communication.
<synopsis>
void PQsetTraceFlags(PGconn *conn, int flags);
</synopsis>
     </para>

     <para>
      <literal>flags</literal> contains flag bits describing the operating mode
      of tracing.
      If <literal>flags</literal> contains <literal>PQTRACE_SUPPRESS_TIMESTAMPS</literal>,
      then the timestamp is not included when printing each message.
      If <literal>flags</literal> contains <literal>PQTRACE_REGRESS_MODE</literal>,
      then some fields are redacted when printing each message, such as object
      OIDs, to make the output more convenient to use in testing frameworks.
      This function must be called after calling <function>PQtrace</function>.
     </para>

    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQuntrace">
    <term><function>PQuntrace</function><indexterm><primary>PQuntrace</primary></indexterm></term>

    <listitem>
     <para>
      Disables tracing started by <xref linkend="libpq-PQtrace"/>.
<synopsis>
void PQuntrace(PGconn *conn);
</synopsis>
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

 </sect1>

 <sect1 id="libpq-misc">
  <title>Miscellaneous Functions</title>

  <para>
   As always, there are some functions that just don't fit anywhere.
  </para>

  <variablelist>
   <varlistentry id="libpq-PQfreemem">
    <term><function>PQfreemem</function><indexterm><primary>PQfreemem</primary></indexterm></term>

    <listitem>
     <para>
      Frees memory allocated by <application>libpq</application>.
<synopsis>
void PQfreemem(void *ptr);
</synopsis>
     </para>

     <para>
      Frees memory allocated by <application>libpq</application>, particularly
      <xref linkend="libpq-PQescapeByteaConn"/>,
      <xref linkend="libpq-PQescapeBytea"/>,
      <xref linkend="libpq-PQunescapeBytea"/>,
      and <function>PQnotifies</function>.
      It is particularly important that this function, rather than
      <function>free()</function>, be used on Microsoft Windows.  This is because
      allocating memory in a DLL and releasing it in the application works
      only if multithreaded/single-threaded, release/debug, and static/dynamic
      flags are the same for the DLL and the application.  On non-Microsoft
      Windows platforms, this function is the same as the standard library
      function <function>free()</function>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQconninfoFree">
    <term><function>PQconninfoFree</function><indexterm><primary>PQconninfoFree</primary></indexterm></term>

    <listitem>
     <para>
      Frees the data structures allocated by
      <xref linkend="libpq-PQconndefaults"/> or <xref linkend="libpq-PQconninfoParse"/>.
<synopsis>
void PQconninfoFree(PQconninfoOption *connOptions);
</synopsis>
      If the argument is a <symbol>NULL</symbol> pointer, no operation is
      performed.
     </para>

     <para>
      A simple <xref linkend="libpq-PQfreemem"/> will not do for this, since
      the array contains references to subsidiary strings.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQencryptPasswordConn">
    <term><function>PQencryptPasswordConn</function><indexterm><primary>PQencryptPasswordConn</primary></indexterm></term>

    <listitem>
     <para>
      Prepares the encrypted form of a <productname>PostgreSQL</productname> password.
<synopsis>
char *PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user, const char *algorithm);
</synopsis>
      This function is intended to be used by client applications that
      wish to send commands like <literal>ALTER USER joe PASSWORD
      'pwd'</literal>.  It is good practice not to send the original cleartext
      password in such a command, because it might be exposed in command
      logs, activity displays, and so on.  Instead, use this function to
      convert the password to encrypted form before it is sent.
     </para>

     <para>
      The <parameter>passwd</parameter> and <parameter>user</parameter> arguments
      are the cleartext password, and the SQL name of the user it is for.
      <parameter>algorithm</parameter> specifies the encryption algorithm
      to use to encrypt the password. Currently supported algorithms are
      <literal>md5</literal> and <literal>scram-sha-256</literal> (<literal>on</literal> and
      <literal>off</literal> are also accepted as aliases for <literal>md5</literal>, for
      compatibility with older server versions). Note that support for
      <literal>scram-sha-256</literal> was introduced in <productname>PostgreSQL</productname>
      version 10, and will not work correctly with older server versions. If
      <parameter>algorithm</parameter> is <symbol>NULL</symbol>, this function will query
      the server for the current value of the
      <xref linkend="guc-password-encryption"/> setting. That can block, and
      will fail if the current transaction is aborted, or if the connection
      is busy executing another query. If you wish to use the default
      algorithm for the server but want to avoid blocking, query
      <varname>password_encryption</varname> yourself before calling
      <xref linkend="libpq-PQencryptPasswordConn"/>, and pass that value as the
      <parameter>algorithm</parameter>.
     </para>

     <para>
      The return value is a string allocated by <function>malloc</function>.
      The caller can assume the string doesn't contain any special characters
      that would require escaping.  Use <xref linkend="libpq-PQfreemem"/> to free the
      result when done with it. On error, returns <symbol>NULL</symbol>, and
      a suitable message is stored in the connection object.
     </para>

    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQencryptPassword">
    <term><function>PQencryptPassword</function><indexterm><primary>PQencryptPassword</primary></indexterm></term>

    <listitem>
     <para>
      Prepares the md5-encrypted form of a <productname>PostgreSQL</productname> password.
<synopsis>
char *PQencryptPassword(const char *passwd, const char *user);
</synopsis>
      <xref linkend="libpq-PQencryptPassword"/> is an older, deprecated version of
      <xref linkend="libpq-PQencryptPasswordConn"/>. The difference is that
      <xref linkend="libpq-PQencryptPassword"/> does not
      require a connection object, and <literal>md5</literal> is always used as the
      encryption algorithm.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQmakeEmptyPGresult">
    <term><function>PQmakeEmptyPGresult</function><indexterm><primary>PQmakeEmptyPGresult</primary></indexterm></term>

    <listitem>
     <para>
      Constructs an empty <structname>PGresult</structname> object with the given status.
<synopsis>
PGresult *PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
</synopsis>
     </para>

     <para>
      This is <application>libpq</application>'s internal function to allocate and
      initialize an empty <structname>PGresult</structname> object.  This
      function returns <symbol>NULL</symbol> if memory could not be allocated. It is
      exported because some applications find it useful to generate result
      objects (particularly objects with error status) themselves.  If
      <parameter>conn</parameter> is not null and <parameter>status</parameter>
      indicates an error, the current error message of the specified
      connection is copied into the <structname>PGresult</structname>.
      Also, if <parameter>conn</parameter> is not null, any event procedures
      registered in the connection are copied into the
      <structname>PGresult</structname>.  (They do not get
      <literal>PGEVT_RESULTCREATE</literal> calls, but see
      <xref linkend="libpq-PQfireResultCreateEvents"/>.)
      Note that <xref linkend="libpq-PQclear"/> should eventually be called
      on the object, just as with a <structname>PGresult</structname>
      returned by <application>libpq</application> itself.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQfireResultCreateEvents">
    <term><function>PQfireResultCreateEvents</function><indexterm><primary>PQfireResultCreateEvents</primary></indexterm></term>
    <listitem>
     <para>
      Fires a <literal>PGEVT_RESULTCREATE</literal> event (see <xref
      linkend="libpq-events"/>) for each event procedure registered in the
      <structname>PGresult</structname> object.  Returns non-zero for success,
      zero if any event procedure fails.

<synopsis>
int PQfireResultCreateEvents(PGconn *conn, PGresult *res);
</synopsis>
     </para>

     <para>
      The <literal>conn</literal> argument is passed through to event procedures
      but not used directly.  It can be <symbol>NULL</symbol> if the event
      procedures won't use it.
     </para>

     <para>
      Event procedures that have already received a
      <literal>PGEVT_RESULTCREATE</literal> or <literal>PGEVT_RESULTCOPY</literal> event
      for this object are not fired again.
     </para>

     <para>
      The main reason that this function is separate from
      <xref linkend="libpq-PQmakeEmptyPGresult"/> is that it is often appropriate
      to create a <structname>PGresult</structname> and fill it with data
      before invoking the event procedures.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQcopyResult">
    <term><function>PQcopyResult</function><indexterm><primary>PQcopyResult</primary></indexterm></term>

    <listitem>
     <para>
      Makes a copy of a <structname>PGresult</structname> object.  The copy is
      not linked to the source result in any way and
      <xref linkend="libpq-PQclear"/> must be called when the copy is no longer
      needed.  If the function fails, <symbol>NULL</symbol> is returned.

<synopsis>
PGresult *PQcopyResult(const PGresult *src, int flags);
</synopsis>
     </para>

     <para>
      This is not intended to make an exact copy.  The returned result is
      always put into <literal>PGRES_TUPLES_OK</literal> status, and does not
      copy any error message in the source.  (It does copy the command status
      string, however.)  The <parameter>flags</parameter> argument determines
      what else is copied.  It is a bitwise OR of several flags.
      <literal>PG_COPYRES_ATTRS</literal> specifies copying the source
      result's attributes (column definitions).
      <literal>PG_COPYRES_TUPLES</literal> specifies copying the source
      result's tuples.  (This implies copying the attributes, too.)
      <literal>PG_COPYRES_NOTICEHOOKS</literal> specifies
      copying the source result's notify hooks.
      <literal>PG_COPYRES_EVENTS</literal> specifies copying the source
      result's events.  (But any instance data associated with the source
      is not copied.)
      The event procedures receive <literal>PGEVT_RESULTCOPY</literal> events.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQsetResultAttrs">
    <term><function>PQsetResultAttrs</function><indexterm><primary>PQsetResultAttrs</primary></indexterm></term>

    <listitem>
     <para>
      Sets the attributes of a <structname>PGresult</structname> object.
<synopsis>
int PQsetResultAttrs(PGresult *res, int numAttributes, PGresAttDesc *attDescs);
</synopsis>
     </para>

     <para>
      The provided <parameter>attDescs</parameter> are copied into the result.
      If the <parameter>attDescs</parameter> pointer is <symbol>NULL</symbol> or
      <parameter>numAttributes</parameter> is less than one, the request is
      ignored and the function succeeds.  If <parameter>res</parameter>
      already contains attributes, the function will fail.  If the function
      fails, the return value is zero.  If the function succeeds, the return
      value is non-zero.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQsetvalue">
    <term><function>PQsetvalue</function><indexterm><primary>PQsetvalue</primary></indexterm></term>

    <listitem>
     <para>
      Sets a tuple field value of a <structname>PGresult</structname> object.
<synopsis>
int PQsetvalue(PGresult *res, int tup_num, int field_num, char *value, int len);
</synopsis>
     </para>

     <para>
      The function will automatically grow the result's internal tuples array
      as needed.  However, the <parameter>tup_num</parameter> argument must be
      less than or equal to <xref linkend="libpq-PQntuples"/>, meaning this
      function can only grow the tuples array one tuple at a time.  But any
      field of any existing tuple can be modified in any order.  If a value at
      <parameter>field_num</parameter> already exists, it will be overwritten.
      If <parameter>len</parameter> is -1 or
      <parameter>value</parameter> is <symbol>NULL</symbol>, the field value
      will be set to an SQL null value.  The
      <parameter>value</parameter> is copied into the result's private storage,
      thus is no longer needed after the function
      returns.  If the function fails, the return value is zero.  If the
      function succeeds, the return value is non-zero.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQresultAlloc">
    <term><function>PQresultAlloc</function><indexterm><primary>PQresultAlloc</primary></indexterm></term>

    <listitem>
     <para>
      Allocate subsidiary storage for a <structname>PGresult</structname> object.
<synopsis>
void *PQresultAlloc(PGresult *res, size_t nBytes);
</synopsis>
     </para>

     <para>
      Any memory allocated with this function will be freed when
      <parameter>res</parameter> is cleared.  If the function fails,
      the return value is <symbol>NULL</symbol>.  The result is
      guaranteed to be adequately aligned for any type of data,
      just as for <function>malloc</function>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQresultMemorySize">
    <term><function>PQresultMemorySize</function><indexterm><primary>PQresultMemorySize</primary></indexterm></term>

    <listitem>
     <para>
      Retrieves the number of bytes allocated for
      a <structname>PGresult</structname> object.
<synopsis>
size_t PQresultMemorySize(const PGresult *res);
</synopsis>
     </para>

     <para>
      This value is the sum of all <function>malloc</function> requests
      associated with the <structname>PGresult</structname> object, that is,
      all the memory that will be freed by <xref linkend="libpq-PQclear"/>.
      This information can be useful for managing memory consumption.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="libpq-PQlibVersion">
    <term><function>PQlibVersion</function><indexterm
    ><primary>PQlibVersion</primary><seealso>PQserverVersion</seealso></indexterm></term>

    <listitem>
     <para>
      Return the version of <productname>libpq</productname> that is being used.
<synopsis>
int PQlibVersion(void);
</synopsis>
     </para>

     <para>
      The result of this function can be used to determine, at
      run time, whether specific functionality is available in the currently
      loaded version of libpq. The function can be used, for example,
      to determine which connection options are available in
      <xref linkend="libpq-PQconnectdb"/>.
     </para>

     <para>
      The result is formed by multiplying the library's major version
      number by 10000 and adding the minor version number.  For example,
      version 10.1 will be returned as 100001, and version 11.0 will be
      returned as 110000.
     </para>

     <para>
      Prior to major version 10, <productname>PostgreSQL</productname> used
      three-part version numbers in which the first two parts together
      represented the major version.  For those
      versions, <xref linkend="libpq-PQlibVersion"/> uses two digits for each
      part; for example version 9.1.5 will be returned as 90105, and
      version 9.2.0 will be returned as 90200.
     </para>

     <para>
      Therefore, for purposes of determining feature compatibility,
      applications should divide the result of <xref linkend="libpq-PQlibVersion"/>
      by 100 not 10000 to determine a logical major version number.
      In all release series, only the last two digits differ between
      minor releases (bug-fix releases).
     </para>

     <note>
      <para>
       This function appeared in <productname>PostgreSQL</productname> version 9.1, so
       it cannot be used to detect required functionality in earlier
       versions, since calling it will create a link dependency
       on version 9.1 or later.
      </para>
     </note>
    </listitem>
   </varlistentry>

  </variablelist>

 </sect1>

 <sect1 id="libpq-notice-processing">
  <title>Notice Processing</title>

  <indexterm zone="libpq-notice-processing">
   <primary>notice processing</primary>
   <secondary>in libpq</secondary>
  </indexterm>

  <para>
   Notice and warning messages generated by the server are not returned
   by the query execution functions, since they do not imply failure of
   the query.  Instead they are passed to a notice handling function, and
   execution continues normally after the handler returns.  The default
   notice handling function prints the message on
   <filename>stderr</filename>, but the application can override this
   behavior by supplying its own handling function.
  </para>

  <para>
   For historical reasons, there are two levels of notice handling, called
   the notice receiver and notice processor.  The default behavior is for
   the notice receiver to format the notice and pass a string to the notice
   processor for printing.  However, an application that chooses to provide
   its own notice receiver will typically ignore the notice processor
   layer and just do all the work in the notice receiver.
  </para>

  <para>
   The function <function id="libpq-PQsetNoticeReceiver">PQsetNoticeReceiver</function>
   <indexterm><primary>notice receiver</primary></indexterm>
   <indexterm><primary>PQsetNoticeReceiver</primary></indexterm> sets or
   examines the current notice receiver for a connection object.
   Similarly, <function id="libpq-PQsetNoticeProcessor">PQsetNoticeProcessor</function>
   <indexterm><primary>notice processor</primary></indexterm>
   <indexterm><primary>PQsetNoticeProcessor</primary></indexterm> sets or
   examines the current notice processor.

<synopsis>
typedef void (*PQnoticeReceiver) (void *arg, const PGresult *res);

PQnoticeReceiver
PQsetNoticeReceiver(PGconn *conn,
                    PQnoticeReceiver proc,
                    void *arg);

typedef void (*PQnoticeProcessor) (void *arg, const char *message);

PQnoticeProcessor
PQsetNoticeProcessor(PGconn *conn,
                     PQnoticeProcessor proc,
                     void *arg);
</synopsis>

   Each of these functions returns the previous notice receiver or
   processor function pointer, and sets the new value.  If you supply a
   null function pointer, no action is taken, but the current pointer is
   returned.
  </para>

  <para>
   When a notice or warning message is received from the server, or
   generated internally by <application>libpq</application>, the notice
   receiver function is called.  It is passed the message in the form of
   a <symbol>PGRES_NONFATAL_ERROR</symbol>
   <structname>PGresult</structname>.  (This allows the receiver to extract
   individual fields using <xref linkend="libpq-PQresultErrorField"/>, or obtain a
   complete preformatted message using <xref linkend="libpq-PQresultErrorMessage"/>
   or <xref linkend="libpq-PQresultVerboseErrorMessage"/>.)  The same
   void pointer passed to <function>PQsetNoticeReceiver</function> is also
   passed.  (This pointer can be used to access application-specific state
   if needed.)
  </para>

  <para>
   The default notice receiver simply extracts the message (using
   <xref linkend="libpq-PQresultErrorMessage"/>) and passes it to the notice
   processor.
  </para>

  <para>
   The notice processor is responsible for handling a notice or warning
   message given in text form.  It is passed the string text of the message
   (including a trailing newline), plus a void pointer that is the same
   one passed to <function>PQsetNoticeProcessor</function>.  (This pointer
   can be used to access application-specific state if needed.)
  </para>

  <para>
   The default notice processor is simply:
<programlisting>
static void
defaultNoticeProcessor(void *arg, const char *message)
{
    fprintf(stderr, "%s", message);
}
</programlisting>
  </para>

  <para>
   Once you have set a notice receiver or processor, you should expect
   that that function could be called as long as either the
   <structname>PGconn</structname> object or <structname>PGresult</structname> objects made
   from it exist.  At creation of a <structname>PGresult</structname>, the
   <structname>PGconn</structname>'s current notice handling pointers are copied
   into the <structname>PGresult</structname> for possible use by functions like
   <xref linkend="libpq-PQgetvalue"/>.
  </para>

 </sect1>

 <sect1 id="libpq-events">
  <title>Event System</title>

  <para>
   <application>libpq</application>'s event system is designed to notify
   registered event handlers about interesting
   <application>libpq</application> events, such as the creation or
   destruction of <structname>PGconn</structname> and
   <structname>PGresult</structname> objects.  A principal use case is that
   this allows applications to associate their own data with a
   <structname>PGconn</structname> or <structname>PGresult</structname>
   and ensure that that data is freed at an appropriate time.
  </para>

  <para>
   Each registered event handler is associated with two pieces of data,
   known to <application>libpq</application> only as opaque <literal>void *</literal>
   pointers.  There is a <firstterm>pass-through</firstterm> pointer that is provided
   by the application when the event handler is registered with a
   <structname>PGconn</structname>.  The pass-through pointer never changes for the
   life of the <structname>PGconn</structname> and all <structname>PGresult</structname>s
   generated from it; so if used, it must point to long-lived data.
   In addition there is an <firstterm>instance data</firstterm> pointer, which starts
   out <symbol>NULL</symbol> in every <structname>PGconn</structname> and <structname>PGresult</structname>.
   This pointer can be manipulated using the
   <xref linkend="libpq-PQinstanceData"/>,
   <xref linkend="libpq-PQsetInstanceData"/>,
   <xref linkend="libpq-PQresultInstanceData"/> and
   <xref linkend="libpq-PQresultSetInstanceData"/> functions.  Note that
   unlike the pass-through pointer, instance data of a <structname>PGconn</structname>
   is not automatically inherited by <structname>PGresult</structname>s created from
   it.  <application>libpq</application> does not know what pass-through
   and instance data pointers point to (if anything) and will never attempt
   to free them &mdash; that is the responsibility of the event handler.
  </para>

  <sect2 id="libpq-events-types">
   <title>Event Types</title>

   <para>
    The enum <literal>PGEventId</literal> names the types of events handled by
    the event system.  All its values have names beginning with
    <literal>PGEVT</literal>.  For each event type, there is a corresponding
    event info structure that carries the parameters passed to the event
    handlers.  The event types are:
   </para>

   <variablelist>
    <varlistentry id="libpq-pgevt-register">
     <term><literal>PGEVT_REGISTER</literal></term>
     <listitem>
      <para>
       The register event occurs when <xref linkend="libpq-PQregisterEventProc"/>
       is called.  It is the ideal time to initialize any
       <literal>instanceData</literal> an event procedure may need.  Only one
       register event will be fired per event handler per connection.  If the
       event procedure fails (returns zero), the registration is cancelled.

<synopsis>
typedef struct
{
    PGconn *conn;
} PGEventRegister;
</synopsis>

       When a <literal>PGEVT_REGISTER</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventRegister *</structname>.  This structure contains a
       <structname>PGconn</structname> that should be in the
       <literal>CONNECTION_OK</literal> status; guaranteed if one calls
       <xref linkend="libpq-PQregisterEventProc"/> right after obtaining a good
       <structname>PGconn</structname>.  When returning a failure code, all
       cleanup must be performed as no <literal>PGEVT_CONNDESTROY</literal>
       event will be sent.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-pgevt-connreset">
     <term><literal>PGEVT_CONNRESET</literal></term>
     <listitem>
      <para>
       The connection reset event is fired on completion of
       <xref linkend="libpq-PQreset"/> or <function>PQresetPoll</function>.  In
       both cases, the event is only fired if the reset was successful.
       The return value of the event procedure is ignored
       in <productname>PostgreSQL</productname> v15 and later.
       With earlier versions, however, it's important to return success
       (nonzero) or the connection will be aborted.

<synopsis>
typedef struct
{
    PGconn *conn;
} PGEventConnReset;
</synopsis>

       When a <literal>PGEVT_CONNRESET</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventConnReset *</structname>.  Although the contained
       <structname>PGconn</structname> was just reset, all event data remains
       unchanged.  This event should be used to reset/reload/requery any
       associated <literal>instanceData</literal>.  Note that even if the
       event procedure fails to process <literal>PGEVT_CONNRESET</literal>, it will
       still receive a <literal>PGEVT_CONNDESTROY</literal> event when the connection
       is closed.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-pgevt-conndestroy">
     <term><literal>PGEVT_CONNDESTROY</literal></term>
     <listitem>
      <para>
       The connection destroy event is fired in response to
       <xref linkend="libpq-PQfinish"/>.  It is the event procedure's
       responsibility to properly clean up its event data as libpq has no
       ability to manage this memory.  Failure to clean up will lead
       to memory leaks.

<synopsis>
typedef struct
{
    PGconn *conn;
} PGEventConnDestroy;
</synopsis>

       When a <literal>PGEVT_CONNDESTROY</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventConnDestroy *</structname>.  This event is fired
       prior to <xref linkend="libpq-PQfinish"/> performing any other cleanup.
       The return value of the event procedure is ignored since there is no
       way of indicating a failure from <xref linkend="libpq-PQfinish"/>.  Also,
       an event procedure failure should not abort the process of cleaning up
       unwanted memory.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-pgevt-resultcreate">
     <term><literal>PGEVT_RESULTCREATE</literal></term>
     <listitem>
      <para>
       The result creation event is fired in response to any query execution
       function that generates a result, including
       <xref linkend="libpq-PQgetResult"/>.  This event will only be fired after
       the result has been created successfully.

<synopsis>
typedef struct
{
    PGconn *conn;
    PGresult *result;
} PGEventResultCreate;
</synopsis>

       When a <literal>PGEVT_RESULTCREATE</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventResultCreate *</structname>.  The
       <parameter>conn</parameter> is the connection used to generate the
       result.  This is the ideal place to initialize any
       <literal>instanceData</literal> that needs to be associated with the
       result.  If an event procedure fails (returns zero), that event
       procedure will be ignored for the remaining lifetime of the result;
       that is, it will not receive <literal>PGEVT_RESULTCOPY</literal>
       or <literal>PGEVT_RESULTDESTROY</literal> events for this result or
       results copied from it.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-pgevt-resultcopy">
     <term><literal>PGEVT_RESULTCOPY</literal></term>
     <listitem>
      <para>
       The result copy event is fired in response to
       <xref linkend="libpq-PQcopyResult"/>.  This event will only be fired after
       the copy is complete.  Only event procedures that have
       successfully handled the <literal>PGEVT_RESULTCREATE</literal>
       or <literal>PGEVT_RESULTCOPY</literal> event for the source result
       will receive <literal>PGEVT_RESULTCOPY</literal> events.

<synopsis>
typedef struct
{
    const PGresult *src;
    PGresult *dest;
} PGEventResultCopy;
</synopsis>

       When a <literal>PGEVT_RESULTCOPY</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventResultCopy *</structname>.  The
       <parameter>src</parameter> result is what was copied while the
       <parameter>dest</parameter> result is the copy destination.  This event
       can be used to provide a deep copy of <literal>instanceData</literal>,
       since <literal>PQcopyResult</literal> cannot do that.  If an event
       procedure fails (returns zero), that event procedure will be
       ignored for the remaining lifetime of the new result; that is, it
       will not receive <literal>PGEVT_RESULTCOPY</literal>
       or <literal>PGEVT_RESULTDESTROY</literal> events for that result or
       results copied from it.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-pgevt-resultdestroy">
     <term><literal>PGEVT_RESULTDESTROY</literal></term>
     <listitem>
      <para>
       The result destroy event is fired in response to a
       <xref linkend="libpq-PQclear"/>.  It is the event procedure's
       responsibility to properly clean up its event data as libpq has no
       ability to manage this memory.  Failure to clean up will lead
       to memory leaks.

<synopsis>
typedef struct
{
    PGresult *result;
} PGEventResultDestroy;
</synopsis>

       When a <literal>PGEVT_RESULTDESTROY</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventResultDestroy *</structname>.  This event is fired
       prior to <xref linkend="libpq-PQclear"/> performing any other cleanup.
       The return value of the event procedure is ignored since there is no
       way of indicating a failure from <xref linkend="libpq-PQclear"/>.  Also,
       an event procedure failure should not abort the process of cleaning up
       unwanted memory.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-events-proc">
   <title>Event Callback Procedure</title>

   <variablelist>
    <varlistentry id="libpq-PGEventProc">
     <term><literal>PGEventProc</literal><indexterm><primary>PGEventProc</primary></indexterm></term>

     <listitem>
      <para>
       <literal>PGEventProc</literal> is a typedef for a pointer to an
       event procedure, that is, the user callback function that receives
       events from libpq.  The signature of an event procedure must be

<synopsis>
int eventproc(PGEventId evtId, void *evtInfo, void *passThrough)
</synopsis>

       The <parameter>evtId</parameter> parameter indicates which
       <literal>PGEVT</literal> event occurred.  The
       <parameter>evtInfo</parameter> pointer must be cast to the appropriate
       structure type to obtain further information about the event.
       The <parameter>passThrough</parameter> parameter is the pointer
       provided to <xref linkend="libpq-PQregisterEventProc"/> when the event
       procedure was registered.  The function should return a non-zero value
       if it succeeds and zero if it fails.
      </para>

      <para>
       A particular event procedure can be registered only once in any
       <structname>PGconn</structname>.  This is because the address of the procedure
       is used as a lookup key to identify the associated instance data.
      </para>

      <caution>
       <para>
        On Windows, functions can have two different addresses: one visible
        from outside a DLL and another visible from inside the DLL.  One
        should be careful that only one of these addresses is used with
        <application>libpq</application>'s event-procedure functions, else confusion will
        result.  The simplest rule for writing code that will work is to
        ensure that event procedures are declared <literal>static</literal>.  If the
        procedure's address must be available outside its own source file,
        expose a separate function to return the address.
       </para>
      </caution>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-events-funcs">
   <title>Event Support Functions</title>

    <variablelist>
    <varlistentry id="libpq-PQregisterEventProc">
     <term><function>PQregisterEventProc</function><indexterm><primary>PQregisterEventProc</primary></indexterm></term>

     <listitem>
      <para>
       Registers an event callback procedure with libpq.

<synopsis>
int PQregisterEventProc(PGconn *conn, PGEventProc proc,
                        const char *name, void *passThrough);
</synopsis>
      </para>

      <para>
       An event procedure must be registered once on each
       <structname>PGconn</structname> you want to receive events about.  There is no
       limit, other than memory, on the number of event procedures that
       can be registered with a connection.  The function returns a non-zero
       value if it succeeds and zero if it fails.
      </para>

      <para>
       The <parameter>proc</parameter> argument will be called when a libpq
       event is fired.  Its memory address is also used to lookup
       <literal>instanceData</literal>.  The <parameter>name</parameter>
       argument is used to refer to the event procedure in error messages.
       This value cannot be <symbol>NULL</symbol> or a zero-length string.  The name string is
       copied into the <structname>PGconn</structname>, so what is passed need not be
       long-lived.  The <parameter>passThrough</parameter> pointer is passed
       to the <parameter>proc</parameter> whenever an event occurs. This
       argument can be <symbol>NULL</symbol>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsetInstanceData">
     <term><function>PQsetInstanceData</function><indexterm><primary>PQsetInstanceData</primary></indexterm></term>
     <listitem>
      <para>
       Sets the connection <parameter>conn</parameter>'s <literal>instanceData</literal>
       for procedure <parameter>proc</parameter> to <parameter>data</parameter>.  This
       returns non-zero for success and zero for failure.  (Failure is
       only possible if <parameter>proc</parameter> has not been properly
       registered in <parameter>conn</parameter>.)

<synopsis>
int PQsetInstanceData(PGconn *conn, PGEventProc proc, void *data);
</synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQinstanceData">
     <term><function>PQinstanceData</function><indexterm><primary>PQinstanceData</primary></indexterm></term>
     <listitem>
      <para>
       Returns the
       connection <parameter>conn</parameter>'s <literal>instanceData</literal>
       associated with procedure <parameter>proc</parameter>,
       or <symbol>NULL</symbol> if there is none.

<synopsis>
void *PQinstanceData(const PGconn *conn, PGEventProc proc);
</synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQresultSetInstanceData">
     <term><function>PQresultSetInstanceData</function><indexterm><primary>PQresultSetInstanceData</primary></indexterm></term>
     <listitem>
      <para>
       Sets the result's <literal>instanceData</literal>
       for <parameter>proc</parameter> to <parameter>data</parameter>.  This returns
       non-zero for success and zero for failure.  (Failure is only
       possible if <parameter>proc</parameter> has not been properly registered
       in the result.)

<synopsis>
int PQresultSetInstanceData(PGresult *res, PGEventProc proc, void *data);
</synopsis>
      </para>

      <para>
       Beware that any storage represented by <parameter>data</parameter>
       will not be accounted for by <xref linkend="libpq-PQresultMemorySize"/>,
       unless it is allocated using <xref linkend="libpq-PQresultAlloc"/>.
       (Doing so is recommendable because it eliminates the need to free
       such storage explicitly when the result is destroyed.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQresultInstanceData">
     <term><function>PQresultInstanceData</function><indexterm><primary>PQresultInstanceData</primary></indexterm></term>
     <listitem>
      <para>
       Returns the result's <literal>instanceData</literal> associated with <parameter>proc</parameter>, or <symbol>NULL</symbol>
       if there is none.

<synopsis>
void *PQresultInstanceData(const PGresult *res, PGEventProc proc);
</synopsis>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-events-example">
   <title>Event Example</title>

   <para>
    Here is a skeleton example of managing private data associated with
    libpq connections and results.
   </para>

<programlisting>
<![CDATA[
/* required header for libpq events (note: includes libpq-fe.h) */
#include <libpq-events.h>

/* The instanceData */
typedef struct
{
    int n;
    char *str;
} mydata;

/* PGEventProc */
static int myEventProc(PGEventId evtId, void *evtInfo, void *passThrough);

int
main(void)
{
    mydata *data;
    PGresult *res;
    PGconn *conn =
        PQconnectdb("dbname=postgres options=-csearch_path=");

    if (PQstatus(conn) != CONNECTION_OK)
    {
        /* PQerrorMessage's result includes a trailing newline */
        fprintf(stderr, "%s", PQerrorMessage(conn));
        PQfinish(conn);
        return 1;
    }

    /* called once on any connection that should receive events.
     * Sends a PGEVT_REGISTER to myEventProc.
     */
    if (!PQregisterEventProc(conn, myEventProc, "mydata_proc", NULL))
    {
        fprintf(stderr, "Cannot register PGEventProc\n");
        PQfinish(conn);
        return 1;
    }

    /* conn instanceData is available */
    data = PQinstanceData(conn, myEventProc);

    /* Sends a PGEVT_RESULTCREATE to myEventProc */
    res = PQexec(conn, "SELECT 1 + 1");

    /* result instanceData is available */
    data = PQresultInstanceData(res, myEventProc);

    /* If PG_COPYRES_EVENTS is used, sends a PGEVT_RESULTCOPY to myEventProc */
    res_copy = PQcopyResult(res, PG_COPYRES_TUPLES | PG_COPYRES_EVENTS);

    /* result instanceData is available if PG_COPYRES_EVENTS was
     * used during the PQcopyResult call.
     */
    data = PQresultInstanceData(res_copy, myEventProc);

    /* Both clears send a PGEVT_RESULTDESTROY to myEventProc */
    PQclear(res);
    PQclear(res_copy);

    /* Sends a PGEVT_CONNDESTROY to myEventProc */
    PQfinish(conn);

    return 0;
}

static int
myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
{
    switch (evtId)
    {
        case PGEVT_REGISTER:
        {
            PGEventRegister *e = (PGEventRegister *)evtInfo;
            mydata *data = get_mydata(e->conn);

            /* associate app specific data with connection */
            PQsetInstanceData(e->conn, myEventProc, data);
            break;
        }

        case PGEVT_CONNRESET:
        {
            PGEventConnReset *e = (PGEventConnReset *)evtInfo;
            mydata *data = PQinstanceData(e->conn, myEventProc);

            if (data)
              memset(data, 0, sizeof(mydata));
            break;
        }

        case PGEVT_CONNDESTROY:
        {
            PGEventConnDestroy *e = (PGEventConnDestroy *)evtInfo;
            mydata *data = PQinstanceData(e->conn, myEventProc);

            /* free instance data because the conn is being destroyed */
            if (data)
              free_mydata(data);
            break;
        }

        case PGEVT_RESULTCREATE:
        {
            PGEventResultCreate *e = (PGEventResultCreate *)evtInfo;
            mydata *conn_data = PQinstanceData(e->conn, myEventProc);
            mydata *res_data = dup_mydata(conn_data);

            /* associate app specific data with result (copy it from conn) */
            PQresultSetInstanceData(e->result, myEventProc, res_data);
            break;
        }

        case PGEVT_RESULTCOPY:
        {
            PGEventResultCopy *e = (PGEventResultCopy *)evtInfo;
            mydata *src_data = PQresultInstanceData(e->src, myEventProc);
            mydata *dest_data = dup_mydata(src_data);

            /* associate app specific data with result (copy it from a result) */
            PQresultSetInstanceData(e->dest, myEventProc, dest_data);
            break;
        }

        case PGEVT_RESULTDESTROY:
        {
            PGEventResultDestroy *e = (PGEventResultDestroy *)evtInfo;
            mydata *data = PQresultInstanceData(e->result, myEventProc);

            /* free instance data because the result is being destroyed */
            if (data)
              free_mydata(data);
            break;
        }

        /* unknown event ID, just return true. */
        default:
            break;
    }

    return true; /* event processing succeeded */
}
]]>
</programlisting>
  </sect2>
 </sect1>

 <sect1 id="libpq-envars">
  <title>Environment Variables</title>

  <indexterm zone="libpq-envars">
   <primary>environment variable</primary>
  </indexterm>

  <para>
   The following environment variables can be used to select default
   connection parameter values, which will be used by
   <xref linkend="libpq-PQconnectdb"/>, <xref linkend="libpq-PQsetdbLogin"/> and
   <xref linkend="libpq-PQsetdb"/> if no value is directly specified by the calling
   code.  These are useful to avoid hard-coding database connection
   information into simple client applications, for example.

   <itemizedlist>
    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGHOST</envar></primary>
      </indexterm>
      <envar>PGHOST</envar> behaves the same as the <xref
      linkend="libpq-connect-host"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGHOSTADDR</envar></primary>
      </indexterm>
      <envar>PGHOSTADDR</envar> behaves the same as the <xref
      linkend="libpq-connect-hostaddr"/> connection parameter.
      This can be set instead of or in addition to <envar>PGHOST</envar>
      to avoid DNS lookup overhead.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGPORT</envar></primary>
      </indexterm>
      <envar>PGPORT</envar> behaves the same as the <xref
      linkend="libpq-connect-port"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGDATABASE</envar></primary>
      </indexterm>
      <envar>PGDATABASE</envar> behaves the same as the <xref
      linkend="libpq-connect-dbname"/> connection parameter.
      </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGUSER</envar></primary>
      </indexterm>
      <envar>PGUSER</envar> behaves the same as the <xref
      linkend="libpq-connect-user"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGPASSWORD</envar></primary>
      </indexterm>
      <envar>PGPASSWORD</envar> behaves the same as the <xref
      linkend="libpq-connect-password"/> connection parameter.
      Use of this environment variable
      is not recommended for security reasons, as some operating systems
      allow non-root users to see process environment variables via
      <application>ps</application>; instead consider using a password file
      (see <xref linkend="libpq-pgpass"/>).
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGPASSFILE</envar></primary>
      </indexterm>
      <envar>PGPASSFILE</envar> behaves the same as the <xref
      linkend="libpq-connect-passfile"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGREQUIREAUTH</envar></primary>
      </indexterm>
      <envar>PGREQUIREAUTH</envar> behaves the same as the <xref
      linkend="libpq-connect-require-auth"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGCHANNELBINDING</envar></primary>
      </indexterm>
      <envar>PGCHANNELBINDING</envar> behaves the same as the <xref
      linkend="libpq-connect-channel-binding"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSERVICE</envar></primary>
      </indexterm>
      <envar>PGSERVICE</envar> behaves the same as the <xref
      linkend="libpq-connect-service"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSERVICEFILE</envar></primary>
      </indexterm>
      <envar>PGSERVICEFILE</envar> specifies the name of the per-user
      connection service file
      (see <xref linkend="libpq-pgservice"/>).
      Defaults to <filename>~/.pg_service.conf</filename>, or
      <filename>%APPDATA%\postgresql\.pg_service.conf</filename> on
      Microsoft Windows.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGOPTIONS</envar></primary>
      </indexterm>
      <envar>PGOPTIONS</envar> behaves the same as the <xref
      linkend="libpq-connect-options"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGAPPNAME</envar></primary>
      </indexterm>
      <envar>PGAPPNAME</envar> behaves the same as the <xref
      linkend="libpq-connect-application-name"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLMODE</envar></primary>
      </indexterm>
      <envar>PGSSLMODE</envar> behaves the same as the <xref
      linkend="libpq-connect-sslmode"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGREQUIRESSL</envar></primary>
      </indexterm>
      <envar>PGREQUIRESSL</envar> behaves the same as the <xref
      linkend="libpq-connect-requiressl"/> connection parameter.
      This environment variable is deprecated in favor of the
      <envar>PGSSLMODE</envar> variable; setting both variables suppresses the
      effect of this one.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLCOMPRESSION</envar></primary>
      </indexterm>
      <envar>PGSSLCOMPRESSION</envar> behaves the same as the <xref
      linkend="libpq-connect-sslcompression"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLCERT</envar></primary>
      </indexterm>
      <envar>PGSSLCERT</envar> behaves the same as the <xref
      linkend="libpq-connect-sslcert"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLKEY</envar></primary>
      </indexterm>
      <envar>PGSSLKEY</envar> behaves the same as the <xref
      linkend="libpq-connect-sslkey"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLCERTMODE</envar></primary>
      </indexterm>
      <envar>PGSSLCERTMODE</envar> behaves the same as the <xref
      linkend="libpq-connect-sslcertmode"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLROOTCERT</envar></primary>
      </indexterm>
      <envar>PGSSLROOTCERT</envar>  behaves the same as the <xref
      linkend="libpq-connect-sslrootcert"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLCRL</envar></primary>
      </indexterm>
      <envar>PGSSLCRL</envar>  behaves the same as the <xref
      linkend="libpq-connect-sslcrl"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLCRLDIR</envar></primary>
      </indexterm>
      <envar>PGSSLCRLDIR</envar> behaves the same as the <xref
      linkend="libpq-connect-sslcrldir"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLSNI</envar></primary>
      </indexterm>
      <envar>PGSSLSNI</envar>  behaves the same as the <xref
      linkend="libpq-connect-sslsni"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGREQUIREPEER</envar></primary>
      </indexterm>
      <envar>PGREQUIREPEER</envar> behaves the same as the <xref
      linkend="libpq-connect-requirepeer"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLMINPROTOCOLVERSION</envar></primary>
      </indexterm>
      <envar>PGSSLMINPROTOCOLVERSION</envar> behaves the same as the <xref
      linkend="libpq-connect-ssl-min-protocol-version"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLMAXPROTOCOLVERSION</envar></primary>
      </indexterm>
      <envar>PGSSLMAXPROTOCOLVERSION</envar> behaves the same as the <xref
      linkend="libpq-connect-ssl-max-protocol-version"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGGSSENCMODE</envar></primary>
      </indexterm>
      <envar>PGGSSENCMODE</envar> behaves the same as the <xref
      linkend="libpq-connect-gssencmode"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGKRBSRVNAME</envar></primary>
      </indexterm>
      <envar>PGKRBSRVNAME</envar>  behaves the same as the <xref
      linkend="libpq-connect-krbsrvname"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGGSSLIB</envar></primary>
      </indexterm>
      <envar>PGGSSLIB</envar> behaves the same as the <xref
      linkend="libpq-connect-gsslib"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGGSSDELEGATION</envar></primary>
      </indexterm>
      <envar>PGGSSDELEGATION</envar> behaves the same as the <xref
      linkend="libpq-connect-gssdelegation"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGCONNECT_TIMEOUT</envar></primary>
      </indexterm>
      <envar>PGCONNECT_TIMEOUT</envar>  behaves the same as the <xref
      linkend="libpq-connect-connect-timeout"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGCLIENTENCODING</envar></primary>
      </indexterm>
      <envar>PGCLIENTENCODING</envar> behaves the same as the <xref
      linkend="libpq-connect-client-encoding"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGTARGETSESSIONATTRS</envar></primary>
      </indexterm>
      <envar>PGTARGETSESSIONATTRS</envar> behaves the same as the <xref
      linkend="libpq-connect-target-session-attrs"/> connection parameter.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGLOADBALANCEHOSTS</envar></primary>
      </indexterm>
      <envar>PGLOADBALANCEHOSTS</envar> behaves the same as the <xref
      linkend="libpq-connect-load-balance-hosts"/> connection parameter.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   The following environment variables can be used to specify default
   behavior for each <productname>PostgreSQL</productname> session.  (See
   also the <xref linkend="sql-alterrole"/>
   and <xref linkend="sql-alterdatabase"/>
   commands for ways to set default behavior on a per-user or per-database
   basis.)

   <itemizedlist>
    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGDATESTYLE</envar></primary>
      </indexterm>
      <envar>PGDATESTYLE</envar> sets the default style of date/time
      representation.  (Equivalent to <literal>SET datestyle TO
      ...</literal>.)
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGTZ</envar></primary>
      </indexterm>
      <envar>PGTZ</envar> sets the default time zone.  (Equivalent to
      <literal>SET timezone TO ...</literal>.)
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGGEQO</envar></primary>
      </indexterm>
      <envar>PGGEQO</envar> sets the default mode for the genetic query
      optimizer.  (Equivalent to <literal>SET geqo TO ...</literal>.)
     </para>
    </listitem>
   </itemizedlist>

   Refer to the <acronym>SQL</acronym> command <xref linkend="sql-set"/>
   for information on correct values for these
   environment variables.
  </para>

  <para>
   The following environment variables determine internal behavior of
   <application>libpq</application>; they override compiled-in defaults.

   <itemizedlist>
    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSYSCONFDIR</envar></primary>
      </indexterm>
      <envar>PGSYSCONFDIR</envar> sets the directory containing the
      <filename>pg_service.conf</filename> file and in a future version
      possibly other system-wide configuration files.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGLOCALEDIR</envar></primary>
      </indexterm>
      <envar>PGLOCALEDIR</envar> sets the directory containing the
      <literal>locale</literal> files for message localization.
     </para>
    </listitem>
   </itemizedlist>
  </para>

 </sect1>


 <sect1 id="libpq-pgpass">
  <title>The Password File</title>

  <indexterm zone="libpq-pgpass">
   <primary>password file</primary>
  </indexterm>
  <indexterm zone="libpq-pgpass">
   <primary>.pgpass</primary>
  </indexterm>

  <para>
   The file <filename>.pgpass</filename> in a user's home directory can
   contain passwords to
   be used if the connection requires a password (and no password has been
   specified otherwise). On Microsoft Windows the file is named
   <filename>%APPDATA%\postgresql\pgpass.conf</filename> (where
   <filename>%APPDATA%</filename> refers to the Application Data subdirectory in
   the user's profile).
   Alternatively, the password file to use can be specified
   using the connection parameter <xref linkend="libpq-connect-passfile"/>
   or the environment variable <envar>PGPASSFILE</envar>.
  </para>

  <para>
   This file should contain lines of the following format:
<synopsis>
<replaceable>hostname</replaceable>:<replaceable>port</replaceable>:<replaceable>database</replaceable>:<replaceable>username</replaceable>:<replaceable>password</replaceable>
</synopsis>
   (You can add a reminder comment to the file by copying the line above and
   preceding it with <literal>#</literal>.)
   Each of the first four fields can be a literal value, or
   <literal>*</literal>, which matches anything.  The password field from
   the first line that matches the current connection parameters will be
   used.  (Therefore, put more-specific entries first when you are using
   wildcards.) If an entry needs to contain <literal>:</literal> or
   <literal>\</literal>, escape this character with <literal>\</literal>.
   The host name field is matched to the <literal>host</literal> connection
   parameter if that is specified, otherwise to
   the <literal>hostaddr</literal> parameter if that is specified; if neither
   are given then the host name <literal>localhost</literal> is searched for.
   The host name <literal>localhost</literal> is also searched for when
   the connection is a Unix-domain socket connection and
   the <literal>host</literal> parameter
   matches <application>libpq</application>'s default socket directory path.
   In a standby server, a database field of <literal>replication</literal>
   matches streaming replication connections made to the primary server.
   The database field is of limited usefulness otherwise, because users have
   the same password for all databases in the same cluster.
  </para>

  <para>
   On Unix systems, the permissions on a password file must
   disallow any access to world or group; achieve this by a command such as
   <command>chmod 0600 ~/.pgpass</command>.  If the permissions are less
   strict than this, the file will be ignored.  On Microsoft Windows, it
   is assumed that the file is stored in a directory that is secure, so
   no special permissions check is made.
  </para>
 </sect1>


 <sect1 id="libpq-pgservice">
  <title>The Connection Service File</title>

  <indexterm zone="libpq-pgservice">
   <primary>connection service file</primary>
  </indexterm>
  <indexterm zone="libpq-pgservice">
   <primary>pg_service.conf</primary>
  </indexterm>
  <indexterm zone="libpq-pgservice">
   <primary>.pg_service.conf</primary>
  </indexterm>

  <para>
   The connection service file allows libpq connection parameters to be
   associated with a single service name. That service name can then be
   specified in a libpq connection string, and the associated settings will be
   used. This allows connection parameters to be modified without requiring
   a recompile of the libpq-using application. The service name can also be
   specified using the <envar>PGSERVICE</envar> environment variable.
  </para>

  <para>
   Service names can be defined in either a per-user service file or a
   system-wide file.  If the same service name exists in both the user
   and the system file, the user file takes precedence.
   By default, the per-user service file is named
   <filename>~/.pg_service.conf</filename>.
   On Microsoft Windows, it is named
   <filename>%APPDATA%\postgresql\.pg_service.conf</filename> (where
   <filename>%APPDATA%</filename> refers to the Application Data subdirectory
   in the user's profile).  A different file name can be specified by
   setting the environment variable <envar>PGSERVICEFILE</envar>.
   The system-wide file is named <filename>pg_service.conf</filename>.
   By default it is sought in the <filename>etc</filename> directory
   of the <productname>PostgreSQL</productname> installation
   (use <literal>pg_config --sysconfdir</literal> to identify this
   directory precisely).  Another directory, but not a different file
   name, can be specified by setting the environment variable
   <envar>PGSYSCONFDIR</envar>.
  </para>

  <para>
   Either service file uses an <quote>INI file</quote> format where the section
   name is the service name and the parameters are connection
   parameters; see <xref linkend="libpq-paramkeywords"/> for a list.  For
   example:
<programlisting>
# comment
[mydb]
host=somehost
port=5433
user=admin
</programlisting>
   An example file is provided in
   the <productname>PostgreSQL</productname> installation at
   <filename>share/pg_service.conf.sample</filename>.
  </para>

  <para>
   Connection parameters obtained from a service file are combined with
   parameters obtained from other sources.  A service file setting
   overrides the corresponding environment variable, and in turn can be
   overridden by a value given directly in the connection string.
   For example, using the above service file, a connection string
   <literal>service=mydb port=5434</literal> will use
   host <literal>somehost</literal>, port <literal>5434</literal>,
   user <literal>admin</literal>, and other parameters as set by
   environment variables or built-in defaults.
  </para>
 </sect1>


 <sect1 id="libpq-ldap">
  <title>LDAP Lookup of Connection Parameters</title>

  <indexterm zone="libpq-ldap">
   <primary>LDAP connection parameter lookup</primary>
  </indexterm>

  <para>
   If <application>libpq</application> has been compiled with LDAP support (option
   <literal><option>--with-ldap</option></literal> for <command>configure</command>)
   it is possible to retrieve connection options like <literal>host</literal>
   or <literal>dbname</literal> via LDAP from a central server.
   The advantage is that if the connection parameters for a database change,
   the connection information doesn't have to be updated on all client machines.
  </para>

  <para>
   LDAP connection parameter lookup uses the connection service file
   <filename>pg_service.conf</filename> (see <xref
   linkend="libpq-pgservice"/>).  A line in a
   <filename>pg_service.conf</filename> stanza that starts with
   <literal>ldap://</literal> will be recognized as an LDAP URL and an
   LDAP query will be performed. The result must be a list of
   <literal>keyword = value</literal> pairs which will be used to set
   connection options.  The URL must conform to
   <ulink url="https://tools.ietf.org/html/rfc1959">RFC 1959</ulink>
   and be of the form
<synopsis>
ldap://[<replaceable>hostname</replaceable>[:<replaceable>port</replaceable>]]/<replaceable>search_base</replaceable>?<replaceable>attribute</replaceable>?<replaceable>search_scope</replaceable>?<replaceable>filter</replaceable>
</synopsis>
   where <replaceable>hostname</replaceable> defaults to
   <literal>localhost</literal> and <replaceable>port</replaceable>
   defaults to 389.
  </para>

  <para>
   Processing of <filename>pg_service.conf</filename> is terminated after
   a successful LDAP lookup, but is continued if the LDAP server cannot
   be contacted.  This is to provide a fallback with further LDAP URL
   lines that point to different LDAP servers, classical <literal>keyword
   = value</literal> pairs, or default connection options.  If you would
   rather get an error message in this case, add a syntactically incorrect
   line after the LDAP URL.
  </para>

  <para>
   A sample LDAP entry that has been created with the LDIF file
<programlisting>
version:1
dn:cn=mydatabase,dc=mycompany,dc=com
changetype:add
objectclass:top
objectclass:device
cn:mydatabase
description:host=dbserver.mycompany.com
description:port=5439
description:dbname=mydb
description:user=mydb_user
description:sslmode=require
</programlisting>
   might be queried with the following LDAP URL:
<programlisting>
ldap://ldap.mycompany.com/dc=mycompany,dc=com?description?one?(cn=mydatabase)
</programlisting>
  </para>

  <para>
   You can also mix regular service file entries with LDAP lookups.
   A complete example for a stanza in <filename>pg_service.conf</filename>
   would be:
<programlisting>
# only host and port are stored in LDAP, specify dbname and user explicitly
[customerdb]
dbname=customer
user=appuser
ldap://ldap.acme.com/cn=dbserver,cn=hosts?pgconnectinfo?base?(objectclass=*)
</programlisting>
  </para>

 </sect1>


 <sect1 id="libpq-ssl">
  <title>SSL Support</title>

  <indexterm zone="libpq-ssl">
   <primary>SSL</primary>
   <secondary>TLS</secondary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> has native support for using <acronym>SSL</acronym>
   connections to encrypt client/server communications using
   <acronym>TLS</acronym> protocols for increased security.
   See <xref linkend="ssl-tcp"/> for details about the server-side
   <acronym>SSL</acronym> functionality.
  </para>

  <para>
   <application>libpq</application> reads the system-wide
   <productname>OpenSSL</productname> configuration file. By default, this
   file is named <filename>openssl.cnf</filename> and is located in the
   directory reported by <literal>openssl version -d</literal>.  This default
   can be overridden by setting environment variable
   <envar>OPENSSL_CONF</envar> to the name of the desired configuration
   file.
  </para>

 <sect2 id="libq-ssl-certificates">
  <title>Client Verification of Server Certificates</title>

  <para>
   By default, <productname>PostgreSQL</productname> will not perform any verification of
   the server certificate. This means that it is possible to spoof the server
   identity (for example by modifying a DNS record or by taking over the server
   IP address) without the client knowing. In order to prevent spoofing,
   the client must be able to verify the server's identity via a chain of
   trust.  A chain of trust is established by placing a root (self-signed)
   certificate authority (<acronym>CA</acronym>) certificate on one
   computer and a leaf certificate <emphasis>signed</emphasis> by the
   root certificate on another computer.  It is also possible to use an
   <quote>intermediate</quote> certificate which is signed by the root
   certificate and signs leaf certificates.
  </para>

  <para>
   To allow the client to verify the identity of the server, place a root
   certificate on the client and a leaf certificate signed by the root
   certificate on the server.  To allow the server to verify the identity
   of the client, place a root certificate on the server and a leaf
   certificate signed by the root certificate on the client.  One or more
   intermediate certificates (usually stored with the leaf certificate)
   can also be used to link the leaf certificate to the root certificate.
  </para>

  <para>
   Once a chain of trust has been established, there are two ways for
   the client to validate the leaf certificate sent by the server.
   If the parameter <literal>sslmode</literal> is set to <literal>verify-ca</literal>,
   libpq will verify that the server is trustworthy by checking the
   certificate chain up to the root certificate stored on the client.
   If <literal>sslmode</literal> is set to <literal>verify-full</literal>,
   libpq will <emphasis>also</emphasis> verify that the server host
   name matches the name stored in the server certificate. The
   SSL connection will fail if the server certificate cannot be
   verified. <literal>verify-full</literal> is recommended in most
   security-sensitive environments.
  </para>

  <para>
   In <literal>verify-full</literal> mode, the host name is matched against the
   certificate's Subject Alternative Name attribute(s) (SAN), or against the
   Common Name attribute if no SAN of type <literal>dNSName</literal> is
   present.  If the certificate's name attribute starts with an asterisk
   (<literal>*</literal>), the asterisk will be treated as
   a wildcard, which will match all characters <emphasis>except</emphasis> a dot
   (<literal>.</literal>). This means the certificate will not match subdomains.
   If the connection is made using an IP address instead of a host name, the
   IP address will be matched (without doing any DNS lookups) against SANs of
   type <literal>iPAddress</literal> or <literal>dNSName</literal>.  If no
   <literal>iPAddress</literal> SAN is present and no
   matching <literal>dNSName</literal> SAN is present, the host IP address is
   matched against the Common Name attribute.
  </para>

  <note>
   <para>
    For backward compatibility with earlier versions of PostgreSQL, the host
    IP address is verified in a manner different
    from <ulink url="https://tools.ietf.org/html/rfc6125">RFC 6125</ulink>.
    The host IP address is always matched against <literal>dNSName</literal>
    SANs as well as <literal>iPAddress</literal> SANs, and can be matched
    against the Common Name attribute if no relevant SANs exist.
   </para>
  </note>

  <para>
   To allow server certificate verification, one or more root certificates
   must be placed in the file <filename>~/.postgresql/root.crt</filename>
   in the user's home directory.  (On Microsoft Windows the file is named
   <filename>%APPDATA%\postgresql\root.crt</filename>.)  Intermediate
   certificates should also be added to the file if they are needed to link
   the certificate chain sent by the server to the root certificates
   stored on the client.
  </para>

  <para>
   Certificate Revocation List (CRL) entries are also checked
   if the file <filename>~/.postgresql/root.crl</filename> exists
   (<filename>%APPDATA%\postgresql\root.crl</filename> on Microsoft
   Windows).
  </para>

  <para>
   The location of the root certificate file and the CRL can be changed by
   setting
   the connection parameters <literal>sslrootcert</literal> and <literal>sslcrl</literal>
   or the environment variables <envar>PGSSLROOTCERT</envar> and <envar>PGSSLCRL</envar>.
   <literal>sslcrldir</literal> or the environment variable <envar>PGSSLCRLDIR</envar>
   can also be used to specify a directory containing CRL files.
  </para>

  <note>
   <para>
    For backwards compatibility with earlier versions of PostgreSQL, if a
    root CA file exists, the behavior of
    <literal>sslmode</literal>=<literal>require</literal> will be the same
    as that of <literal>verify-ca</literal>, meaning the server certificate
    is validated against the CA. Relying on this behavior is discouraged,
    and applications that need certificate validation should always use
    <literal>verify-ca</literal> or <literal>verify-full</literal>.
   </para>
  </note>
 </sect2>

 <sect2 id="libpq-ssl-clientcert">
  <title>Client Certificates</title>

  <para>
   If the server attempts to verify the identity of the
   client by requesting the client's leaf certificate,
   <application>libpq</application> will send the certificate(s) stored in
   file <filename>~/.postgresql/postgresql.crt</filename> in the user's home
   directory.  The certificates must chain to the root certificate trusted
   by the server.  A matching
   private key file <filename>~/.postgresql/postgresql.key</filename> must also
   be present.
   On Microsoft Windows these files are named
   <filename>%APPDATA%\postgresql\postgresql.crt</filename> and
   <filename>%APPDATA%\postgresql\postgresql.key</filename>.
   The location of the certificate and key files can be overridden by the
   connection parameters <literal>sslcert</literal>
   and <literal>sslkey</literal>, or by the
   environment variables <envar>PGSSLCERT</envar> and <envar>PGSSLKEY</envar>.
  </para>

  <para>
   On Unix systems, the permissions on the private key file must disallow
   any access to world or group; achieve this by a command such as
   <command>chmod 0600 ~/.postgresql/postgresql.key</command>.
   Alternatively, the file can be owned by root and have group read access
   (that is, <literal>0640</literal> permissions).  That setup is intended
   for installations where certificate and key files are managed by the
   operating system.  The user of <application>libpq</application> should
   then be made a member of the group that has access to those certificate
   and key files.  (On Microsoft Windows, there is no file permissions
   check, since the <filename>%APPDATA%\postgresql</filename> directory is
   presumed secure.)
  </para>

  <para>
   The first certificate in <filename>postgresql.crt</filename> must be the
   client's certificate because it must match the client's private key.
   <quote>Intermediate</quote> certificates can be optionally appended
   to the file &mdash; doing so avoids requiring storage of intermediate
   certificates on the server (<xref linkend="guc-ssl-ca-file"/>).
  </para>

  <para>
   The certificate and key may be in PEM or ASN.1 DER format.
  </para>

  <para>
   The key may be
   stored in cleartext or encrypted with a passphrase using any algorithm
   supported by <productname>OpenSSL</productname>, like AES-128. If the key
   is stored encrypted, then the passphrase may be provided in the
   <xref linkend="libpq-connect-sslpassword"/> connection option. If an
   encrypted key is supplied and the <literal>sslpassword</literal> option
   is absent or blank, a password will be prompted for interactively by
   <productname>OpenSSL</productname> with a
   <literal>Enter PEM pass phrase:</literal> prompt if a TTY is available.
   Applications can override the client certificate prompt and the handling
   of the <literal>sslpassword</literal> parameter by supplying their own
   key password callback; see
   <xref linkend="libpq-pqsetsslkeypasshook-openssl"/>.
  </para>

  <para>
   For instructions on creating certificates, see <xref
   linkend="ssl-certificate-creation"/>.
  </para>
 </sect2>

 <sect2 id="libpq-ssl-protection">
  <title>Protection Provided in Different Modes</title>

  <para>
   The different values for the <literal>sslmode</literal> parameter provide different
   levels of protection. SSL can provide
   protection against three types of attacks:

   <variablelist>
    <varlistentry>
     <term>Eavesdropping</term>
     <listitem>
      <para>If a third party can examine the network traffic between the
       client and the server, it can read both connection information (including
       the user name and password) and the data that is passed. <acronym>SSL</acronym>
       uses encryption to prevent this.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>Man-in-the-middle (<acronym>MITM</acronym>)</term>
     <listitem>
      <para>If a third party can modify the data while passing between the
       client and server, it can pretend to be the server and therefore see and
       modify data <emphasis>even if it is encrypted</emphasis>. The third party can then
       forward the connection information and data to the original server,
       making it impossible to detect this attack. Common vectors to do this
       include DNS poisoning and address hijacking, whereby the client is directed
       to a different server than intended. There are also several other
       attack methods that can accomplish this. <acronym>SSL</acronym> uses certificate
       verification to prevent this, by authenticating the server to the client.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>Impersonation</term>
     <listitem>
      <para>If a third party can pretend to be an authorized client, it can
       simply access data it should not have access to. Typically this can
       happen through insecure password management. <acronym>SSL</acronym> uses
       client certificates to prevent this, by making sure that only holders
       of valid certificates can access the server.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   For a connection to be known SSL-secured, SSL usage must be configured
   on <emphasis>both the client and the server</emphasis> before the connection
   is made. If it is only configured on the server, the client may end up
   sending sensitive information (e.g., passwords) before
   it knows that the server requires high security. In libpq, secure
   connections can be ensured
   by setting the <literal>sslmode</literal> parameter to <literal>verify-full</literal> or
   <literal>verify-ca</literal>, and providing the system with a root certificate to
   verify against. This is analogous to using an <literal>https</literal>
   <acronym>URL</acronym> for encrypted web browsing.
  </para>

  <para>
   Once the server has been authenticated, the client can pass sensitive data.
   This means that up until this point, the client does not need to know if
   certificates will be used for authentication, making it safe to specify that
   only in the server configuration.
  </para>

  <para>
   All <acronym>SSL</acronym> options carry overhead in the form of encryption and
   key-exchange, so there is a trade-off that has to be made between performance
   and security. <xref linkend="libpq-ssl-sslmode-statements"/>
   illustrates the risks the different <literal>sslmode</literal> values
   protect against, and what statement they make about security and overhead.
  </para>

  <table id="libpq-ssl-sslmode-statements">
   <title>SSL Mode Descriptions</title>
   <tgroup cols="4">
    <colspec colname="col1" colwidth="1*"/>
    <colspec colname="col2" colwidth="1*"/>
    <colspec colname="col3" colwidth="1*"/>
    <colspec colname="col4" colwidth="2*"/>
    <thead>
     <row>
      <entry><literal>sslmode</literal></entry>
      <entry>Eavesdropping protection</entry>
      <entry><acronym>MITM</acronym> protection</entry>
      <entry>Statement</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><literal>disable</literal></entry>
      <entry>No</entry>
      <entry>No</entry>
      <entry>I don't care about security, and I don't want to pay the overhead
       of encryption.
      </entry>
     </row>

     <row>
      <entry><literal>allow</literal></entry>
      <entry>Maybe</entry>
      <entry>No</entry>
      <entry>I don't care about security, but I will pay the overhead of
       encryption if the server insists on it.
      </entry>
     </row>

     <row>
      <entry><literal>prefer</literal></entry>
      <entry>Maybe</entry>
      <entry>No</entry>
      <entry>I don't care about encryption, but I wish to pay the overhead of
       encryption if the server supports it.
      </entry>
     </row>

     <row>
      <entry><literal>require</literal></entry>
      <entry>Yes</entry>
      <entry>No</entry>
      <entry>I want my data to be encrypted, and I accept the overhead. I trust
       that the network will make sure I always connect to the server I want.
      </entry>
     </row>

     <row>
      <entry><literal>verify-ca</literal></entry>
      <entry>Yes</entry>
      <entry>Depends on CA policy</entry>
      <entry>I want my data encrypted, and I accept the overhead. I want to be
       sure that I connect to a server that I trust.
      </entry>
     </row>

     <row>
      <entry><literal>verify-full</literal></entry>
       <entry>Yes</entry>
       <entry>Yes</entry>
       <entry>I want my data encrypted, and I accept the overhead. I want to be
        sure that I connect to a server I trust, and that it's the one I
        specify.
       </entry>
      </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   The difference between <literal>verify-ca</literal> and <literal>verify-full</literal>
   depends on the policy of the root <acronym>CA</acronym>. If a public
   <acronym>CA</acronym> is used, <literal>verify-ca</literal> allows connections to a server
   that <emphasis>somebody else</emphasis> may have registered with the <acronym>CA</acronym>.
   In this case, <literal>verify-full</literal> should always be used. If
   a local <acronym>CA</acronym> is used, or even a self-signed certificate, using
   <literal>verify-ca</literal> often provides enough protection.
  </para>

  <para>
   The default value for <literal>sslmode</literal> is <literal>prefer</literal>. As is shown
   in the table, this makes no sense from a security point of view, and it only
   promises performance overhead if possible. It is only provided as the default
   for backward compatibility, and is not recommended in secure deployments.
  </para>

 </sect2>

 <sect2 id="libpq-ssl-fileusage">
  <title>SSL Client File Usage</title>

  <para>
   <xref linkend="libpq-ssl-file-usage"/> summarizes the files that are
   relevant to the SSL setup on the client.
  </para>

  <table id="libpq-ssl-file-usage">
   <title>Libpq/Client SSL File Usage</title>
   <tgroup cols="3">
    <thead>
     <row>
      <entry>File</entry>
      <entry>Contents</entry>
      <entry>Effect</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry><filename>~/.postgresql/postgresql.crt</filename></entry>
      <entry>client certificate</entry>
      <entry>sent to server</entry>
     </row>

     <row>
      <entry><filename>~/.postgresql/postgresql.key</filename></entry>
      <entry>client private key</entry>
      <entry>proves client certificate sent by owner; does not indicate
      certificate owner is trustworthy</entry>
     </row>

     <row>
      <entry><filename>~/.postgresql/root.crt</filename></entry>
      <entry>trusted certificate authorities</entry>
      <entry>checks that server certificate is signed by a trusted certificate
      authority</entry>
     </row>

     <row>
      <entry><filename>~/.postgresql/root.crl</filename></entry>
      <entry>certificates revoked by certificate authorities</entry>
      <entry>server certificate must not be on this list</entry>
     </row>

    </tbody>
   </tgroup>
  </table>
 </sect2>

 <sect2 id="libpq-ssl-initialize">
  <title>SSL Library Initialization</title>

  <para>
   If your application initializes <literal>libssl</literal> and/or
   <literal>libcrypto</literal> libraries and <application>libpq</application>
   is built with <acronym>SSL</acronym> support, you should call
   <xref linkend="libpq-PQinitOpenSSL"/> to tell <application>libpq</application>
   that the <literal>libssl</literal> and/or <literal>libcrypto</literal> libraries
   have been initialized by your application, so that
   <application>libpq</application> will not also initialize those libraries.
   However, this is unnecessary when using <productname>OpenSSL</productname>
   version 1.1.0 or later, as duplicate initializations are no longer problematic.
  </para>

  <para>
   <variablelist>
    <varlistentry id="libpq-PQinitOpenSSL">
     <term><function>PQinitOpenSSL</function><indexterm><primary>PQinitOpenSSL</primary></indexterm></term>

     <listitem>
      <para>
       Allows applications to select which security libraries to initialize.
<synopsis>
void PQinitOpenSSL(int do_ssl, int do_crypto);
</synopsis>
      </para>

      <para>
       When <parameter>do_ssl</parameter> is non-zero, <application>libpq</application>
       will initialize the <productname>OpenSSL</productname> library before first
       opening a database connection.  When <parameter>do_crypto</parameter> is
       non-zero, the <literal>libcrypto</literal> library will be initialized.  By
       default (if <xref linkend="libpq-PQinitOpenSSL"/> is not called), both libraries
       are initialized.  When SSL support is not compiled in, this function is
       present but does nothing.
      </para>

      <para>
       If your application uses and initializes either <productname>OpenSSL</productname>
       or its underlying <literal>libcrypto</literal> library, you <emphasis>must</emphasis>
       call this function with zeroes for the appropriate parameter(s)
       before first opening a database connection.  Also be sure that you
       have done that initialization before opening a database connection.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQinitSSL">
     <term><function>PQinitSSL</function><indexterm><primary>PQinitSSL</primary></indexterm></term><listitem>
      <para>
       Allows applications to select which security libraries to initialize.
<synopsis>
void PQinitSSL(int do_ssl);
</synopsis>
      </para>

      <para>
       This function is equivalent to
       <literal>PQinitOpenSSL(do_ssl, do_ssl)</literal>.
       It is sufficient for applications that initialize both or neither
       of <productname>OpenSSL</productname> and <literal>libcrypto</literal>.
      </para>

      <para>
       <xref linkend="libpq-PQinitSSL"/> has been present since
       <productname>PostgreSQL</productname> 8.0, while <xref linkend="libpq-PQinitOpenSSL"/>
       was added in <productname>PostgreSQL</productname> 8.4, so <xref linkend="libpq-PQinitSSL"/>
       might be preferable for applications that need to work with older
       versions of <application>libpq</application>.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </sect2>

 </sect1>


 <sect1 id="libpq-threading">
  <title>Behavior in Threaded Programs</title>

  <indexterm zone="libpq-threading">
   <primary>threads</primary>
   <secondary>with libpq</secondary>
  </indexterm>

  <para>
   As of version 17, <application>libpq</application> is always reentrant and thread-safe.
   However, one restriction is that no two threads attempt to manipulate
   the same <structname>PGconn</structname> object at the same time. In particular,
   you cannot issue concurrent commands from different threads through
   the same connection object. (If you need to run concurrent commands,
   use multiple connections.)
  </para>

  <para>
   <structname>PGresult</structname> objects are normally read-only after creation,
   and so can be passed around freely between threads.  However, if you use
   any of the <structname>PGresult</structname>-modifying functions described in
   <xref linkend="libpq-misc"/> or <xref linkend="libpq-events"/>, it's up
   to you to avoid concurrent operations on the same <structname>PGresult</structname>,
   too.
  </para>

  <para>
   In earlier versions, <application>libpq</application> could be compiled
   with or without thread support, depending on compiler options. This
   function allows the querying of <application>libpq</application>'s
   thread-safe status:
  </para>

  <variablelist>
   <varlistentry id="libpq-PQisthreadsafe">
    <term><function>PQisthreadsafe</function><indexterm><primary>PQisthreadsafe</primary></indexterm></term>

    <listitem>
     <para>
      Returns the thread safety status of the
      <application>libpq</application> library.
<synopsis>
int PQisthreadsafe();
</synopsis>
     </para>

     <para>
      Returns 1 if the <application>libpq</application> is thread-safe
      and 0 if it is not. Always returns 1 on version 17 and above.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The deprecated functions <xref linkend="libpq-PQrequestCancel"/> and
   <xref linkend="libpq-PQoidStatus"/> are not thread-safe and should not be
   used in multithread programs.  <xref linkend="libpq-PQrequestCancel"/>
   can be replaced by <xref linkend="libpq-PQcancel"/>.
   <xref linkend="libpq-PQoidStatus"/> can be replaced by
   <xref linkend="libpq-PQoidValue"/>.
  </para>

  <para>
   If you are using Kerberos inside your application (in addition to inside
   <application>libpq</application>), you will need to do locking around
   Kerberos calls because Kerberos functions are not thread-safe.  See
   function <function>PQregisterThreadLock</function> in the
   <application>libpq</application> source code for a way to do cooperative
   locking between <application>libpq</application> and your application.
  </para>
 </sect1>


 <sect1 id="libpq-build">
  <title>Building <application>libpq</application> Programs</title>

  <indexterm zone="libpq-build">
   <primary>compiling</primary>
   <secondary>libpq applications</secondary>
  </indexterm>

  <para>
   To build (i.e., compile and link) a program using
   <application>libpq</application> you need to do all of the following
   things:

   <itemizedlist>
    <listitem>
     <para>
      Include the <filename>libpq-fe.h</filename> header file:
<programlisting>
#include &lt;libpq-fe.h&gt;
</programlisting>
      If you failed to do that then you will normally get error messages
      from your compiler similar to:
<screen>
foo.c: In function `main':
foo.c:34: `PGconn' undeclared (first use in this function)
foo.c:35: `PGresult' undeclared (first use in this function)
foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)
</screen>
     </para>
    </listitem>

    <listitem>
     <para>
      Point your compiler to the directory where the <productname>PostgreSQL</productname> header
      files were installed, by supplying the
      <literal>-I<replaceable>directory</replaceable></literal> option
      to your compiler.  (In some cases the compiler will look into
      the directory in question by default, so you can omit this
      option.)  For instance, your compile command line could look
      like:
<programlisting>
cc -c -I/usr/local/pgsql/include testprog.c
</programlisting>
      If you are using makefiles then add the option to the
      <varname>CPPFLAGS</varname> variable:
<programlisting>
CPPFLAGS += -I/usr/local/pgsql/include
</programlisting>
     </para>

     <para>
      If there is any chance that your program might be compiled by
      other users then you should not hardcode the directory location
      like that.  Instead, you can run the utility
      <command>pg_config</command><indexterm><primary>pg_config</primary><secondary
      sortas="libpq">with libpq</secondary></indexterm> to find out where the header
      files are on the local system:
<screen>
<prompt>$</prompt> pg_config --includedir
<computeroutput>/usr/local/include</computeroutput>
</screen>
     </para>

     <para>
      If you
      have <command>pkg-config</command><indexterm><primary>pkg-config</primary><secondary sortas="libpq">with
      libpq</secondary></indexterm> installed, you can run instead:
<screen>
<prompt>$</prompt> pkg-config --cflags libpq
<computeroutput>-I/usr/local/include</computeroutput>
</screen>
      Note that this will already include the <option>-I</option> in front of
      the path.
     </para>

     <para>
      Failure to specify the correct option to the compiler will
      result in an error message such as:
<screen>
testlibpq.c:8:22: libpq-fe.h: No such file or directory
</screen>
     </para>
    </listitem>

    <listitem>
     <para>
      When linking the final program, specify the option
      <literal>-lpq</literal> so that the <application>libpq</application>
      library gets pulled in, as well as the option
      <literal>-L<replaceable>directory</replaceable></literal> to point
      the compiler to the directory where the
      <application>libpq</application> library resides.  (Again, the
      compiler will search some directories by default.)  For maximum
      portability, put the <option>-L</option> option before the
      <option>-lpq</option> option.  For example:
<programlisting>
cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
</programlisting>
     </para>

     <para>
      You can find out the library directory using
      <command>pg_config</command> as well:
<screen>
<prompt>$</prompt> pg_config --libdir
<computeroutput>/usr/local/pgsql/lib</computeroutput>
</screen>
     </para>

     <para>
      Or again use <command>pkg-config</command>:
<screen>
<prompt>$</prompt> pkg-config --libs libpq
<computeroutput>-L/usr/local/pgsql/lib -lpq</computeroutput>
</screen>
      Note again that this prints the full options, not only the path.
     </para>

     <para>
      Error messages that point to problems in this area could look like
      the following:
<screen>
testlibpq.o: In function `main':
testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
testlibpq.o(.text+0x71): undefined reference to `PQstatus'
testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'
</screen>
      This means you forgot <option>-lpq</option>.
<screen>
/usr/bin/ld: cannot find -lpq
</screen>
      This means you forgot the <option>-L</option> option or did not
      specify the right directory.
     </para>
    </listitem>
   </itemizedlist>
  </para>

 </sect1>


 <sect1 id="libpq-example">
  <title>Example Programs</title>

  <para>
   These examples and others can be found in the
   directory <filename>src/test/examples</filename> in the source code
   distribution.
  </para>

  <example id="libpq-example-1">
   <title><application>libpq</application> Example Program 1</title>

<programlisting>
<![CDATA[
/*
 * src/test/examples/testlibpq.c
 *
 *
 * testlibpq.c
 *
 *      Test the C version of libpq, the PostgreSQL frontend library.
 */
#include <stdio.h>
#include <stdlib.h>
#include "libpq-fe.h"

static void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

int
main(int argc, char **argv)
{
    const char *conninfo;
    PGconn     *conn;
    PGresult   *res;
    int         nFields;
    int         i,
                j;

    /*
     * If the user supplies a parameter on the command line, use it as the
     * conninfo string; otherwise default to setting dbname=postgres and using
     * environment variables or defaults for all other connection parameters.
     */
    if (argc > 1)
        conninfo = argv[1];
    else
        conninfo = "dbname = postgres";

    /* Make a connection to the database */
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /* Set always-secure search path, so malicious users can't take control. */
    res = PQexec(conn,
                 "SELECT pg_catalog.set_config('search_path', '', false)");
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "SET failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    /*
     * Should PQclear PGresult whenever it is no longer needed to avoid memory
     * leaks
     */
    PQclear(res);

    /*
     * Our test case here involves using a cursor, for which we must be inside
     * a transaction block.  We could do the whole thing with a single
     * PQexec() of "select * from pg_database", but that's too trivial to make
     * a good example.
     */

    /* Start a transaction block */
    res = PQexec(conn, "BEGIN");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);

    /*
     * Fetch rows from pg_database, the system catalog of databases
     */
    res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_database");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "DECLARE CURSOR failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);

    res = PQexec(conn, "FETCH ALL in myportal");
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "FETCH ALL failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    /* first, print out the attribute names */
    nFields = PQnfields(res);
    for (i = 0; i < nFields; i++)
        printf("%-15s", PQfname(res, i));
    printf("\n\n");

    /* next, print out the rows */
    for (i = 0; i < PQntuples(res); i++)
    {
        for (j = 0; j < nFields; j++)
            printf("%-15s", PQgetvalue(res, i, j));
        printf("\n");
    }

    PQclear(res);

    /* close the portal ... we don't bother to check for errors ... */
    res = PQexec(conn, "CLOSE myportal");
    PQclear(res);

    /* end the transaction */
    res = PQexec(conn, "END");
    PQclear(res);

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
]]>
</programlisting>
  </example>

  <example id="libpq-example-2">
   <title><application>libpq</application> Example Program 2</title>

<programlisting>
<![CDATA[
/*
 * src/test/examples/testlibpq2.c
 *
 *
 * testlibpq2.c
 *      Test of the asynchronous notification interface
 *
 * Start this program, then from psql in another window do
 *   NOTIFY TBL2;
 * Repeat four times to get this program to exit.
 *
 * Or, if you want to get fancy, try this:
 * populate a database with the following commands
 * (provided in src/test/examples/testlibpq2.sql):
 *
 *   CREATE SCHEMA TESTLIBPQ2;
 *   SET search_path = TESTLIBPQ2;
 *   CREATE TABLE TBL1 (i int4);
 *   CREATE TABLE TBL2 (i int4);
 *   CREATE RULE r1 AS ON INSERT TO TBL1 DO
 *     (INSERT INTO TBL2 VALUES (new.i); NOTIFY TBL2);
 *
 * Start this program, then from psql do this four times:
 *
 *   INSERT INTO TESTLIBPQ2.TBL1 VALUES (10);
 */

#ifdef WIN32
#include <windows.h>
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>

#include "libpq-fe.h"

static void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

int
main(int argc, char **argv)
{
    const char *conninfo;
    PGconn     *conn;
    PGresult   *res;
    PGnotify   *notify;
    int         nnotifies;

    /*
     * If the user supplies a parameter on the command line, use it as the
     * conninfo string; otherwise default to setting dbname=postgres and using
     * environment variables or defaults for all other connection parameters.
     */
    if (argc > 1)
        conninfo = argv[1];
    else
        conninfo = "dbname = postgres";

    /* Make a connection to the database */
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /* Set always-secure search path, so malicious users can't take control. */
    res = PQexec(conn,
                 "SELECT pg_catalog.set_config('search_path', '', false)");
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "SET failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    /*
     * Should PQclear PGresult whenever it is no longer needed to avoid memory
     * leaks
     */
    PQclear(res);

    /*
     * Issue LISTEN command to enable notifications from the rule's NOTIFY.
     */
    res = PQexec(conn, "LISTEN TBL2");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "LISTEN command failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);

    /* Quit after four notifies are received. */
    nnotifies = 0;
    while (nnotifies < 4)
    {
        /*
         * Sleep until something happens on the connection.  We use select(2)
         * to wait for input, but you could also use poll() or similar
         * facilities.
         */
        int         sock;
        fd_set      input_mask;

        sock = PQsocket(conn);

        if (sock < 0)
            break;              /* shouldn't happen */

        FD_ZERO(&input_mask);
        FD_SET(sock, &input_mask);

        if (select(sock + 1, &input_mask, NULL, NULL, NULL) < 0)
        {
            fprintf(stderr, "select() failed: %s\n", strerror(errno));
            exit_nicely(conn);
        }

        /* Now check for input */
        PQconsumeInput(conn);
        while ((notify = PQnotifies(conn)) != NULL)
        {
            fprintf(stderr,
                    "ASYNC NOTIFY of '%s' received from backend PID %d\n",
                    notify->relname, notify->be_pid);
            PQfreemem(notify);
            nnotifies++;
            PQconsumeInput(conn);
        }
    }

    fprintf(stderr, "Done.\n");

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
]]>
</programlisting>
  </example>

  <example id="libpq-example-3">
   <title><application>libpq</application> Example Program 3</title>

<programlisting>
<![CDATA[
/*
 * src/test/examples/testlibpq3.c
 *
 *
 * testlibpq3.c
 *      Test out-of-line parameters and binary I/O.
 *
 * Before running this, populate a database with the following commands
 * (provided in src/test/examples/testlibpq3.sql):
 *
 * CREATE SCHEMA testlibpq3;
 * SET search_path = testlibpq3;
 * SET standard_conforming_strings = ON;
 * CREATE TABLE test1 (i int4, t text, b bytea);
 * INSERT INTO test1 values (1, 'joe''s place', '\000\001\002\003\004');
 * INSERT INTO test1 values (2, 'ho there', '\004\003\002\001\000');
 *
 * The expected output is:
 *
 * tuple 0: got
 *  i = (4 bytes) 1
 *  t = (11 bytes) 'joe's place'
 *  b = (5 bytes) \000\001\002\003\004
 *
 * tuple 0: got
 *  i = (4 bytes) 2
 *  t = (8 bytes) 'ho there'
 *  b = (5 bytes) \004\003\002\001\000
 */

#ifdef WIN32
#include <windows.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <sys/types.h>
#include "libpq-fe.h"

/* for ntohl/htonl */
#include <netinet/in.h>
#include <arpa/inet.h>


static void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

/*
 * This function prints a query result that is a binary-format fetch from
 * a table defined as in the comment above.  We split it out because the
 * main() function uses it twice.
 */
static void
show_binary_results(PGresult *res)
{
    int         i,
                j;
    int         i_fnum,
                t_fnum,
                b_fnum;

    /* Use PQfnumber to avoid assumptions about field order in result */
    i_fnum = PQfnumber(res, "i");
    t_fnum = PQfnumber(res, "t");
    b_fnum = PQfnumber(res, "b");

    for (i = 0; i < PQntuples(res); i++)
    {
        char       *iptr;
        char       *tptr;
        char       *bptr;
        int         blen;
        int         ival;

        /* Get the field values (we ignore possibility they are null!) */
        iptr = PQgetvalue(res, i, i_fnum);
        tptr = PQgetvalue(res, i, t_fnum);
        bptr = PQgetvalue(res, i, b_fnum);

        /*
         * The binary representation of INT4 is in network byte order, which
         * we'd better coerce to the local byte order.
         */
        ival = ntohl(*((uint32_t *) iptr));

        /*
         * The binary representation of TEXT is, well, text, and since libpq
         * was nice enough to append a zero byte to it, it'll work just fine
         * as a C string.
         *
         * The binary representation of BYTEA is a bunch of bytes, which could
         * include embedded nulls so we have to pay attention to field length.
         */
        blen = PQgetlength(res, i, b_fnum);

        printf("tuple %d: got\n", i);
        printf(" i = (%d bytes) %d\n",
               PQgetlength(res, i, i_fnum), ival);
        printf(" t = (%d bytes) '%s'\n",
               PQgetlength(res, i, t_fnum), tptr);
        printf(" b = (%d bytes) ", blen);
        for (j = 0; j < blen; j++)
            printf("\\%03o", bptr[j]);
        printf("\n\n");
    }
}

int
main(int argc, char **argv)
{
    const char *conninfo;
    PGconn     *conn;
    PGresult   *res;
    const char *paramValues[1];
    int         paramLengths[1];
    int         paramFormats[1];
    uint32_t    binaryIntVal;

    /*
     * If the user supplies a parameter on the command line, use it as the
     * conninfo string; otherwise default to setting dbname=postgres and using
     * environment variables or defaults for all other connection parameters.
     */
    if (argc > 1)
        conninfo = argv[1];
    else
        conninfo = "dbname = postgres";

    /* Make a connection to the database */
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /* Set always-secure search path, so malicious users can't take control. */
    res = PQexec(conn, "SET search_path = testlibpq3");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "SET failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);

    /*
     * The point of this program is to illustrate use of PQexecParams() with
     * out-of-line parameters, as well as binary transmission of data.
     *
     * This first example transmits the parameters as text, but receives the
     * results in binary format.  By using out-of-line parameters we can avoid
     * a lot of tedious mucking about with quoting and escaping, even though
     * the data is text.  Notice how we don't have to do anything special with
     * the quote mark in the parameter value.
     */

    /* Here is our out-of-line parameter value */
    paramValues[0] = "joe's place";

    res = PQexecParams(conn,
                       "SELECT * FROM test1 WHERE t = $1",
                       1,       /* one param */
                       NULL,    /* let the backend deduce param type */
                       paramValues,
                       NULL,    /* don't need param lengths since text */
                       NULL,    /* default to all text params */
                       1);      /* ask for binary results */

    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "SELECT failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    show_binary_results(res);

    PQclear(res);

    /*
     * In this second example we transmit an integer parameter in binary form,
     * and again retrieve the results in binary form.
     *
     * Although we tell PQexecParams we are letting the backend deduce
     * parameter type, we really force the decision by casting the parameter
     * symbol in the query text.  This is a good safety measure when sending
     * binary parameters.
     */

    /* Convert integer value "2" to network byte order */
    binaryIntVal = htonl((uint32_t) 2);

    /* Set up parameter arrays for PQexecParams */
    paramValues[0] = (char *) &binaryIntVal;
    paramLengths[0] = sizeof(binaryIntVal);
    paramFormats[0] = 1;        /* binary */

    res = PQexecParams(conn,
                       "SELECT * FROM test1 WHERE i = $1::int4",
                       1,       /* one param */
                       NULL,    /* let the backend deduce param type */
                       paramValues,
                       paramLengths,
                       paramFormats,
                       1);      /* ask for binary results */

    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "SELECT failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    show_binary_results(res);

    PQclear(res);

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
]]>
</programlisting>
  </example>

 </sect1>
</chapter>