qom: Do not reuse errp after a possible error

[qemu/ar7.git] / docs / writing-qmp-commands.txt

= How to write QMP commands using the QAPI framework =

This document is a step-by-step guide on how to write new QMP commands using
the QAPI framework. It also shows how to implement new style HMP commands.

This document doesn't discuss QMP protocol level details, nor does it dive
into the QAPI framework implementation.

For an in-depth introduction to the QAPI framework, please refer to
docs/qapi-code-gen.txt. For documentation about the QMP protocol, please
check the files in QMP/.

== Overview ==

Generally speaking, the following steps should be taken in order to write a
new QMP command.

1. Write the command's and type(s) specification in the QAPI schema file
   (qapi-schema.json in the root source directory)

2. Write the QMP command itself, which is a regular C function. Preferably,
   the command should be exported by some QEMU subsystem. But it can also be
   added to the qmp.c file

3. At this point the command can be tested under the QMP protocol

4. Write the HMP command equivalent. This is not required and should only be
   done if it does make sense to have the functionality in HMP. The HMP command
   is implemented in terms of the QMP command

The following sections will demonstrate each of the steps above. We will start
very simple and get more complex as we progress.

=== Testing ===

For all the examples in the next sections, the test setup is the same and is
shown here.

First, QEMU should be started as:

# /path/to/your/source/qemu [...] \
    -chardev socket,id=qmp,port=4444,host=localhost,server \
    -mon chardev=qmp,mode=control,pretty=on

Then, in a different terminal:

$ telnet localhost 4444
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
{
    "QMP": {
        "version": {
            "qemu": {
                "micro": 50, 
                "minor": 15, 
                "major": 0
            }, 
            "package": ""
        }, 
        "capabilities": [
        ]
    }
}

The above output is the QMP server saying you're connected. The server is
actually in capabilities negotiation mode. To enter in command mode type:

{ "execute": "qmp_capabilities" }

Then the server should respond:

{
    "return": {
    }
}

Which is QMP's way of saying "the latest command executed OK and didn't return
any data". Now you're ready to enter the QMP example commands as explained in
the following sections.

== Writing a command that doesn't return data ==

That's the most simple QMP command that can be written. Usually, this kind of
command carries some meaningful action in QEMU but here it will just print
"Hello, world" to the standard output.

Our command will be called "hello-world". It takes no arguments, nor does it
return any data.

The first step is to add the following line to the bottom of the
qapi-schema.json file:

{ 'command': 'hello-world' }

The "command" keyword defines a new QMP command. It's an JSON object. All
schema entries are JSON objects. The line above will instruct the QAPI to
generate any prototypes and the necessary code to marshal and unmarshal
protocol data.

The next step is to write the "hello-world" implementation. As explained
earlier, it's preferable for commands to live in QEMU subsystems. But
"hello-world" doesn't pertain to any, so we put its implementation in qmp.c:

void qmp_hello_world(Error **errp)
{
    printf("Hello, world!\n");
}

There are a few things to be noticed:

1. QMP command implementation functions must be prefixed with "qmp_"
2. qmp_hello_world() returns void, this is in accordance with the fact that the
   command doesn't return any data
3. It takes an "Error **" argument. This is required. Later we will see how to
   return errors and take additional arguments. The Error argument should not
   be touched if the command doesn't return errors
4. We won't add the function's prototype. That's automatically done by the QAPI
5. Printing to the terminal is discouraged for QMP commands, we do it here
   because it's the easiest way to demonstrate a QMP command

Now a little hack is needed. As we're still using the old QMP server we need
to add the new command to its internal dispatch table. This step won't be
required in the near future. Open the qmp-commands.hx file and add the
following at the bottom:

    {
        .name       = "hello-world",
        .args_type  = "",
        .mhandler.cmd_new = qmp_marshal_input_hello_world,
    },

You're done. Now build qemu, run it as suggested in the "Testing" section,
and then type the following QMP command:

{ "execute": "hello-world" }

Then check the terminal running qemu and look for the "Hello, world" string. If
you don't see it then something went wrong.

=== Arguments ===

Let's add an argument called "message" to our "hello-world" command. The new
argument will contain the string to be printed to stdout. It's an optional
argument, if it's not present we print our default "Hello, World" string.

The first change we have to do is to modify the command specification in the
schema file to the following:

{ 'command': 'hello-world', 'data': { '*message': 'str' } }

Notice the new 'data' member in the schema. It's an JSON object whose each
element is an argument to the command in question. Also notice the asterisk,
it's used to mark the argument optional (that means that you shouldn't use it
for mandatory arguments). Finally, 'str' is the argument's type, which
stands for "string". The QAPI also supports integers, booleans, enumerations
and user defined types.

Now, let's update our C implementation in qmp.c:

void qmp_hello_world(bool has_message, const char *message, Error **errp)
{
    if (has_message) {
        printf("%s\n", message);
    } else {
        printf("Hello, world\n");
    }
}

There are two important details to be noticed:

1. All optional arguments are accompanied by a 'has_' boolean, which is set
   if the optional argument is present or false otherwise
2. The C implementation signature must follow the schema's argument ordering,
   which is defined by the "data" member

The last step is to update the qmp-commands.hx file:

    {
        .name       = "hello-world",
        .args_type  = "message:s?",
        .mhandler.cmd_new = qmp_marshal_input_hello_world,
    },

Notice that the "args_type" member got our "message" argument. The character
"s" stands for "string" and "?" means it's optional. This too must be ordered
according to the C implementation and schema file. You can look for more
examples in the qmp-commands.hx file if you need to define more arguments.

Again, this step won't be required in the future.

Time to test our new version of the "hello-world" command. Build qemu, run it as
described in the "Testing" section and then send two commands:

{ "execute": "hello-world" }
{
    "return": {
    }
}

{ "execute": "hello-world", "arguments": { "message": "We love qemu" } }
{
    "return": {
    }
}

You should see "Hello, world" and "we love qemu" in the terminal running qemu,
if you don't see these strings, then something went wrong.

=== Errors ===

QMP commands should use the error interface exported by the error.h header
file. Basically, errors are set by calling the error_set() function.

Let's say we don't accept the string "message" to contain the word "love". If
it does contain it, we want the "hello-world" command to return an error:

void qmp_hello_world(bool has_message, const char *message, Error **errp)
{
    if (has_message) {
        if (strstr(message, "love")) {
            error_set(errp, ERROR_CLASS_GENERIC_ERROR,
                      "the word 'love' is not allowed");
            return;
        }
        printf("%s\n", message);
    } else {
        printf("Hello, world\n");
    }
}

The first argument to the error_set() function is the Error pointer to pointer,
which is passed to all QMP functions. The second argument is a ErrorClass
value, which should be ERROR_CLASS_GENERIC_ERROR most of the time (more
details about error classes are given below). The third argument is a human
description of the error, this is a free-form printf-like string.

Let's test the example above. Build qemu, run it as defined in the "Testing"
section, and then issue the following command:

{ "execute": "hello-world", "arguments": { "message": "all you need is love" } }

The QMP server's response should be:

{
    "error": {
        "class": "GenericError",
        "desc": "the word 'love' is not allowed"
    }
}

As a general rule, all QMP errors should use ERROR_CLASS_GENERIC_ERROR. There
are two exceptions to this rule:

 1. A non-generic ErrorClass value exists* for the failure you want to report
    (eg. DeviceNotFound)

 2. Management applications have to take special action on the failure you
    want to report, hence you have to add a new ErrorClass value so that they
    can check for it

If the failure you want to report doesn't fall in one of the two cases above,
just report ERROR_CLASS_GENERIC_ERROR.

 * All existing ErrorClass values are defined in the qapi-schema.json file

=== Command Documentation ===

There's only one step missing to make "hello-world"'s implementation complete,
and that's its documentation in the schema file.

This is very important. No QMP command will be accepted in QEMU without proper
documentation.

There are many examples of such documentation in the schema file already, but
here goes "hello-world"'s new entry for the qapi-schema.json file:

##
# @hello-world
#
# Print a client provided string to the standard output stream.
#
# @message: #optional string to be printed
#
# Returns: Nothing on success.
#
# Notes: if @message is not provided, the "Hello, world" string will
#        be printed instead
#
# Since: <next qemu stable release, eg. 1.0>
##
{ 'command': 'hello-world', 'data': { '*message': 'str' } }

Please, note that the "Returns" clause is optional if a command doesn't return
any data nor any errors.

=== Implementing the HMP command ===

Now that the QMP command is in place, we can also make it available in the human
monitor (HMP).

With the introduction of the QAPI, HMP commands make QMP calls. Most of the
time HMP commands are simple wrappers. All HMP commands implementation exist in
the hmp.c file.

Here's the implementation of the "hello-world" HMP command:

void hmp_hello_world(Monitor *mon, const QDict *qdict)
{
    const char *message = qdict_get_try_str(qdict, "message");
    Error *err = NULL;

    qmp_hello_world(!!message, message, &err);
    if (err) {
        monitor_printf(mon, "%s\n", error_get_pretty(err));
        error_free(err);
        return;
    }
}

Also, you have to add the function's prototype to the hmp.h file.

There are three important points to be noticed:

1. The "mon" and "qdict" arguments are mandatory for all HMP functions. The
   former is the monitor object. The latter is how the monitor passes
   arguments entered by the user to the command implementation
2. hmp_hello_world() performs error checking. In this example we just print
   the error description to the user, but we could do more, like taking
   different actions depending on the error qmp_hello_world() returns
3. The "err" variable must be initialized to NULL before performing the
   QMP call

There's one last step to actually make the command available to monitor users,
we should add it to the hmp-commands.hx file:

    {
        .name       = "hello-world",
        .args_type  = "message:s?",
        .params     = "hello-world [message]",
        .help       = "Print message to the standard output",
        .mhandler.cmd = hmp_hello_world,
    },

STEXI
@item hello_world @var{message}
@findex hello_world
Print message to the standard output
ETEXI

To test this you have to open a user monitor and issue the "hello-world"
command. It might be instructive to check the command's documentation with
HMP's "help" command.

Please, check the "-monitor" command-line option to know how to open a user
monitor.

== Writing a command that returns data ==

A QMP command is capable of returning any data the QAPI supports like integers,
strings, booleans, enumerations and user defined types.

In this section we will focus on user defined types. Please, check the QAPI
documentation for information about the other types.

=== User Defined Types ===

FIXME This example needs to be redone after commit 6d32717

For this example we will write the query-alarm-clock command, which returns
information about QEMU's timer alarm. For more information about it, please
check the "-clock" command-line option.

We want to return two pieces of information. The first one is the alarm clock's
name. The second one is when the next alarm will fire. The former information is
returned as a string, the latter is an integer in nanoseconds (which is not
very useful in practice, as the timer has probably already fired when the
information reaches the client).

The best way to return that data is to create a new QAPI type, as shown below:

##
# @QemuAlarmClock
#
# QEMU alarm clock information.
#
# @clock-name: The alarm clock method's name.
#
# @next-deadline: #optional The time (in nanoseconds) the next alarm will fire.
#
# Since: 1.0
##
{ 'type': 'QemuAlarmClock',
  'data': { 'clock-name': 'str', '*next-deadline': 'int' } }

The "type" keyword defines a new QAPI type. Its "data" member contains the
type's members. In this example our members are the "clock-name" and the
"next-deadline" one, which is optional.

Now let's define the query-alarm-clock command:

##
# @query-alarm-clock
#
# Return information about QEMU's alarm clock.
#
# Returns a @QemuAlarmClock instance describing the alarm clock method
# being currently used by QEMU (this is usually set by the '-clock'
# command-line option).
#
# Since: 1.0
##
{ 'command': 'query-alarm-clock', 'returns': 'QemuAlarmClock' }

Notice the "returns" keyword. As its name suggests, it's used to define the
data returned by a command.

It's time to implement the qmp_query_alarm_clock() function, you can put it
in the qemu-timer.c file:

QemuAlarmClock *qmp_query_alarm_clock(Error **errp)
{
    QemuAlarmClock *clock;
    int64_t deadline;

    clock = g_malloc0(sizeof(*clock));

    deadline = qemu_next_alarm_deadline();
    if (deadline > 0) {
        clock->has_next_deadline = true;
        clock->next_deadline = deadline;
    }
    clock->clock_name = g_strdup(alarm_timer->name);

    return clock;
}

There are a number of things to be noticed:

1. The QemuAlarmClock type is automatically generated by the QAPI framework,
   its members correspond to the type's specification in the schema file
2. As specified in the schema file, the function returns a QemuAlarmClock
   instance and takes no arguments (besides the "errp" one, which is mandatory
   for all QMP functions)
3. The "clock" variable (which will point to our QAPI type instance) is
   allocated by the regular g_malloc0() function. Note that we chose to
   initialize the memory to zero. This is recommended for all QAPI types, as
   it helps avoiding bad surprises (specially with booleans)
4. Remember that "next_deadline" is optional? All optional members have a
   'has_TYPE_NAME' member that should be properly set by the implementation,
   as shown above
5. Even static strings, such as "alarm_timer->name", should be dynamically
   allocated by the implementation. This is so because the QAPI also generates
   a function to free its types and it cannot distinguish between dynamically
   or statically allocated strings
6. You have to include the "qmp-commands.h" header file in qemu-timer.c,
   otherwise qemu won't build

The last step is to add the correspoding entry in the qmp-commands.hx file:

    {
        .name       = "query-alarm-clock",
        .args_type  = "",
        .mhandler.cmd_new = qmp_marshal_input_query_alarm_clock,
    },

Time to test the new command. Build qemu, run it as described in the "Testing"
section and try this:

{ "execute": "query-alarm-clock" }
{
    "return": {
        "next-deadline": 2368219,
        "clock-name": "dynticks"
    }
}

==== The HMP command ====

Here's the HMP counterpart of the query-alarm-clock command:

void hmp_info_alarm_clock(Monitor *mon)
{
    QemuAlarmClock *clock;
    Error *err = NULL;

    clock = qmp_query_alarm_clock(&err);
    if (err) {
        monitor_printf(mon, "Could not query alarm clock information\n");
        error_free(err);
        return;
    }

    monitor_printf(mon, "Alarm clock method in use: '%s'\n", clock->clock_name);
    if (clock->has_next_deadline) {
        monitor_printf(mon, "Next alarm will fire in %" PRId64 " nanoseconds\n",
                       clock->next_deadline);
    }

   qapi_free_QemuAlarmClock(clock); 
}

It's important to notice that hmp_info_alarm_clock() calls
qapi_free_QemuAlarmClock() to free the data returned by qmp_query_alarm_clock().
For user defined types, the QAPI will generate a qapi_free_QAPI_TYPE_NAME()
function and that's what you have to use to free the types you define and
qapi_free_QAPI_TYPE_NAMEList() for list types (explained in the next section).
If the QMP call returns a string, then you should g_free() to free it.

Also note that hmp_info_alarm_clock() performs error handling. That's not
strictly required if you're sure the QMP function doesn't return errors, but
it's good practice to always check for errors.

Another important detail is that HMP's "info" commands don't go into the
hmp-commands.hx. Instead, they go into the info_cmds[] table, which is defined
in the monitor.c file. The entry for the "info alarmclock" follows:

    {
        .name       = "alarmclock",
        .args_type  = "",
        .params     = "",
        .help       = "show information about the alarm clock",
        .mhandler.info = hmp_info_alarm_clock,
    },

To test this, run qemu and type "info alarmclock" in the user monitor.

=== Returning Lists ===

For this example, we're going to return all available methods for the timer
alarm, which is pretty much what the command-line option "-clock ?" does,
except that we're also going to inform which method is in use.

This first step is to define a new type:

##
# @TimerAlarmMethod
#
# Timer alarm method information.
#
# @method-name: The method's name.
#
# @current: true if this alarm method is currently in use, false otherwise
#
# Since: 1.0
##
{ 'type': 'TimerAlarmMethod',
  'data': { 'method-name': 'str', 'current': 'bool' } }

The command will be called "query-alarm-methods", here is its schema
specification:

##
# @query-alarm-methods
#
# Returns information about available alarm methods.
#
# Returns: a list of @TimerAlarmMethod for each method
#
# Since: 1.0
##
{ 'command': 'query-alarm-methods', 'returns': ['TimerAlarmMethod'] }

Notice the syntax for returning lists "'returns': ['TimerAlarmMethod']", this
should be read as "returns a list of TimerAlarmMethod instances".

The C implementation follows:

TimerAlarmMethodList *qmp_query_alarm_methods(Error **errp)
{
    TimerAlarmMethodList *method_list = NULL;
    const struct qemu_alarm_timer *p;
    bool current = true;

    for (p = alarm_timers; p->name; p++) {
        TimerAlarmMethodList *info = g_malloc0(sizeof(*info));
        info->value = g_malloc0(sizeof(*info->value));
        info->value->method_name = g_strdup(p->name);
        info->value->current = current;

        current = false;

        info->next = method_list;
        method_list = info;
    }

    return method_list;
}

The most important difference from the previous examples is the
TimerAlarmMethodList type, which is automatically generated by the QAPI from
the TimerAlarmMethod type.

Each list node is represented by a TimerAlarmMethodList instance. We have to
allocate it, and that's done inside the for loop: the "info" pointer points to
an allocated node. We also have to allocate the node's contents, which is
stored in its "value" member. In our example, the "value" member is a pointer
to an TimerAlarmMethod instance.

Notice that the "current" variable is used as "true" only in the first
iteration of the loop. That's because the alarm timer method in use is the
first element of the alarm_timers array. Also notice that QAPI lists are handled
by hand and we return the head of the list.

To test this you have to add the corresponding qmp-commands.hx entry:

    {
        .name       = "query-alarm-methods",
        .args_type  = "",
        .mhandler.cmd_new = qmp_marshal_input_query_alarm_methods,
    },

Now Build qemu, run it as explained in the "Testing" section and try our new
command:

{ "execute": "query-alarm-methods" }
{
    "return": [
        {
            "current": false, 
            "method-name": "unix"
        }, 
        {
            "current": true, 
            "method-name": "dynticks"
        }
    ]
}

The HMP counterpart is a bit more complex than previous examples because it
has to traverse the list, it's shown below for reference:

void hmp_info_alarm_methods(Monitor *mon)
{
    TimerAlarmMethodList *method_list, *method;
    Error *err = NULL;

    method_list = qmp_query_alarm_methods(&err);
    if (err) {
        monitor_printf(mon, "Could not query alarm methods\n");
        error_free(err);
        return;
    }

    for (method = method_list; method; method = method->next) {
        monitor_printf(mon, "%c %s\n", method->value->current ? '*' : ' ',
                                       method->value->method_name);
    }

    qapi_free_TimerAlarmMethodList(method_list);
}