mingw/html/lib/IPC/Open2.html

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  13 <big><strong><span class="block">&nbsp;open2 - open a process for both reading and writing</span></strong></big>
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  20 <ul>
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  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#synopsis">SYNOPSIS</a></li>
  24         <li><a href="#description">DESCRIPTION</a></li>
  25         <li><a href="#warning">WARNING</a></li>
  26         <li><a href="#see_also">SEE ALSO</a></li>
  27 </ul>
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  31 <p>
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  33 <h1><a name="name">NAME</a></h1>
  34 <p>IPC::Open2, open2 - open a process for both reading and writing</p>
  35 <p>
  36 </p>
  37 <hr />
  38 <h1><a name="synopsis">SYNOPSIS</a></h1>
  39 <pre>
  40     use IPC::Open2;</pre>
  41 <pre>
  42     $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some cmd and args');
  43       # or without using the shell
  44     $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some', 'cmd', 'and', 'args');</pre>
  45 <pre>
  46     # or with handle autovivification
  47     my($chld_out, $chld_in);
  48     $pid = open2($chld_out, $chld_in, 'some cmd and args');
  49       # or without using the shell
  50     $pid = open2($chld_out, $chld_in, 'some', 'cmd', 'and', 'args');</pre>
  51 <p>
  52 </p>
  53 <hr />
  54 <h1><a name="description">DESCRIPTION</a></h1>
  55 <p>The <code>open2()</code> function runs the given $cmd and connects $chld_out for
  56 reading and $chld_in for writing.  It's what you think should work
  57 when you try</p>
  58 <pre>
  59     $pid = open(HANDLE, &quot;|cmd args|&quot;);</pre>
  60 <p>The write filehandle will have autoflush turned on.</p>
  61 <p>If $chld_out is a string (that is, a bareword filehandle rather than a glob
  62 or a reference) and it begins with <code>&gt;&amp;</code>, then the child will send output
  63 directly to that file handle.  If $chld_in is a string that begins with
  64 <code>&lt;&amp;</code>, then $chld_in will be closed in the parent, and the child will
  65 read from it directly.  In both cases, there will be a <code>dup(2)</code> instead of a
  66 <code>pipe(2)</code> made.</p>
  67 <p>If either reader or writer is the null string, this will be replaced
  68 by an autogenerated filehandle.  If so, you must pass a valid lvalue
  69 in the parameter slot so it can be overwritten in the caller, or
  70 an exception will be raised.</p>
  71 <p><code>open2()</code> returns the process ID of the child process.  It doesn't return on
  72 failure: it just raises an exception matching <code>/^open2:/</code>.  However,
  73 <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_exec"><code>exec</code></a> failures in the child are not detected.  You'll have to
  74 trap SIGPIPE yourself.</p>
  75 <p><code>open2()</code> does not wait for and reap the child process after it exits.
  76 Except for short programs where it's acceptable to let the operating system
  77 take care of this, you need to do this yourself.  This is normally as
  78 simple as calling <code>waitpid $pid, 0</code> when you're done with the process.
  79 Failing to do this can result in an accumulation of defunct or ``zombie''
  80 processes.  See <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#waitpid">waitpid in the perlfunc manpage</a> for more information.</p>
  81 <p>This whole affair is quite dangerous, as you may block forever.  It
  82 assumes it's going to talk to something like <strong>bc</strong>, both writing
  83 to it and reading from it.  This is presumably safe because you
  84 ``know'' that commands like <strong>bc</strong> will read a line at a time and
  85 output a line at a time.  Programs like <strong>sort</strong> that read their
  86 entire input stream first, however, are quite apt to cause deadlock.</p>
  87 <p>The big problem with this approach is that if you don't have control
  88 over source code being run in the child process, you can't control
  89 what it does with pipe buffering.  Thus you can't just open a pipe to
  90 <code>cat -v</code> and continually read and write a line from it.</p>
  91 <p>The IO::Pty and Expect modules from CPAN can help with this, as they
  92 provide a real tty (well, a pseudo-tty, actually), which gets you
  93 back to line buffering in the invoked command again.</p>
  94 <p>
  95 </p>
  96 <hr />
  97 <h1><a name="warning">WARNING</a></h1>
  98 <p>The order of arguments differs from that of open3().</p>
  99 <p>
 100 </p>
 101 <hr />
 102 <h1><a name="see_also">SEE ALSO</a></h1>
 103 <p>See <a href="file://C|\msysgit\mingw\html/lib/IPC/Open3.html">the IPC::Open3 manpage</a> for an alternative that handles STDERR as well.  This
 104 function is really just a wrapper around open3().</p>
 105 <table border="0" width="100%" cellspacing="0" cellpadding="3">
 106 <tr><td class="block" style="background-color: #cccccc" valign="middle">
 107 <big><strong><span class="block">&nbsp;open2 - open a process for both reading and writing</span></strong></big>
 108 </td></tr>
 109 </table>
 110
 111 </body>
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