arm-2011.03/share/doc/arm-arm-none-linux-gnueabi/html/libc/Pipe-to-a-Subprocess.html - nest-cam/v366/arm-none-linux-gnueabi-i686-pc-linux-gnu - Git at Google

 <html lang="en">
 <head>
 <title>Pipe to a Subprocess - The GNU C Library</title>
 <meta http-equiv="Content-Type" content="text/html">
 <meta name="description" content="The GNU C Library">
 <meta name="generator" content="makeinfo 4.13">
 <link title="Top" rel="start" href="index.html#Top">
 <link rel="up" href="Pipes-and-FIFOs.html#Pipes-and-FIFOs" title="Pipes and FIFOs">
 <link rel="prev" href="Creating-a-Pipe.html#Creating-a-Pipe" title="Creating a Pipe">
 <link rel="next" href="FIFO-Special-Files.html#FIFO-Special-Files" title="FIFO Special Files">
 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
 <!--
 This file documents the GNU C library.

 This is Edition 0.12, last updated 2007-10-27,
 of `The GNU C Library Reference Manual', for version
 2.8 (Sourcery G++ Lite 2011.03-41).

 Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
 2003, 2007, 2008, 2010 Free Software Foundation, Inc.

 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
 any later version published by the Free Software Foundation; with the
 Invariant Sections being ``Free Software Needs Free Documentation''
 and ``GNU Lesser General Public License'', the Front-Cover texts being
 ``A GNU Manual'', and with the Back-Cover Texts as in (a) below.  A
 copy of the license is included in the section entitled "GNU Free
 Documentation License".

 (a) The FSF's Back-Cover Text is: ``You have the freedom to
 copy and modify this GNU manual.  Buying copies from the FSF
 supports it in developing GNU and promoting software freedom.''-->
 <meta http-equiv="Content-Style-Type" content="text/css">
 <style type="text/css"><!--
   pre.display { font-family:inherit }
   pre.format  { font-family:inherit }
   pre.smalldisplay { font-family:inherit; font-size:smaller }
   pre.smallformat  { font-family:inherit; font-size:smaller }
   pre.smallexample { font-size:smaller }
   pre.smalllisp    { font-size:smaller }
   span.sc    { font-variant:small-caps }
   span.roman { font-family:serif; font-weight:normal; }
   span.sansserif { font-family:sans-serif; font-weight:normal; }
 --></style>
 <link rel="stylesheet" type="text/css" href="../cs.css">
 </head>
 <body>
 <div class="node">
 <a name="Pipe-to-a-Subprocess"></a>
 <p>
 Next:&nbsp;<a rel="next" accesskey="n" href="FIFO-Special-Files.html#FIFO-Special-Files">FIFO Special Files</a>,
 Previous:&nbsp;<a rel="previous" accesskey="p" href="Creating-a-Pipe.html#Creating-a-Pipe">Creating a Pipe</a>,
 Up:&nbsp;<a rel="up" accesskey="u" href="Pipes-and-FIFOs.html#Pipes-and-FIFOs">Pipes and FIFOs</a>
 <hr>
 </div>

 <h3 class="section">15.2 Pipe to a Subprocess</h3>

 <p><a name="index-creating-a-pipe-to-a-subprocess-1621"></a><a name="index-pipe-to-a-subprocess-1622"></a><a name="index-filtering-i_002fo-through-subprocess-1623"></a>
 A common use of pipes is to send data to or receive data from a program
 being run as a subprocess.  One way of doing this is by using a combination of
 <code>pipe</code> (to create the pipe), <code>fork</code> (to create the subprocess),
 <code>dup2</code> (to force the subprocess to use the pipe as its standard input
 or output channel), and <code>exec</code> (to execute the new program).  Or,
 you can use <code>popen</code> and <code>pclose</code>.

    <p>The advantage of using <code>popen</code> and <code>pclose</code> is that the
 interface is much simpler and easier to use.  But it doesn't offer as
 much flexibility as using the low-level functions directly.

 <!-- stdio.h -->
 <!-- POSIX.2, SVID, BSD -->
 <div class="defun">
 &mdash; Function: FILE * <b>popen</b> (<var>const char *command, const char *mode</var>)<var><a name="index-popen-1624"></a></var><br>
 <blockquote><p>The <code>popen</code> function is closely related to the <code>system</code>
 function; see <a href="Running-a-Command.html#Running-a-Command">Running a Command</a>.  It executes the shell command
 <var>command</var> as a subprocess.  However, instead of waiting for the
 command to complete, it creates a pipe to the subprocess and returns a
 stream that corresponds to that pipe.

         <p>If you specify a <var>mode</var> argument of <code>"r"</code>, you can read from the
 stream to retrieve data from the standard output channel of the subprocess.
 The subprocess inherits its standard input channel from the parent process.

         <p>Similarly, if you specify a <var>mode</var> argument of <code>"w"</code>, you can
 write to the stream to send data to the standard input channel of the
 subprocess.  The subprocess inherits its standard output channel from
 the parent process.

         <p>In the event of an error <code>popen</code> returns a null pointer.  This
 might happen if the pipe or stream cannot be created, if the subprocess
 cannot be forked, or if the program cannot be executed.
 </p></blockquote></div>

 <!-- stdio.h -->
 <!-- POSIX.2, SVID, BSD -->
 <div class="defun">
 &mdash; Function: int <b>pclose</b> (<var>FILE *stream</var>)<var><a name="index-pclose-1625"></a></var><br>
 <blockquote><p>The <code>pclose</code> function is used to close a stream created by <code>popen</code>.
 It waits for the child process to terminate and returns its status value,
 as for the <code>system</code> function.
 </p></blockquote></div>

    <p>Here is an example showing how to use <code>popen</code> and <code>pclose</code> to
 filter output through another program, in this case the paging program
 <code>more</code>.

 <pre class="smallexample">     #include &lt;stdio.h&gt;
      #include &lt;stdlib.h&gt;

      void
      write_data (FILE * stream)
      {
        int i;
        for (i = 0; i &lt; 100; i++)
          fprintf (stream, "%d\n", i);
        if (ferror (stream))
          {
            fprintf (stderr, "Output to stream failed.\n");
            exit (EXIT_FAILURE);
          }
      }

      int
      main (void)
      {
        FILE *output;

        output = popen ("more", "w");
        if (!output)
          {
            fprintf (stderr,
                     "incorrect parameters or too many files.\n");
            return EXIT_FAILURE;
          }
        write_data (output);
        if (pclose (output) != 0)
          {
            fprintf (stderr,
                     "Could not run more or other error.\n");
          }
        return EXIT_SUCCESS;
      }
 </pre>
    </body></html>
	<html lang="en">
	<head>
	<title>Pipe to a Subprocess - The GNU C Library</title>
	<meta http-equiv="Content-Type" content="text/html">
	<meta name="description" content="The GNU C Library">
	<meta name="generator" content="makeinfo 4.13">
	<link title="Top" rel="start" href="index.html#Top">
	<link rel="up" href="Pipes-and-FIFOs.html#Pipes-and-FIFOs" title="Pipes and FIFOs">
	<link rel="prev" href="Creating-a-Pipe.html#Creating-a-Pipe" title="Creating a Pipe">
	<link rel="next" href="FIFO-Special-Files.html#FIFO-Special-Files" title="FIFO Special Files">
	<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
	<!--
	This file documents the GNU C library.

	This is Edition 0.12, last updated 2007-10-27,
	of `The GNU C Library Reference Manual', for version
	2.8 (Sourcery G++ Lite 2011.03-41).

	Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
	2003, 2007, 2008, 2010 Free Software Foundation, Inc.

	Permission is granted to copy, distribute and/or modify this document
	under the terms of the GNU Free Documentation License, Version 1.3 or
	any later version published by the Free Software Foundation; with the
	Invariant Sections being ``Free Software Needs Free Documentation''
	and ``GNU Lesser General Public License'', the Front-Cover texts being
	``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A
	copy of the license is included in the section entitled "GNU Free
	Documentation License".

	(a) The FSF's Back-Cover Text is: ``You have the freedom to
	copy and modify this GNU manual. Buying copies from the FSF
	supports it in developing GNU and promoting software freedom.''-->
	<meta http-equiv="Content-Style-Type" content="text/css">
	<style type="text/css"><!--
	pre.display { font-family:inherit }
	pre.format { font-family:inherit }
	pre.smalldisplay { font-family:inherit; font-size:smaller }
	pre.smallformat { font-family:inherit; font-size:smaller }
	pre.smallexample { font-size:smaller }
	pre.smalllisp { font-size:smaller }
	span.sc { font-variant:small-caps }
	span.roman { font-family:serif; font-weight:normal; }
	span.sansserif { font-family:sans-serif; font-weight:normal; }
	--></style>
	<link rel="stylesheet" type="text/css" href="../cs.css">
	</head>
	<body>
	<div class="node">
	<a name="Pipe-to-a-Subprocess"></a>
	<p>
	Next: <a rel="next" accesskey="n" href="FIFO-Special-Files.html#FIFO-Special-Files">FIFO Special Files</a>,
	Previous: <a rel="previous" accesskey="p" href="Creating-a-Pipe.html#Creating-a-Pipe">Creating a Pipe</a>,
	Up: <a rel="up" accesskey="u" href="Pipes-and-FIFOs.html#Pipes-and-FIFOs">Pipes and FIFOs</a>
	<hr>
	</div>

	<h3 class="section">15.2 Pipe to a Subprocess</h3>

	<p><a name="index-creating-a-pipe-to-a-subprocess-1621"></a><a name="index-pipe-to-a-subprocess-1622"></a><a name="index-filtering-i_002fo-through-subprocess-1623"></a>
	A common use of pipes is to send data to or receive data from a program
	being run as a subprocess. One way of doing this is by using a combination of
	<code>pipe</code> (to create the pipe), <code>fork</code> (to create the subprocess),
	<code>dup2</code> (to force the subprocess to use the pipe as its standard input
	or output channel), and <code>exec</code> (to execute the new program). Or,
	you can use <code>popen</code> and <code>pclose</code>.

	<p>The advantage of using <code>popen</code> and <code>pclose</code> is that the
	interface is much simpler and easier to use. But it doesn't offer as
	much flexibility as using the low-level functions directly.

	<!-- stdio.h -->
	<!-- POSIX.2, SVID, BSD -->
	<div class="defun">
	— Function: FILE * <b>popen</b> (<var>const char command, const char mode</var>)<var><a name="index-popen-1624"></a></var><br>
	<blockquote><p>The <code>popen</code> function is closely related to the <code>system</code>
	function; see <a href="Running-a-Command.html#Running-a-Command">Running a Command</a>. It executes the shell command
	<var>command</var> as a subprocess. However, instead of waiting for the
	command to complete, it creates a pipe to the subprocess and returns a
	stream that corresponds to that pipe.

	<p>If you specify a <var>mode</var> argument of <code>"r"</code>, you can read from the
	stream to retrieve data from the standard output channel of the subprocess.
	The subprocess inherits its standard input channel from the parent process.

	<p>Similarly, if you specify a <var>mode</var> argument of <code>"w"</code>, you can
	write to the stream to send data to the standard input channel of the
	subprocess. The subprocess inherits its standard output channel from
	the parent process.

	<p>In the event of an error <code>popen</code> returns a null pointer. This
	might happen if the pipe or stream cannot be created, if the subprocess
	cannot be forked, or if the program cannot be executed.
	</p></blockquote></div>

	<!-- stdio.h -->
	<!-- POSIX.2, SVID, BSD -->
	<div class="defun">
	— Function: int <b>pclose</b> (<var>FILE *stream</var>)<var><a name="index-pclose-1625"></a></var><br>
	<blockquote><p>The <code>pclose</code> function is used to close a stream created by <code>popen</code>.
	It waits for the child process to terminate and returns its status value,
	as for the <code>system</code> function.
	</p></blockquote></div>

	<p>Here is an example showing how to use <code>popen</code> and <code>pclose</code> to
	filter output through another program, in this case the paging program
	<code>more</code>.

	<pre class="smallexample"> #include <stdio.h>
	#include <stdlib.h>

	void
	write_data (FILE * stream)
	{
	int i;
	for (i = 0; i < 100; i++)
	fprintf (stream, "%d\n", i);
	if (ferror (stream))
	{
	fprintf (stderr, "Output to stream failed.\n");
	exit (EXIT_FAILURE);
	}
	}

	int
	main (void)
	{
	FILE *output;

	output = popen ("more", "w");
	if (!output)
	{
	fprintf (stderr,
	"incorrect parameters or too many files.\n");
	return EXIT_FAILURE;
	}
	write_data (output);
	if (pclose (output) != 0)
	{
	fprintf (stderr,
	"Could not run more or other error.\n");
	}
	return EXIT_SUCCESS;
	}
	</pre>
	</body></html>