222 lines
8.9 KiB
HTML
222 lines
8.9 KiB
HTML
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>popen</title>
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<basefont size="3"> <a name="popen"></a> <a name="tag_03_420"></a><!-- popen -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_420_01"></a>NAME</h4>
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<blockquote>popen - initiate pipe streams to or from a process</blockquote>
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<h4><a name="tag_03_420_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/stdio.h.html">stdio.h</a>><br>
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<br>
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FILE *popen(const char *</tt><i>command</i><tt>, const char *</tt><i>mode</i><tt>); <img src="../images/opt-end.gif" alt=
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"[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_420_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>popen</i>() function shall execute the command specified by the string <i>command</i>. It shall create a pipe between the
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calling program and the executed command, and shall return a pointer to a stream that can be used to either read from or write to
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the pipe.</p>
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<p>The environment of the executed command shall be as if a child process were created within the <i>popen</i>() call using the <a
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href="../functions/fork.html"><i>fork</i>()</a> function, and the child invoked the <a href="../utilities/sh.html"><i>sh</i></a>
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utility using the call:</p>
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<pre>
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<tt>execl(</tt><i>shell path</i><tt>, "sh", "-c",</tt> <i>command</i><tt>, (char *)0);
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</tt>
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</pre>
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<p>where <i>shell path</i> is an unspecified pathname for the <a href="../utilities/sh.html"><i>sh</i></a> utility.</p>
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<p>The <i>popen</i>() function shall ensure that any streams from previous <i>popen</i>() calls that remain open in the parent
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process are closed in the new child process.</p>
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<p>The <i>mode</i> argument to <i>popen</i>() is a string that specifies I/O mode:</p>
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<ol>
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<li>
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<p>If <i>mode</i> is <i>r</i>, when the child process is started, its file descriptor STDOUT_FILENO shall be the writable end of
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the pipe, and the file descriptor <i>fileno</i>(<i>stream</i>) in the calling process, where <i>stream</i> is the stream pointer
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returned by <i>popen</i>(), shall be the readable end of the pipe.</p>
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</li>
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<li>
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<p>If <i>mode</i> is <i>w</i>, when the child process is started its file descriptor STDIN_FILENO shall be the readable end of the
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pipe, and the file descriptor <i>fileno</i>(<i>stream</i>) in the calling process, where <i>stream</i> is the stream pointer
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returned by <i>popen</i>(), shall be the writable end of the pipe.</p>
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</li>
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<li>
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<p>If <i>mode</i> is any other value, the result is undefined.</p>
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</li>
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</ol>
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<p>After <i>popen</i>(), both the parent and the child process shall be capable of executing independently before either
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terminates.</p>
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<p>Pipe streams are byte-oriented.</p>
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</blockquote>
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<h4><a name="tag_03_420_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, <i>popen</i>() shall return a pointer to an open stream that can be used to read or write to the
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pipe. Otherwise, it shall return a null pointer and may set <i>errno</i> to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_420_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>popen</i>() function may fail if:</p>
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<dl compact>
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<dt>[EMFILE]</dt>
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<dd>{FOPEN_MAX} or {STREAM_MAX} streams are currently open in the calling process.</dd>
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<dt>[EINVAL]</dt>
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<dd>The <i>mode</i> argument is invalid.</dd>
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</dl>
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<p>The <i>popen</i>() function may also set <i>errno</i> values as described by <a href="fork.html"><i>fork</i>()</a> or <a href=
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"pipe.html"><i>pipe</i>()</a> .</p>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_420_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_420_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>Since open files are shared, a mode <i>r</i> command can be used as an input filter and a mode <i>w</i> command as an output
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filter.</p>
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<p>Buffered reading before opening an input filter may leave the standard input of that filter mispositioned. Similar problems with
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an output filter may be prevented by careful buffer flushing; for example, with <a href="fflush.html"><i>fflush</i>()</a> .</p>
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<p>A stream opened by <i>popen</i>() should be closed by <a href="../functions/pclose.html"><i>pclose</i>()</a>.</p>
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<p>The behavior of <i>popen</i>() is specified for values of <i>mode</i> of <i>r</i> and <i>w</i>. Other modes such as <i>rb</i>
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and <i>wb</i> might be supported by specific implementations, but these would not be portable features. Note that historical
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implementations of <i>popen</i>() only check to see if the first character of <i>mode</i> is <i>r</i>. Thus, a <i>mode</i> of
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<i>robert the robot</i> would be treated as <i>mode</i> <i>r</i>, and a <i>mode</i> of <i>anything else</i> would be treated as
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<i>mode</i> <i>w</i>.</p>
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<p>If the application calls <a href="../functions/waitpid.html"><i>waitpid</i>()</a> or <a href=
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"../functions/waitid.html"><i>waitid</i>()</a> with a <i>pid</i> argument greater than 0, and it still has a stream that was called
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with <i>popen</i>() open, it must ensure that <i>pid</i> does not refer to the process started by <i>popen</i>().</p>
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<p>To determine whether or not the environment specified in the Shell and Utilities volume of IEEE Std 1003.1-2001 is
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present, use the function call:</p>
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<pre>
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<tt>sysconf(_SC_2_VERSION)
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</tt>
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</pre>
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<p>(See <a href="sysconf.html"><i>sysconf</i>()</a> ).</p>
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</blockquote>
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<h4><a name="tag_03_420_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The <i>popen</i>() function should not be used by programs that have set user (or group) ID privileges. The <a href=
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"../functions/fork.html"><i>fork</i>()</a> and <i><a href="../functions/exec.html">exec</a></i> family of functions (except <a
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href="../functions/execlp.html"><i>execlp</i>()</a> and <a href="../functions/execvp.html"><i>execvp</i>()</a>), should be used
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instead. This prevents any unforeseen manipulation of the environment of the user that could cause execution of commands not
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anticipated by the calling program.</p>
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<p>If the original and <i>popen</i>()ed processes both intend to read or write or read and write a common file, and either will be
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using FILE-type C functions ( <a href="../functions/fread.html"><i>fread</i>()</a>, <a href=
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"../functions/fwrite.html"><i>fwrite</i>()</a>, and so on), the rules for sharing file handles must be observed (see <a href=
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"xsh_chap02_05.html#tag_02_05_01"><i>Interaction of File Descriptors and Standard I/O Streams</i></a> ).</p>
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</blockquote>
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<h4><a name="tag_03_420_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_420_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="pclose.html"><i>pclose</i>()</a> , <a href="pipe.html"><i>pipe</i>()</a> , <a href="sysconf.html"><i>sysconf</i>()</a>
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, <a href="system.html"><i>system</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/stdio.h.html"><i><stdio.h></i></a>, the Shell and Utilities volume of IEEE Std 1003.1-2001, <a href=
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"../utilities/sh.html"><i>sh</i></a></p>
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</blockquote>
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<h4><a name="tag_03_420_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 1. Derived from Issue 1 of the SVID.</p>
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</blockquote>
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<h4><a name="tag_03_420_12"></a>Issue 5</h4>
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<blockquote>
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<p>A statement is added to the DESCRIPTION indicating that pipe streams are byte-oriented.</p>
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</blockquote>
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<h4><a name="tag_03_420_13"></a>Issue 6</h4>
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<blockquote>
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<p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>
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<ul>
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<li>
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<p>The optional [EMFILE] error condition is added.</p>
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</li>
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</ul>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<hr size="2" noshade>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
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"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
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