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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>popen</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_007_997"> </a>NAME</h4><blockquote>popen - initiate pipe streams to or from a process</blockquote><h4><a name = "tag_000_007_998"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="stdio.h.html">stdio.h</a>>FILE *popen(const char *<i>command</i>, const char *<i>mode</i>);</code></pre></blockquote><h4><a name = "tag_000_007_999"> </a>DESCRIPTION</h4><blockquote>The<i>popen()</i>function executes the command specified by the string<i>command</i>.It creates a pipe between the calling program and the executed command, andreturns a pointer to a stream that can be used to either read from or write tothe pipe.<p>If the implementation supports the referenced <b>XCU</b> specification, the environment of theexecuted command will be as if a child process were created within the<i>popen()</i>call using<i><a href="fork.html">fork()</a></i>,and the child invoked the<i><a href="../xcu/sh.html">sh</a></i>utility using the call:<pre><code>execl(<i>shell path</i>, "sh", "-c", <i>command</i>, (char *)0);</code></pre><p>where<i>shell path</i>is an unspecified pathname for the<i><a href="../xcu/sh.html">sh</a></i>utility.<p>The<i>popen()</i>function ensures that any streams from previous<i>popen()</i>calls that remain open in the parent process are closed in the new childprocess.<p>The<i>mode</i>argument to<i>popen()</i>is a string that specifies I/O mode:<ol><p><li>If<i>mode</i>is <b>r</b>, when the child process is started its file descriptorSTDOUT_FILENOwill be the writable end of the pipe, and the file descriptor<i>fileno</i>(<i>stream</i>) in the calling process, where<i>stream</i>is the stream pointer returned by<i>popen()</i>,will be the readable end of the pipe.<p><li>If<i>mode</i>is <b>w</b>, when the child process is started its file descriptorSTDIN_FILENOwill be the readable end of the pipe, and the file descriptor<i>fileno</i>(<i>stream</i>) in the calling process, where<i>stream</i>is the stream pointer returned by<i>popen()</i>,will be the writable end of the pipe.<p><li>If<i>mode</i>is any other value, the result is undefined.<p></ol><p>After<i>popen()</i>,both the parent and the child process will becapable of executing independently before either terminates.<p>Pipe streams are byte oriented.</blockquote><h4><a name = "tag_000_007_1000"> </a>RETURN VALUE</h4><blockquote>On successful completion,<i>popen()</i>returns a pointer to an open stream that can be used to read or write to thepipe. Otherwise, it returns a null pointer and may set<i>errno</i>to indicate the error.</blockquote><h4><a name = "tag_000_007_1001"> </a>ERRORS</h4><blockquote>The<i>popen()</i>function may fail if:<dl compact><dt>[EMFILE]<dd>{FOPEN_MAX} or {STREAM_MAX} streams are currently open in thecalling process.<dt>[EINVAL]<dd>The<i>mode</i>argument is invalid.</dl><p>The<i>popen()</i>function may also set<i>errno</i>values as described by<i><a href="fork.html">fork()</a></i>or<i><a href="pipe.html">pipe()</a></i>.<br></blockquote><h4><a name = "tag_000_007_1002"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_1003"> </a>APPLICATION USAGE</h4><blockquote>Because open files are shared, a mode <b>r</b>command can be used as an input filter and a mode <b>w</b>command as an output filter.<p>Buffered reading before opening an input filtermay leave the standard input of that filter mispositioned.Similar problems with an output filter may be preventedby careful buffer flushing, for example, with<i><a href="fflush.html">fflush()</a></i>.<p>A stream opened by<i>popen()</i>should be closed by<i><a href="pclose.html">pclose()</a></i>.<p>The behaviour of<i>popen()</i>is specified for values of<i>mode</i>of <b>r</b> and <b>w</b>.Other modes such as <b>rb</b> and <b>wb</b>might be supported by specific implementations,but these would not be portable features.Note that historical implementations of<i>popen()</i>only check to see if the first character of<i>mode</i>is r. Thus, a<i>mode</i>of <b>robert the robot</b> would be treated as<i>mode</i><b>r</b>, and a<i>mode</i>of <b>anything else</b> would be treated as<i>mode</i><b>w</b>.<p>If the application calls<i><a href="waitpid.html">waitpid()</a></i>or<i><a href="waitid.html">waitid()</a></i>with a<i>pid</i>argument greater than 0, and it still has a stream that was called with<i>popen()</i>open, it must ensure that<i>pid</i>does not refer to the process started by<i>popen()</i>.<p>To determine whether or not the <b>XCU</b> specification environment is present, use thefunction call:<pre><code>sysconf(_SC_2_VERSION)</code></pre><p>(See<i><a href="sysconf.html">sysconf()</a></i>).</blockquote><h4><a name = "tag_000_007_1004"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_1005"> </a>SEE ALSO</h4><blockquote><i><a href="../xcu/sh.html">sh</a></i>,<i><a href="pclose.html">pclose()</a></i>,<i><a href="pipe.html">pipe()</a></i>,<i><a href="sysconf.html">sysconf()</a></i>,<i><a href="system.html">system()</a></i>,<i><a href="stdio.h.html"><stdio.h></a></i>.</blockquote><h4>DERIVATION</h4><blockquote>Derived from Issue 1 of the SVID.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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