325 lines
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325 lines
7.0 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of POPEN</TITLE>
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</HEAD><BODY>
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<H1>POPEN</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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popen, pclose - pipe stream to or from a process
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/stdio.h">stdio.h</A>></B>
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<B>FILE *popen(const char *</B><I>command</I><B>, const char *</B><I>type</I><B>);</B>
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<B>int pclose(FILE *</B><I>stream</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>popen</B>(),
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<B>pclose</B>():
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<DL COMPACT><DT id="1"><DD>
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_POSIX_C_SOURCE >= 2
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<BR> || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>popen</B>()
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function opens a process by creating a pipe, forking, and invoking the
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shell.
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Since a pipe is by definition unidirectional, the
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<I>type</I>
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argument may specify only reading or writing, not both; the resulting
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stream is correspondingly read-only or write-only.
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<P>
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The
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<I>command</I>
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argument is a pointer to a null-terminated string containing a shell
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command line.
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This command is passed to
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<I>/bin/sh</I>
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using the
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<B>-c</B>
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flag; interpretation, if any, is performed by the shell.
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<P>
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The
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<I>type</I>
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argument is a pointer to a null-terminated string which must contain
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either the letter 'r' for reading or the letter 'w' for writing.
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Since glibc 2.9,
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this argument can additionally include the letter 'e',
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which causes the close-on-exec flag
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(<B>FD_CLOEXEC</B>)
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to be set on the underlying file descriptor;
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see the description of the
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<B>O_CLOEXEC</B>
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flag in
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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for reasons why this may be useful.
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<P>
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The return value from
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<B>popen</B>()
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is a normal standard I/O stream in all respects save that it must be closed
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with
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<B>pclose</B>()
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rather than
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<B><A HREF="/cgi-bin/man/man2html?3+fclose">fclose</A></B>(3).
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Writing to such a stream writes to the standard input of the command; the
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command's standard output is the same as that of the process that called
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<B>popen</B>(),
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unless this is altered by the command itself.
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Conversely, reading from
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the stream reads the command's standard output, and the command's
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standard input is the same as that of the process that called
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<B>popen</B>().
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<P>
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Note that output
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<B>popen</B>()
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streams are block buffered by default.
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<P>
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The
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<B>pclose</B>()
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function waits for the associated process to terminate and returns the exit
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status of the command as returned by
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<B><A HREF="/cgi-bin/man/man2html?2+wait4">wait4</A></B>(2).
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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<B>popen</B>():
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on success, returns a pointer to an open stream that
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can be used to read or write to the pipe;
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if the
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<B><A HREF="/cgi-bin/man/man2html?2+fork">fork</A></B>(2)
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or
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<B><A HREF="/cgi-bin/man/man2html?2+pipe">pipe</A></B>(2)
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calls fail, or if the function cannot allocate memory,
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NULL is returned.
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<P>
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<B>pclose</B>():
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on success, returns the exit status of the command; if
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<B><A HREF="/cgi-bin/man/man2html?2+wait4">wait4</A></B>(2)
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returns an error, or some other error is detected,
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-1 is returned.
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<P>
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Both functions set
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<I>errno</I>
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to an appropriate value in the case of an error.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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The
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<B>popen</B>()
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function does not set
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<I>errno</I>
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if memory allocation fails.
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If the underlying
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<B><A HREF="/cgi-bin/man/man2html?2+fork">fork</A></B>(2)
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or
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<B><A HREF="/cgi-bin/man/man2html?2+pipe">pipe</A></B>(2)
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fails,
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<I>errno</I>
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is set appropriately.
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If the
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<I>type</I>
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argument is invalid, and this condition is detected,
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<I>errno</I>
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is set to
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<B>EINVAL</B>.
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<P>
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If
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<B>pclose</B>()
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cannot obtain the child status,
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<I>errno</I>
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is set to
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<B>ECHILD</B>.
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<A NAME="lbAG"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>popen</B>(),
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<B>pclose</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<P>
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008.
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<P>
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The 'e' value for
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<I>type</I>
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is a Linux extension.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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<B>Note</B>:
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carefully read Caveats in
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<B><A HREF="/cgi-bin/man/man2html?3+system">system</A></B>(3).
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<A NAME="lbAJ"> </A>
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<H2>BUGS</H2>
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Since the standard input of a command opened for reading shares its seek
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offset with the process that called
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<B>popen</B>(),
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if the original process has done a buffered read, the command's input
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position may not be as expected.
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Similarly, the output from a command
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opened for writing may become intermingled with that of the original
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process.
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The latter can be avoided by calling
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<B><A HREF="/cgi-bin/man/man2html?3+fflush">fflush</A></B>(3)
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before
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<B>popen</B>().
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<P>
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Failure to execute the shell is indistinguishable from the shell's failure
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to execute command, or an immediate exit of the command.
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The only hint is an exit status of 127.
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?1+sh">sh</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?2+fork">fork</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+pipe">pipe</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+wait4">wait4</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+fclose">fclose</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+fflush">fflush</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+fopen">fopen</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+stdio">stdio</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+system">system</A></B>(3)
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<A NAME="lbAL"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="2"><A HREF="#lbAB">NAME</A><DD>
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<DT id="3"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="4"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="5"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="6"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="7"><A HREF="#lbAG">ATTRIBUTES</A><DD>
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<DT id="8"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="9"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="10"><A HREF="#lbAJ">BUGS</A><DD>
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<DT id="11"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="12"><A HREF="#lbAL">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:52 GMT, March 31, 2021
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</BODY>
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</HTML>
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