252 lines
5.7 KiB
HTML
252 lines
5.7 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 GETPID</TITLE>
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</HEAD><BODY>
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<H1>GETPID</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-03-06<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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getpid, getppid - get process identification
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>></B>
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<BR>
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<B>#include <<A HREF="file:///usr/include/unistd.h">unistd.h</A>></B>
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<P>
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<B>pid_t getpid(void);</B>
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<BR>
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<B>pid_t getppid(void);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>getpid</B>()
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returns the process ID (PID) of the calling process.
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(This is often used by
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routines that generate unique temporary filenames.)
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<P>
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<B>getppid</B>()
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returns the process ID of the parent of the calling process.
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This will be either the ID of the process that created this process using
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<B>fork</B>(),
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or, if that process has already terminated,
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the ID of the process to which this process has been reparented (either
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<B><A HREF="/cgi-bin/man/man2html?1+init">init</A></B>(1)
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or a "subreaper" process defined via the
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<B><A HREF="/cgi-bin/man/man2html?2+prctl">prctl</A></B>(2)
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<B>PR_SET_CHILD_SUBREAPER</B>
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operation).
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<A NAME="lbAE"> </A>
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<H2>ERRORS</H2>
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These functions are always successful.
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<A NAME="lbAF"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
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<A NAME="lbAG"> </A>
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<H2>NOTES</H2>
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If the caller's parent is in a different PID namespace (see
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<B><A HREF="/cgi-bin/man/man2html?7+pid_namespaces">pid_namespaces</A></B>(7)),
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<B>getppid</B>()
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returns 0.
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<P>
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From a kernel perspective,
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the PID (which is shared by all of the threads in a multithreaded process)
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is sometimes also known as the thread group ID (TGID).
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This contrasts with the kernel thread ID (TID),
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which is unique for each thread.
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For further details, see
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<B><A HREF="/cgi-bin/man/man2html?2+gettid">gettid</A></B>(2)
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and the discussion of the
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<B>CLONE_THREAD</B>
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flag in
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<B><A HREF="/cgi-bin/man/man2html?2+clone">clone</A></B>(2).
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<A NAME="lbAH"> </A>
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<H3>C library/kernel differences</H3>
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From glibc version 2.3.4 up to and including version 2.24,
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the glibc wrapper function for
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<B>getpid</B>()
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cached PIDs,
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with the goal of avoiding additional system calls when a process calls
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<B>getpid</B>()
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repeatedly.
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Normally this caching was invisible,
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but its correct operation relied on support in the wrapper functions for
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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+vfork">vfork</A></B>(2),
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and
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<B><A HREF="/cgi-bin/man/man2html?2+clone">clone</A></B>(2):
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if an application bypassed the glibc wrappers for these system calls by using
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<B><A HREF="/cgi-bin/man/man2html?2+syscall">syscall</A></B>(2),
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then a call to
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<B>getpid</B>()
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in the child would return the wrong value
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(to be precise: it would return the PID of the parent process).
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In addition, there were cases where
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<B>getpid</B>()
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could return the wrong value even when invoking
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<B><A HREF="/cgi-bin/man/man2html?2+clone">clone</A></B>(2)
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via the glibc wrapper function.
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(For a discussion of one such case, see BUGS in
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<B><A HREF="/cgi-bin/man/man2html?2+clone">clone</A></B>(2).)
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Furthermore, the complexity of the caching code had been
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the source of a few bugs within glibc over the years.
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<P>
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Because of the aforementioned problems,
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since glibc version 2.25, the PID cache is removed:
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calls to
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<B>getpid</B>()
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always invoke the actual system call, rather than returning a cached value.
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<P>
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On Alpha, instead of a pair of
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<B>getpid</B>()
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and
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<B>getppid</B>()
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system calls, a single
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<B>getxpid</B>()
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system call is provided, which returns a pair of PID and parent PID.
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The glibc
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<B>getpid</B>()
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and
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<B>getppid</B>()
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wrapper functions transparently deal with this.
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See
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<B><A HREF="/cgi-bin/man/man2html?2+syscall">syscall</A></B>(2)
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for details regarding register mapping.
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<A NAME="lbAI"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+clone">clone</A></B>(2),
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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+gettid">gettid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+kill">kill</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+exec">exec</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+mkstemp">mkstemp</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+tempnam">tempnam</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+tmpfile">tmpfile</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+tmpnam">tmpnam</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?7+credentials">credentials</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+pid_namespaces">pid_namespaces</A></B>(7)
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<A NAME="lbAJ"> </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="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">ERRORS</A><DD>
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<DT id="5"><A HREF="#lbAF">CONFORMING TO</A><DD>
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<DT id="6"><A HREF="#lbAG">NOTES</A><DD>
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<DL>
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<DT id="7"><A HREF="#lbAH">C library/kernel differences</A><DD>
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</DL>
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<DT id="8"><A HREF="#lbAI">SEE ALSO</A><DD>
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<DT id="9"><A HREF="#lbAJ">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: 00:05:32 GMT, March 31, 2021
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</BODY>
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</HTML>
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