381 lines
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381 lines
8.9 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of PIDFD_OPEN</TITLE>
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</HEAD><BODY>
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<H1>PIDFD_OPEN</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-11-19<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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pidfd_open - obtain a file descriptor that refers to 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/sys/types.h">sys/types.h</A>></B>
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<B>int pidfd_open(pid_t </B><I>pid</I><B>, unsigned int </B><I>flags</I><B>);</B>
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</PRE>
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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>pidfd_open</B>()
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system call creates a file descriptor that refers to
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the process whose PID is specified in
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<I>pid</I>.
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The file descriptor is returned as the function result;
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the close-on-exec flag is set on the file descriptor.
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<P>
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The
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<I>flags</I>
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argument is reserved for future use;
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currently, this argument must be specified as 0.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On success,
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<B>pidfd_open</B>()
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returns a nonnegative file descriptor.
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On error, -1 is returned and
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<I>errno</I>
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is set to indicate the cause of the error.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="1"><B>EINVAL</B>
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<DD>
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<I>flags</I>
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is not 0.
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<DT id="2"><B>EINVAL</B>
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<DD>
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<I>pid</I>
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is not valid.
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<DT id="3"><B>EMFILE</B>
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<DD>
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The per-process limit on the number of open file descriptors has been reached
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(see the description of
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<B>RLIMIT_NOFILE</B>
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in
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<B><A HREF="/cgi-bin/man/man2html?2+getrlimit">getrlimit</A></B>(2)).
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<DT id="4"><B>ENFILE</B>
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<DD>
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The system-wide limit on the total number of open files has been reached.
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<DT id="5"><B>ENODEV</B>
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<DD>
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The anonymous inode filesystem is not available in this kernel.
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<DT id="6"><B>ENOMEM</B>
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<DD>
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Insufficient kernel memory was available.
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<DT id="7"><B>ESRCH</B>
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<DD>
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The process specified by
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<I>pid</I>
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does not exist.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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<B>pidfd_open</B>()
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first appeared in Linux 5.3.
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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<B>pidfd_open</B>()
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is Linux specific.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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Currently, there is no glibc wrapper for this system call; call it using
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<B><A HREF="/cgi-bin/man/man2html?2+syscall">syscall</A></B>(2).
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<P>
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The following code sequence can be used to obtain a file descriptor
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for the child of
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<B><A HREF="/cgi-bin/man/man2html?2+fork">fork</A></B>(2):
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<P>
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pid = fork();
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if (pid > 0) { /* If parent */
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<BR> pidfd = pidfd_open(pid, 0);
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<BR> ...
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}
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<P>
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Even if the child has already terminated by the time of the
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<B>pidfd_open</B>()
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call, its PID will not have been recycled and the returned
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file descriptor will refer to the resulting zombie process.
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Note, however, that this is guaranteed only if the following
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conditions hold true:
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<DL COMPACT>
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<DT id="8">*<DD>
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the disposition of
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<B>SIGCHLD</B>
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has not been explicitly set to
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<B>SIG_IGN</B>
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(see
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2));
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<DT id="9">*<DD>
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the
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<B>SA_NOCLDWAIT</B>
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flag was not specified while establishing a handler for
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<B>SIGCHLD</B>
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or while setting the disposition of that signal to
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<B>SIG_DFL</B>
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(see
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<B><A HREF="/cgi-bin/man/man2html?2+sigaction">sigaction</A></B>(2));
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and
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<DT id="10">*<DD>
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the zombie process was not reaped elsewhere in the program
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(e.g., either by an asynchronously executed signal handler or by
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<B><A HREF="/cgi-bin/man/man2html?2+wait">wait</A></B>(2)
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or similar in another thread).
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</DL>
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<P>
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If any of these conditions does not hold,
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then the child process (along with a PID file descriptor that refers to it)
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should instead be created using
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<B><A HREF="/cgi-bin/man/man2html?2+clone">clone</A></B>(2)
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with the
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<B>CLONE_PIDFD</B>
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flag.
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<A NAME="lbAJ"> </A>
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<H3>Use cases for PID file descriptors</H3>
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<P>
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A PID file descriptor returned by
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<B>pidfd_open</B>()
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(of by
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<B><A HREF="/cgi-bin/man/man2html?2+clone">clone</A></B>(2)
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with the
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<B>CLONE_PID</B>
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flag) can be used for the following purposes:
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<DL COMPACT>
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<DT id="11">*<DD>
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The
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<B><A HREF="/cgi-bin/man/man2html?2+pidfd_send_signal">pidfd_send_signal</A></B>(2)
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system call can be used to send a signal to the process referred to by
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a PID file descriptor.
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<DT id="12">*<DD>
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A PID file descriptor can be monitored using
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<B><A HREF="/cgi-bin/man/man2html?2+poll">poll</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+select">select</A></B>(2),
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and
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<B><A HREF="/cgi-bin/man/man2html?7+epoll">epoll</A></B>(7).
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When the process that it refers to terminates,
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these interfaces indicate the file descriptor as readable.
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Note, however, that in the current implementation,
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nothing can be read from the file descriptor
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(<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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on the file descriptor fails with the error
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<B>EINVAL</B>).
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<DT id="13">*<DD>
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If the PID file descriptor refers to a child of the calling process,
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then it can be waited on using
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<B><A HREF="/cgi-bin/man/man2html?2+waitid">waitid</A></B>(2).
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</DL>
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<P>
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The
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<B>pidfd_open</B>()
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system call is the preferred way of obtaining a PID file descriptor
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for an already existing process.
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The alternative is to obtain a file descriptor by opening a
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<I>/proc/[pid]</I>
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directory.
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However, the latter technique is possible only if the
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<B><A HREF="/cgi-bin/man/man2html?5+proc">proc</A></B>(5)
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filesystem is mounted;
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furthermore, the file descriptor obtained in this way is
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<I>not</I>
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pollable and can't be waited on with
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<B><A HREF="/cgi-bin/man/man2html?2+waitid">waitid</A></B>(2).
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<A NAME="lbAK"> </A>
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<H2>EXAMPLE</H2>
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The program below opens a PID file descriptor for the
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process whose PID is specified as its command-line argument.
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It then uses
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<B><A HREF="/cgi-bin/man/man2html?2+poll">poll</A></B>(2)
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to monitor the file descriptor for process exit, as indicated by an
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<B>EPOLLIN</B>
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event.
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<A NAME="lbAL"> </A>
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<H3>Program source</H3>
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<PRE>
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#define _GNU_SOURCE
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#include <<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>>
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#include <<A HREF="file:///usr/include/sys/syscall.h">sys/syscall.h</A>>
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#include <<A HREF="file:///usr/include/unistd.h">unistd.h</A>>
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#include <<A HREF="file:///usr/include/poll.h">poll.h</A>>
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#include <<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>>
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#include <<A HREF="file:///usr/include/stdio.h">stdio.h</A>>
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#ifndef __NR_pidfd_open
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#define __NR_pidfd_open 434 /* System call # on most architectures */
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#endif
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static int
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pidfd_open(pid_t pid, unsigned int flags)
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{
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return syscall(__NR_pidfd_open, pid, flags);
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}
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int
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main(int argc, char *argv[])
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{
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struct pollfd pollfd;
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int pidfd, ready;
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if (argc != 2) {
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fprintf(stderr, "Usage: %s <pid>\n", argv[0]);
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exit(EXIT_SUCCESS);
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}
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pidfd = pidfd_open(atoi(argv[1]), 0);
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if (pidfd == -1) {
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perror("pidfd_open");
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exit(EXIT_FAILURE);
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}
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pollfd.fd = pidfd;
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pollfd.events = POLLIN;
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ready = poll(&pollfd, 1, -1);
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if (ready == -1) {
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perror("poll");
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exit(EXIT_FAILURE);
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}
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printf("Events (0x%x): POLLIN is %sset\n", pollfd.revents,
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(pollfd.revents & POLLIN) ? "" : "not ");
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exit(EXIT_SUCCESS);
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}
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</PRE>
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<A NAME="lbAM"> </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+kill">kill</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+pidfd_send_signal">pidfd_send_signal</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+poll">poll</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+select">select</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+waitid">waitid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+epoll">epoll</A></B>(7)
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<A NAME="lbAN"> </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="14"><A HREF="#lbAB">NAME</A><DD>
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<DT id="15"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="16"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="17"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="18"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="19"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="20"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="21"><A HREF="#lbAI">NOTES</A><DD>
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<DL>
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<DT id="22"><A HREF="#lbAJ">Use cases for PID file descriptors</A><DD>
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</DL>
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<DT id="23"><A HREF="#lbAK">EXAMPLE</A><DD>
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<DL>
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<DT id="24"><A HREF="#lbAL">Program source</A><DD>
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</DL>
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<DT id="25"><A HREF="#lbAM">SEE ALSO</A><DD>
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<DT id="26"><A HREF="#lbAN">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: 00:05:33 GMT, March 31, 2021
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</BODY>
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</HTML>
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