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8.6 KiB
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409 lines
8.6 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 EPOLL_WAIT</TITLE>
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</HEAD><BODY>
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<H1>EPOLL_WAIT</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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epoll_wait, epoll_pwait - wait for an I/O event on an epoll file descriptor
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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/epoll.h">sys/epoll.h</A>></B>
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<B>int epoll_wait(int </B><I>epfd</I><B>, struct epoll_event *</B><I>events</I><B>,</B>
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<B> int </B><I>maxevents</I><B>, int </B><I>timeout</I><B>);</B>
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<B>int epoll_pwait(int </B><I>epfd</I><B>, struct epoll_event *</B><I>events</I><B>,</B>
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<B> int </B><I>maxevents</I><B>, int </B><I>timeout</I><B>,</B>
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<B> const sigset_t *</B><I>sigmask</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>epoll_wait</B>()
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system call waits for events on the
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<B><A HREF="/cgi-bin/man/man2html?7+epoll">epoll</A></B>(7)
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instance referred to by the file descriptor
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<I>epfd</I>.
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The memory area pointed to by
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<I>events</I>
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will contain the events that will be available for the caller.
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Up to
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<I>maxevents</I>
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are returned by
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<B>epoll_wait</B>().
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The
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<I>maxevents</I>
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argument must be greater than zero.
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<P>
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The
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<I>timeout</I>
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argument specifies the number of milliseconds that
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<B>epoll_wait</B>()
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will block.
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Time is measured against the
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<B>CLOCK_MONOTONIC</B>
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clock.
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The call will block until either:
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<DL COMPACT>
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<DT id="1">*<DD>
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a file descriptor delivers an event;
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<DT id="2">*<DD>
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the call is interrupted by a signal handler; or
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<DT id="3">*<DD>
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the timeout expires.
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</DL>
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<P>
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Note that the
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<I>timeout</I>
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interval will be rounded up to the system clock granularity,
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and kernel scheduling delays mean that the blocking interval
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may overrun by a small amount.
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Specifying a
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<I>timeout</I>
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of -1 causes
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<B>epoll_wait</B>()
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to block indefinitely, while specifying a
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<I>timeout</I>
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equal to zero cause
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<B>epoll_wait</B>()
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to return immediately, even if no events are available.
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<P>
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The
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<I>struct epoll_event</I>
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is defined as:
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<P>
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typedef union epoll_data {
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<BR> void *ptr;
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<BR> int fd;
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<BR> uint32_t u32;
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<BR> uint64_t u64;
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} epoll_data_t;
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<P>
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struct epoll_event {
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<BR> uint32_t events; /* Epoll events */
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<BR> epoll_data_t data; /* User data variable */
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};
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<P>
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The
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<I>data</I>
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field of each returned structure contains the same data as was specified
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in the most recent call to
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<B><A HREF="/cgi-bin/man/man2html?2+epoll_ctl">epoll_ctl</A></B>(2)
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(<B>EPOLL_CTL_ADD</B>, <B>EPOLL_CTL_MOD</B>)
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for the corresponding open file description.
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The
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<I>events</I>
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field contains the returned event bit field.
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<A NAME="lbAE"> </A>
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<H3>epoll_pwait()</H3>
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The relationship between
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<B>epoll_wait</B>()
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and
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<B>epoll_pwait</B>()
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is analogous to the relationship between
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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?2+pselect">pselect</A></B>(2):
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like
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<B><A HREF="/cgi-bin/man/man2html?2+pselect">pselect</A></B>(2),
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<B>epoll_pwait</B>()
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allows an application to safely wait until either a file descriptor
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becomes ready or until a signal is caught.
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<P>
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The following
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<B>epoll_pwait</B>()
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call:
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<P>
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ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
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<P>
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is equivalent to
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<I>atomically</I>
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executing the following calls:
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<P>
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sigset_t origmask;
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<P>
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pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
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ready = epoll_wait(epfd, &events, maxevents, timeout);
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pthread_sigmask(SIG_SETMASK, &origmask, NULL);
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<P>
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The
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<I>sigmask</I>
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argument may be specified as NULL, in which case
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<B>epoll_pwait</B>()
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is equivalent to
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<B>epoll_wait</B>().
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<A NAME="lbAF"> </A>
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<H2>RETURN VALUE</H2>
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When successful,
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<B>epoll_wait</B>()
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returns the number of file descriptors ready for the requested I/O, or zero
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if no file descriptor became ready during the requested
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<I>timeout</I>
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milliseconds.
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When an error occurs,
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<B>epoll_wait</B>()
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returns -1 and
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<I>errno</I>
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is set appropriately.
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<A NAME="lbAG"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="4"><B>EBADF</B>
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<DD>
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<I>epfd</I>
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is not a valid file descriptor.
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<DT id="5"><B>EFAULT</B>
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<DD>
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The memory area pointed to by
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<I>events</I>
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is not accessible with write permissions.
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<DT id="6"><B>EINTR</B>
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<DD>
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The call was interrupted by a signal handler before either (1) any of the
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requested events occurred or (2) the
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<I>timeout</I>
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expired; see
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<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7).
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<DT id="7"><B>EINVAL</B>
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<DD>
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<I>epfd</I>
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is not an
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<B>epoll</B>
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file descriptor, or
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<I>maxevents</I>
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is less than or equal to zero.
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</DL>
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<A NAME="lbAH"> </A>
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<H2>VERSIONS</H2>
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<B>epoll_wait</B>()
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was added to the kernel in version 2.6.
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Library support is provided in glibc starting with version 2.3.2.
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<P>
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<B>epoll_pwait</B>()
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was added to Linux in kernel 2.6.19.
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Library support is provided in glibc starting with version 2.6.
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<A NAME="lbAI"> </A>
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<H2>CONFORMING TO</H2>
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<B>epoll_wait</B>()
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is Linux-specific.
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<A NAME="lbAJ"> </A>
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<H2>NOTES</H2>
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While one thread is blocked in a call to
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<B>epoll_wait</B>(),
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it is possible for another thread to add a file descriptor to the waited-upon
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<B>epoll</B>
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instance.
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If the new file descriptor becomes ready,
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it will cause the
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<B>epoll_wait</B>()
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call to unblock.
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<P>
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If more than
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<I>maxevents</I>
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file descriptors are ready when
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<B>epoll_wait</B>()
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is called, then successive
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<B>epoll_wait</B>()
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calls will round robin through the set of ready file descriptors.
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This behavior helps avoid starvation scenarios,
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where a process fails to notice that additional file descriptors
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are ready because it focuses on a set of file descriptors that
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are already known to be ready.
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<P>
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Note that it is possible to call
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<B>epoll_wait</B>()
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on an
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<B>epoll</B>
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instance whose interest list is currently empty
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(or whose interest list becomes empty because file descriptors are closed
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or removed from the interest in another thread).
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The call will block until some file descriptor is later added to the
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interest list (in another thread) and that file descriptor becomes ready.
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<A NAME="lbAK"> </A>
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<H2>BUGS</H2>
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In kernels before 2.6.37, a
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<I>timeout</I>
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value larger than approximately
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<I>LONG_MAX / HZ</I>
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milliseconds is treated as -1 (i.e., infinity).
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Thus, for example, on a system where
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<I>sizeof(long)</I>
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is 4 and the kernel
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<I>HZ</I>
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value is 1000,
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this means that timeouts greater than 35.79 minutes are treated as infinity.
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<A NAME="lbAL"> </A>
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<H3>C library/kernel differences</H3>
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The raw
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<B>epoll_pwait</B>()
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system call has a sixth argument,
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<I>size_t sigsetsize</I>,
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which specifies the size in bytes of the
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<I>sigmask</I>
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argument.
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The glibc
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<B>epoll_pwait</B>()
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wrapper function specifies this argument as a fixed value
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(equal to
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<I>sizeof(sigset_t)</I>).
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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+epoll_create">epoll_create</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+epoll_ctl">epoll_ctl</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="8"><A HREF="#lbAB">NAME</A><DD>
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<DT id="9"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="10"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DL>
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<DT id="11"><A HREF="#lbAE">epoll_pwait()</A><DD>
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</DL>
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<DT id="12"><A HREF="#lbAF">RETURN VALUE</A><DD>
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<DT id="13"><A HREF="#lbAG">ERRORS</A><DD>
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<DT id="14"><A HREF="#lbAH">VERSIONS</A><DD>
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<DT id="15"><A HREF="#lbAI">CONFORMING TO</A><DD>
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<DT id="16"><A HREF="#lbAJ">NOTES</A><DD>
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<DT id="17"><A HREF="#lbAK">BUGS</A><DD>
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<DL>
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<DT id="18"><A HREF="#lbAL">C library/kernel differences</A><DD>
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</DL>
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<DT id="19"><A HREF="#lbAM">SEE ALSO</A><DD>
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<DT id="20"><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: 00:05:32 GMT, March 31, 2021
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