epoll_wait(2) — Linux manual page


epoll_wait(2)              System Calls Manual             epoll_wait(2)

NAME         top

       epoll_wait, epoll_pwait, epoll_pwait2 - wait for an I/O event on
       an epoll file descriptor

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/epoll.h>

       int epoll_wait(int epfd, struct epoll_event *events,
                      int maxevents, int timeout);
       int epoll_pwait(int epfd, struct epoll_event *events,
                      int maxevents, int timeout,
                      const sigset_t *_Nullable sigmask);
       int epoll_pwait2(int epfd, struct epoll_event *events,
                      int maxevents, const struct timespec *_Nullable timeout,
                      const sigset_t *_Nullable sigmask);

DESCRIPTION         top

       The epoll_wait() system call waits for events on the epoll(7)
       instance referred to by the file descriptor epfd.  The buffer
       pointed to by events is used to return information from the ready
       list about file descriptors in the interest list that have some
       events available.  Up to maxevents are returned by epoll_wait().
       The maxevents argument must be greater than zero.

       The timeout argument specifies the number of milliseconds that
       epoll_wait() will block.  Time is measured against the
       CLOCK_MONOTONIC clock.

       A call to epoll_wait() will block until either:

       •  a file descriptor delivers an event;

       •  the call is interrupted by a signal handler; or

       •  the timeout expires.

       Note that the timeout interval will be rounded up to the system
       clock granularity, and kernel scheduling delays mean that the
       blocking interval may overrun by a small amount.  Specifying a
       timeout of -1 causes epoll_wait() to block indefinitely, while
       specifying a timeout equal to zero causes epoll_wait() to return
       immediately, even if no events are available.

       The struct epoll_event is described in epoll_event(3type).

       The data field of each returned epoll_event structure contains
       the same data as was specified in the most recent call to
       epoll_ctl(2) (EPOLL_CTL_ADD, EPOLL_CTL_MOD) for the corresponding
       open file descriptor.

       The events field is a bit mask that indicates the events that
       have occurred for the corresponding open file description.  See
       epoll_ctl(2) for a list of the bits that may appear in this mask.

       The relationship between epoll_wait() and epoll_pwait() is
       analogous to the relationship between select(2) and pselect(2):
       like pselect(2), epoll_pwait() allows an application to safely
       wait until either a file descriptor becomes ready or until a
       signal is caught.

       The following epoll_pwait() call:

           ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;

           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
           ready = epoll_wait(epfd, &events, maxevents, timeout);
           pthread_sigmask(SIG_SETMASK, &origmask, NULL);

       The sigmask argument may be specified as NULL, in which case
       epoll_pwait() is equivalent to epoll_wait().

       The epoll_pwait2() system call is equivalent to epoll_pwait()
       except for the timeout argument.  It takes an argument of type
       timespec to be able to specify nanosecond resolution timeout.
       This argument functions the same as in pselect(2) and ppoll(2).
       If timeout is NULL, then epoll_pwait2() can block indefinitely.

RETURN VALUE         top

       On success, epoll_wait() returns the number of file descriptors
       ready for the requested I/O operation, or zero if no file
       descriptor became ready during the requested timeout
       milliseconds.  On failure, epoll_wait() returns -1 and errno is
       set to indicate the error.

ERRORS         top

       EBADF  epfd is not a valid file descriptor.

       EFAULT The memory area pointed to by events is not accessible
              with write permissions.

       EINTR  The call was interrupted by a signal handler before either
              (1) any of the requested events occurred or (2) the
              timeout expired; see signal(7).

       EINVAL epfd is not an epoll file descriptor, or maxevents is less
              than or equal to zero.

STANDARDS         top


HISTORY         top

              Linux 2.6, glibc 2.3.2.

              Linux 2.6.19, glibc 2.6.

              Linux 5.11.

NOTES         top

       While one thread is blocked in a call to epoll_wait(), it is
       possible for another thread to add a file descriptor to the
       waited-upon epoll instance.  If the new file descriptor becomes
       ready, it will cause the epoll_wait() call to unblock.

       If more than maxevents file descriptors are ready when
       epoll_wait() is called, then successive epoll_wait() calls will
       round robin through the set of ready file descriptors.  This
       behavior helps avoid starvation scenarios, where a process fails
       to notice that additional file descriptors are ready because it
       focuses on a set of file descriptors that are already known to be

       Note that it is possible to call epoll_wait() on an epoll
       instance whose interest list is currently empty (or whose
       interest list becomes empty because file descriptors are closed
       or removed from the interest in another thread).  The call will
       block until some file descriptor is later added to the interest
       list (in another thread) and that file descriptor becomes ready.

   C library/kernel differences
       The raw epoll_pwait() and epoll_pwait2() system calls have a
       sixth argument, size_t sigsetsize, which specifies the size in
       bytes of the sigmask argument.  The glibc epoll_pwait() wrapper
       function specifies this argument as a fixed value (equal to

BUGS         top

       Before Linux 2.6.37, a timeout value larger than approximately
       LONG_MAX / HZ milliseconds is treated as -1 (i.e., infinity).
       Thus, for example, on a system where sizeof(long) is 4 and the
       kernel HZ value is 1000, this means that timeouts greater than
       35.79 minutes are treated as infinity.

SEE ALSO         top

       epoll_create(2), epoll_ctl(2), epoll(7)

COLOPHON         top

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       This page was obtained from the tarball man-pages-6.9.1.tar.gz
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Linux man-pages 6.9.1          2024-05-02                  epoll_wait(2)

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