epoll_wait(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

EPOLL_WAIT(2)           Linux Programmer's Manual          EPOLL_WAIT(2)

NAME         top

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

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 *sigmask);
       int epoll_pwait2(int epfd, struct epoll_event *events,
                      int maxevents, const struct timespec *timeout,
                      const sigset_t *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 cause epoll_wait() to return
       immediately, even if no events are available.

       The struct epoll_event is defined as:

           typedef union epoll_data {
               void    *ptr;
               int      fd;
               uint32_t u32;
               uint64_t u64;
           } epoll_data_t;

           struct epoll_event {
               uint32_t     events;    /* Epoll events */
               epoll_data_t data;      /* User data variable */
           };

       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.

   epoll_pwait()
       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().

   epoll_pwait2()
       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, 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.

VERSIONS         top

       epoll_wait() was added to the kernel in version 2.6.  Library
       support is provided in glibc starting with version 2.3.2.

       epoll_pwait() was added to Linux in kernel 2.6.19.  Library
       support is provided in glibc starting with version 2.6.

       epoll_pwait2() was added to Linux in kernel 5.11.

CONFORMING TO         top

       epoll_wait(), epoll_pwait(), and epoll_pwait2() are Linux-
       specific.

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
       ready.

       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.

BUGS         top

       In kernels before 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.

   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
       sizeof(sigset_t)).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                  EPOLL_WAIT(2)

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