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

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

       #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);

       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

       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

       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

       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.

       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.

       EBADF  epfd is not a valid file descriptor.

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

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

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

       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.

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

       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.

       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

   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)).

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

       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

Linux                              2021-03-22                      EPOLL_WAIT(2)