EPOLL_WAIT(2)             (2020-04-11)              EPOLL_WAIT(2)

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

     SYNOPSIS
          #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);

     DESCRIPTION
          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 informa-
          tion 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 argu-
          ment 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:

          +o a file descriptor delivers an event;

          +o the call is interrupted by a signal handler; or

          +o 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;

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

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

          The events field is a bit mask that indicates the events
          that have occurred for the corresponding open file descrip-
          tion.  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 appli-
          cation 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().

     RETURN VALUE
          When successful, 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 mil-
          liseconds.  When an error occurs, epoll_wait() returns -1
          and errno is set appropriately.

     ERRORS
          EBADF
               epfd is not a valid file descriptor.

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

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          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
          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.

     CONFORMING TO
          epoll_wait() is Linux-specific.

     NOTES
          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 pro-
          cess 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 descrip-
          tor becomes ready.

     BUGS
          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

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          The raw epoll_pwait() system call has 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
          epoll_create(2), epoll_ctl(2), epoll(7)

     COLOPHON
          This page is part of release 5.10 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/.

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