epoll_wait

EPOLL_WAIT(2)              Linux Programmer's Manual             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 memory area pointed to by
       events will contain the events that will be available for the caller.
       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.  The call 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 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 description.  The events
       field contains the returned event bit field.

   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().

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 milliseconds.  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.

       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_pwait(), 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
       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() 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 5.02 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                             2019-03-06                     EPOLL_WAIT(2)