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  -  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);

DESCRIPTION         top

       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.  The call will block until either:

       *  a file descriptor delivers an event;

       *  the call is interrupted by a signal handler; or

       *  the timout 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 of each returned structure will contain the same data the
       user set with an epoll_ctl(2) (EPOLL_CTL_ADD, EPOLL_CTL_MOD) while
       the events member will contain 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;

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

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

RETURN VALUE         top

       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         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.

CONFORMING TO         top

       epoll_wait() is Linux-specific.

NOTES         top

       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.

       For a discussion of what may happen if a file descriptor in an epoll
       instance being monitored by epoll_wait() is closed in another thread,
       see select(2).

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 the 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

       This page is part of release 3.64 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                            2014-01-31                    EPOLL_WAIT(2)