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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 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 call waits for a maximum time of timeout milliseconds.  Specifying  a  timeout  of  -1
       makes  epoll_wait()  wait  indefinitely,  while  specifying  a timeout equal to zero makes
       epoll_wait() to return immediately even if no events are available (return code  equal  to
       zero).

       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

       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 any of the requested events
              occurred or 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.

SEE ALSO

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

COLOPHON

       This page is part of release 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.