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.   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  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;

           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.

       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

       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.

   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 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                                              2015-05-07                                      EPOLL_WAIT(2)