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 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 indefi-
       nitely, 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  pse-
       lect(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_pwait() was added to Linux in kernel 2.6.19.

       Glibc support for epoll_pwait() is provided starting with version 2.6.

CONFORMING TO
       epoll_wait() is Linux-specific, and was introduced in kernel 2.5.44.

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

COLOPHON
       This  page  is  part of release 3.25 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                                                      2009-01-17                                              EPOLL_WAIT(2)