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EPOLL_WAIT(2)              Linux Programmer's Manual             EPOLL_WAIT(2)

       epoll_wait, epoll_pwait - wait for an I/O event on an epoll file descriptor

       #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);

       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 max-
       events 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.

       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().

       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.

       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.

       epoll_pwait() was added to Linux in kernel 2.6.19.

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

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

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

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

Linux                             2009-01-17                     EPOLL_WAIT(2)