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


       #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  file
       descriptor epfd for a maximum time of timeout milliseconds.  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  parameter  must  be  greater  than  zero.
       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);


       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.

       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)


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