#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(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 minimum number of milliseconds  that
       epoll_wait() will block.  (This interval will be rounded up to the sys-
       tem 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.

       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);

       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  either  any
              of  the  requested  events  occurred or the timeout expired; see

       EINVAL epfd is not an epoll file descriptor, or maxevents is less  than
              or equal to zero.

       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.

       epoll_wait() is Linux-specific.

       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).

       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.

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

       This page is part of release 3.54 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/.
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