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 minimum number of milliseconds that epoll_wait()  will  block.   (This
       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;

           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 either 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_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.

SEE ALSO

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

COLOPHON

       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/.

Linux                                              2012-08-17                                      EPOLL_WAIT(2)