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/.