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 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 argument 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.

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 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_pwait() was added to Linux in kernel 2.6.19.

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

CONFORMING TO

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

SEE ALSO

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

COLOPHON

This page is part of release 3.12 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/.


 epoll by Davide Libenzi ( efficient event notification retrieval )
 Copyright (C) 2003  Davide Libenzi

 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
 the Free Software Foundation; either version 2 of the License, or
 (at your option) any later version.

 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.

 You should have received a copy of the GNU General Public License
 along with this program; if not, write to the Free Software
 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

 Davide Libenzi <davidelxmailserver.org>

2007-04-30: mtk, Added description of epoll_pwait()