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 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 call waits for a maximum time of timeout milliseconds. 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 3.24 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()