epoll_wait(2) — Linux manual pages

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(2). The maxevents parameter must be greater than zero. Specifying a timeout of −1 makes epoll_wait(2) wait indefinitely, while specifying a timeout equal to zero makes epoll_wait(2) 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 a 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);

RETURN VALUE

When successful, epoll_wait(2) 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(2) 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.
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(2) is Linux specific, and was introduced in kernel 2.5.44.

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 <davidel@xmailserver.org>

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

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