epoll_wait (2)
Leading comments
Copyright (C) 2003 Davide Libenzi Davide Libenzi <davidel@xmailserver.org> %%%LICENSE_START(GPLv2+_SW_3_PARA) 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 MERCHANT...
NAME
epoll_wait, epoll_pwait - wait for an I/O event on an epoll file descriptorSYNOPSIS
#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 number of milliseconds that epoll_wait() will block. The call will block until either:
- *
- a file descriptor delivers an event;
- *
- the call is interrupted by a signal handler; or
- *
- the timeout expires.
Note that the
timeout
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;
pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = epoll_wait(epfd, &events, maxevents, timeout);
pthread_sigmask(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 (1) any of the requested events occurred or (2) 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).