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EPOLL_WAIT

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

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 number of milliseconds that epoll_wait() will block. Time is measured against the CLOCK_MONOTONIC clock. 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 field of each returned structure contains the same data as was specified in the most recent call to epoll_ctl(2) (EPOLL_CTL_ADD, EPOLL_CTL_MOD) for the corresponding open file description. The events field contains 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).

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 sizeof(long) is 4 and the kernel HZ value is 1000, this means that timeouts greater than 35.79 minutes are treated as infinity.

C library/kernel differences

The raw epoll_pwait() system call has a sixth argument, size_t sigsetsize, which specifies the size in bytes of the sigmask argument. The glibc epoll_pwait() wrapper function specifies this argument as a fixed value (equal to sizeof(sigset_t)).

COLOPHON

This page is part of release 4.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.