EPOLL_WAIT(2)

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 3.15 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/.