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(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);

VERSIONS
epoll_pwait() was added to Linux in kernel 2.6.19.

NOTES

  Glibc Notes
Support for epoll_pwait() is provided starting with glibc 2.6.

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.

CONFORMING TO
epoll_wait(2) is Linux specific, and was introduced in kernel 2.5.44.

SEE ALSO
epoll_create(2), epoll_ctl(2), epoll(7)