epoll_wait, epoll_pwait - wait for an I/O event on an epoll file descriptor


#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);


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.


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);


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.


epfd is not a valid file descriptor.
The memory area pointed to by events is not accessible with write permissions.
The call was interrupted by a signal handler before any of the requested events occurred or the timeout expired.
epfd is not an epoll file descriptor, or maxevents is less than or equal to zero.


epoll_pwait() was added to Linux in kernel 2.6.19.

Glibc support for epoll_pwait() is provided starting with version 2.6.


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


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