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(). The maxevents parameter 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.


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() 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.


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


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


This page is part of release 2.78 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/.