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epoll_wait(2) System Calls Manual epoll_wait(2)


epoll_wait, epoll_pwait, epoll_pwait2 - wait for an I/O event on an epoll file descriptor


Standard C library (libc, -lc)


#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 *_Nullable sigmask);
int epoll_pwait2(int epfd, struct epoll_event *events,
               int maxevents, const struct timespec *_Nullable timeout,
               const sigset_t *_Nullable sigmask);


The epoll_wait() system call waits for events on the epoll(7) instance referred to by the file descriptor epfd. The buffer pointed to by events is used to return information from the ready list about file descriptors in the interest list that have some events available. 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.

A call to epoll_wait() 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 described in epoll_event(3type).

The data field of each returned epoll_event 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 descriptor.

The events field is a bit mask that indicates the events that have occurred for the corresponding open file description. See epoll_ctl(2) for a list of the bits that may appear in this mask.


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().


The epoll_pwait2() system call is equivalent to epoll_pwait() except for the timeout argument. It takes an argument of type timespec to be able to specify nanosecond resolution timeout. This argument functions the same as in pselect(2) and ppoll(2). If timeout is NULL, then epoll_pwait2() can block indefinitely.


On success, 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. On failure, epoll_wait() returns -1 and errno is set to indicate the error.


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 either (1) any of the requested events occurred or (2) the timeout expired; see signal(7).
epfd is not an epoll file descriptor, or maxevents is less than or equal to zero.


epoll_wait() was added in Linux 2.6. Library support is provided in glibc 2.3.2.

epoll_pwait() was added in Linux 2.6.19. Library support is provided in glibc 2.6.

epoll_pwait2() was added in Linux 5.11.


epoll_wait(), epoll_pwait(), and epoll_pwait2() are Linux-specific.


While one thread is blocked in a call to epoll_wait(), 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.

If more than maxevents file descriptors are ready when epoll_wait() is called, then successive epoll_wait() calls will round robin through the set of ready file descriptors. This behavior helps avoid starvation scenarios, where a process fails to notice that additional file descriptors are ready because it focuses on a set of file descriptors that are already known to be ready.

Note that it is possible to call epoll_wait() on an epoll instance whose interest list is currently empty (or whose interest list becomes empty because file descriptors are closed or removed from the interest in another thread). The call will block until some file descriptor is later added to the interest list (in another thread) and that file descriptor becomes ready.

C library/kernel differences

The raw epoll_pwait() and epoll_pwait2() system calls have 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)).


Before Linux 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.


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

2023-02-05 Linux man-pages 6.03