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(7) instance referred to by the file descriptor epfd. 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.

The timeout argument specifies the minimum number of milliseconds that epoll_wait() will block. (This 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 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);

The sigmask argument may be specified as NULL, in which case epoll_pwait() is equivalent to epoll_wait().


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 either any of the requested events occurred or 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 to the kernel in version 2.6. Library support is provided in glibc starting with version 2.3.2.

epoll_pwait() was added to Linux in kernel 2.6.19. Library support is provided in glibc starting with version 2.6.


epoll_wait() is Linux-specific.


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

For a discussion of what may happen if a file descriptor in an epoll instance being monitored by epoll_wait() is closed in another thread, see select(2).


In kernels before 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 the 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)


This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

   Copyright (C) 2003  Davide Libenzi
 Davide Libenzi <>

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2007-04-30: mtk, Added description of epoll_pwait()