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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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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 maxevents;
int epfd, struct epoll_event events[maxevents],
int maxevents, int timeout);
int epoll_pwait(int maxevents;
int epfd, struct epoll_event events[maxevents],
int maxevents, int timeout,
const sigset_t *_Nullable sigmask);
int epoll_pwait2(int maxevents;
int epfd, struct epoll_event events[maxevents],
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 causes 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.
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;
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().
epoll_pwait2()
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 operation, 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.
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 either
(1) any of the requested events occurred or (2) the timeout
expired; see signal(7).
EINVAL epfd is not an epoll file descriptor, or maxevents is less
than or equal to zero.
Linux.
epoll_wait()
Linux 2.6, glibc 2.3.2.
epoll_pwait()
Linux 2.6.19, glibc 2.6.
epoll_pwait2()
Linux 5.11.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 epoll_wait(2)
Pages that refer to this page: epoll_create(2), epoll_ctl(2), PR_SET_TIMERSLACK(2const), ptrace(2), seccomp_unotify(2), signalfd(2), syscalls(2), epoll_event(3type), io_uring_prep_epoll_wait(3), pcap_get_required_select_timeout(3pcap), pcap_get_selectable_fd(3pcap), proc_pid_mounts(5), epoll(7), signal(7), socket(7), system_data_types(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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