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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |NOTES |BUGS |SEE ALSO |COLOPHON | |
epoll_wait(2) System Calls Manualepoll_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;intepfd, struct epoll_eventevents[maxevents],intmaxevents, inttimeout);int epoll_pwait(int maxevents;intepfd, struct epoll_eventevents[maxevents],intmaxevents, inttimeout,const sigset_t *_Nullablesigmask);int epoll_pwait2(int maxevents;intepfd, struct epoll_eventevents[maxevents],intmaxevents, const struct timespec *_Nullabletimeout,const sigset_t *_Nullablesigmask);
Theepoll_wait() system call waits for events on theepoll(7) instance referred to by the file descriptorepfd. The buffer pointed to byevents is used to return information from the ready list about file descriptors in the interest list that have some events available. Up tomaxevents are returned byepoll_wait(). Themaxevents argument must be greater than zero. Thetimeout argument specifies the number of milliseconds thatepoll_wait() will block. Time is measured against theCLOCK_MONOTONICclock. A call toepoll_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 thetimeout 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 atimeout of -1 causesepoll_wait() to block indefinitely, while specifying atimeout equal to zero causesepoll_wait() to return immediately, even if no events are available. Thestruct epoll_event is described inepoll_event(3type). Thedata field of each returnedepoll_event structure contains the same data as was specified in the most recent call toepoll_ctl(2) (EPOLL_CTL_ADD,EPOLL_CTL_MOD) for the corresponding open file descriptor. Theevents field is a bit mask that indicates the events that have occurred for the corresponding open file description. Seeepoll_ctl(2) for a list of the bits that may appear in this mask.epoll_pwait() The relationship betweenepoll_wait() andepoll_pwait() is analogous to the relationship betweenselect(2) andpselect(2): likepselect(2),epoll_pwait() allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught. The followingepoll_pwait() call: ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask); is equivalent toatomically 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); Thesigmask argument may be specified as NULL, in which caseepoll_pwait() is equivalent toepoll_wait().epoll_pwait2() Theepoll_pwait2() system call is equivalent toepoll_pwait() except for thetimeout argument. It takes an argument of typetimespec to be able to specify nanosecond resolution timeout. This argument functions the same as inpselect(2) andppoll(2). Iftimeout is NULL, thenepoll_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 requestedtimeout milliseconds. On failure,epoll_wait() returns -1 anderrno is set to indicate the error.
EBADFepfd is not a valid file descriptor.EFAULTThe memory area pointed to byevents is not accessible with write permissions.EINTRThe call was interrupted by a signal handler before either (1) any of the requested events occurred or (2) thetimeout expired; seesignal(7).EINVALepfd is not anepollfile descriptor, ormaxevents 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 toepoll_wait(), it is possible for another thread to add a file descriptor to the waited-uponepollinstance. If the new file descriptor becomes ready, it will cause theepoll_wait() call to unblock. If more thanmaxevents file descriptors are ready whenepoll_wait() is called, then successiveepoll_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 callepoll_wait() on anepollinstance 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 rawepoll_pwait() andepoll_pwait2() system calls have a sixth argument,size_t sigsetsize, which specifies the size in bytes of thesigmask argument. The glibcepoll_pwait() wrapper function specifies this argument as a fixed value (equal tosizeof(sigset_t)).
Before Linux 2.6.37, atimeout value larger than approximatelyLONG_MAX / HZ milliseconds is treated as -1 (i.e., infinity). Thus, for example, on a system wheresizeof(long) is 4 and the kernelHZ 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 theman-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 isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-06-28epoll_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)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |