.\" Copyright (C) 2003 Davide Libenzi .\" Davide Libenzi .\" and Copyright 2007, 2012, 2014, 2018 Michael Kerrisk .\" .\" %%%LICENSE_START(GPLv2+_SW_3_PARA) .\" This program is free software; you can redistribute it and/or modify .\" it under the terms of the GNU General Public License as published by .\" the Free Software Foundation; either version 2 of the License, or .\" (at your option) any later version. .\" .\" This program is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, see .\" . .\" %%%LICENSE_END .\" .\" 2007-04-30: mtk, Added description of epoll_pwait() .\" .TH EPOLL_WAIT 2 2020-04-11 "Linux" "Linux Programmer's Manual" .SH NAME epoll_wait, epoll_pwait \- wait for an I/O event on an epoll file descriptor .SH SYNOPSIS .nf .B #include .PP .BI "int epoll_wait(int " epfd ", struct epoll_event *" events , .BI " int " maxevents ", int " timeout ); .BI "int epoll_pwait(int " epfd ", struct epoll_event *" events , .BI " int " maxevents ", int " timeout , .BI " const sigset_t *" sigmask ); .fi .SH DESCRIPTION The .BR epoll_wait () system call waits for events on the .BR epoll (7) instance referred to by the file descriptor .IR epfd . The buffer pointed to by .I events is used to return information from the ready list about file descriptors in the interest list that have some events available. Up to .I maxevents are returned by .BR epoll_wait (). The .I maxevents argument must be greater than zero. .PP The .I timeout argument specifies the number of milliseconds that .BR epoll_wait () will block. Time is measured against the .B CLOCK_MONOTONIC clock. .PP A call to .BR epoll_wait () will block until either: .IP \(bu 2 a file descriptor delivers an event; .IP \(bu the call is interrupted by a signal handler; or .IP \(bu the timeout expires. .PP Note that the .I 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 .I timeout of \-1 causes .BR epoll_wait () to block indefinitely, while specifying a .I timeout equal to zero cause .BR epoll_wait () to return immediately, even if no events are available. .PP The .I struct epoll_event is defined as: .PP .in +4n .EX 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 */ }; .EE .in .PP The .I data field of each returned .I epoll_event structure contains the same data as was specified in the most recent call to .BR epoll_ctl (2) .RB ( EPOLL_CTL_ADD ", " EPOLL_CTL_MOD ) for the corresponding open file descriptor. .PP The .I events field is a bit mask that indicates the events that have occurred for the corresponding open file description. See .BR epoll_ctl (2) for a list of the bits that may appear in this mask. .\" .SS epoll_pwait() The relationship between .BR epoll_wait () and .BR epoll_pwait () is analogous to the relationship between .BR select (2) and .BR pselect (2): like .BR pselect (2), .BR epoll_pwait () allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught. .PP The following .BR epoll_pwait () call: .PP .in +4n .EX ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask); .EE .in .PP is equivalent to .I atomically executing the following calls: .PP .in +4n .EX sigset_t origmask; pthread_sigmask(SIG_SETMASK, &sigmask, &origmask); ready = epoll_wait(epfd, &events, maxevents, timeout); pthread_sigmask(SIG_SETMASK, &origmask, NULL); .EE .in .PP The .I sigmask argument may be specified as NULL, in which case .BR epoll_pwait () is equivalent to .BR epoll_wait (). .SH RETURN VALUE When successful, .BR 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 .I timeout milliseconds. When an error occurs, .BR epoll_wait () returns \-1 and .I errno is set appropriately. .SH ERRORS .TP .B EBADF .I epfd is not a valid file descriptor. .TP .B EFAULT The memory area pointed to by .I events is not accessible with write permissions. .TP .B EINTR The call was interrupted by a signal handler before either (1) any of the requested events occurred or (2) the .I timeout expired; see .BR signal (7). .TP .B EINVAL .I epfd is not an .B epoll file descriptor, or .I maxevents is less than or equal to zero. .SH VERSIONS .BR epoll_wait () was added to the kernel in version 2.6. .\" To be precise: kernel 2.5.44. .\" The interface should be finalized by Linux kernel 2.5.66. Library support is provided in glibc starting with version 2.3.2. .PP .BR epoll_pwait () was added to Linux in kernel 2.6.19. Library support is provided in glibc starting with version 2.6. .SH CONFORMING TO .BR epoll_wait () is Linux-specific. .SH NOTES While one thread is blocked in a call to .BR epoll_wait (), it is possible for another thread to add a file descriptor to the waited-upon .B epoll instance. If the new file descriptor becomes ready, it will cause the .BR epoll_wait () call to unblock. .PP If more than .I maxevents file descriptors are ready when .BR epoll_wait () is called, then successive .BR 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. .PP Note that it is possible to call .BR epoll_wait () on an .B 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. .SH BUGS In kernels before 2.6.37, a .I timeout value larger than approximately .I LONG_MAX / HZ milliseconds is treated as \-1 (i.e., infinity). Thus, for example, on a system where .I sizeof(long) is 4 and the kernel .I HZ value is 1000, this means that timeouts greater than 35.79 minutes are treated as infinity. .SS C library/kernel differences The raw .BR epoll_pwait () system call has a sixth argument, .IR "size_t sigsetsize" , which specifies the size in bytes of the .IR sigmask argument. The glibc .BR epoll_pwait () wrapper function specifies this argument as a fixed value (equal to .IR sizeof(sigset_t) ). .SH SEE ALSO .BR epoll_create (2), .BR epoll_ctl (2), .BR epoll (7) .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.