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1 .\" Copyright (C) 2003 Davide Libenzi
2 .\" Davide Libenzi <davidel@xmailserver.org>
3 .\" and Copyright 2007, 2012, 2014, 2018 Michael Kerrisk <tk.manpages@gmail.com>
4 .\"
5 .\" SPDX-License-Identifier: GPL-2.0-or-later
6 .\"
7 .\" 2007-04-30: mtk, Added description of epoll_pwait()
8 .\"
9 .TH epoll_wait 2 (date) "Linux man-pages (unreleased)"
10 .SH NAME
11 epoll_wait, epoll_pwait, epoll_pwait2 \-
12 wait for an I/O event on an epoll file descriptor
13 .SH LIBRARY
14 Standard C library
15 .RI ( libc ", " \-lc )
16 .SH SYNOPSIS
17 .nf
18 .B #include <sys/epoll.h>
19 .PP
20 .BI "int epoll_wait(int " epfd ", struct epoll_event *" events ,
21 .BI " int " maxevents ", int " timeout );
22 .BI "int epoll_pwait(int " epfd ", struct epoll_event *" events ,
23 .BI " int " maxevents ", int " timeout ,
24 .BI " const sigset_t *_Nullable " sigmask );
25 .BI "int epoll_pwait2(int " epfd ", struct epoll_event *" events ,
26 .BI " int " maxevents ", \
27 const struct timespec *_Nullable " timeout ,
28 .BI " const sigset_t *_Nullable " sigmask );
29 .fi
30 .SH DESCRIPTION
31 The
32 .BR epoll_wait ()
33 system call waits for events on the
34 .BR epoll (7)
35 instance referred to by the file descriptor
36 .IR epfd .
37 The buffer pointed to by
38 .I events
39 is used to return information from the ready list
40 about file descriptors in the interest list that
41 have some events available.
42 Up to
43 .I maxevents
44 are returned by
45 .BR epoll_wait ().
46 The
47 .I maxevents
48 argument must be greater than zero.
49 .PP
50 The
51 .I timeout
52 argument specifies the number of milliseconds that
53 .BR epoll_wait ()
54 will block.
55 Time is measured against the
56 .B CLOCK_MONOTONIC
57 clock.
58 .PP
59 A call to
60 .BR epoll_wait ()
61 will block until either:
62 .IP \[bu] 3
63 a file descriptor delivers an event;
64 .IP \[bu]
65 the call is interrupted by a signal handler; or
66 .IP \[bu]
67 the timeout expires.
68 .PP
69 Note that the
70 .I timeout
71 interval will be rounded up to the system clock granularity,
72 and kernel scheduling delays mean that the blocking interval
73 may overrun by a small amount.
74 Specifying a
75 .I timeout
76 of \-1 causes
77 .BR epoll_wait ()
78 to block indefinitely, while specifying a
79 .I timeout
80 equal to zero cause
81 .BR epoll_wait ()
82 to return immediately, even if no events are available.
83 .PP
84 The
85 .I struct epoll_event
86 is described in
87 .BR epoll_event (3type).
88 .PP
89 The
90 .I data
91 field of each returned
92 .I epoll_event
93 structure contains the same data as was specified
94 in the most recent call to
95 .BR epoll_ctl (2)
96 .RB ( EPOLL_CTL_ADD ", " EPOLL_CTL_MOD )
97 for the corresponding open file descriptor.
98 .PP
99 The
100 .I events
101 field is a bit mask that indicates the events that have occurred for the
102 corresponding open file description.
103 See
104 .BR epoll_ctl (2)
105 for a list of the bits that may appear in this mask.
106 .\"
107 .SS epoll_pwait()
108 The relationship between
109 .BR epoll_wait ()
110 and
111 .BR epoll_pwait ()
112 is analogous to the relationship between
113 .BR select (2)
114 and
115 .BR pselect (2):
116 like
117 .BR pselect (2),
118 .BR epoll_pwait ()
119 allows an application to safely wait until either a file descriptor
120 becomes ready or until a signal is caught.
121 .PP
122 The following
123 .BR epoll_pwait ()
124 call:
125 .PP
126 .in +4n
127 .EX
128 ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
129 .EE
130 .in
131 .PP
132 is equivalent to
133 .I atomically
134 executing the following calls:
135 .PP
136 .in +4n
137 .EX
138 sigset_t origmask;
140 pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
141 ready = epoll_wait(epfd, &events, maxevents, timeout);
142 pthread_sigmask(SIG_SETMASK, &origmask, NULL);
143 .EE
144 .in
145 .PP
146 The
147 .I sigmask
148 argument may be specified as NULL, in which case
149 .BR epoll_pwait ()
150 is equivalent to
151 .BR epoll_wait ().
152 .\"
153 .SS epoll_pwait2()
154 The
155 .BR epoll_pwait2 ()
156 system call is equivalent to
157 .BR epoll_pwait ()
158 except for the
159 .I timeout
160 argument.
161 It takes an argument of type
162 .I timespec
163 to be able to specify nanosecond resolution timeout.
164 This argument functions the same as in
165 .BR pselect (2)
166 and
167 .BR ppoll (2).
168 If
169 .I timeout
170 is NULL, then
171 .BR epoll_pwait2 ()
172 can block indefinitely.
173 .SH RETURN VALUE
174 On success,
175 .BR epoll_wait ()
176 returns the number of file descriptors ready for the requested I/O, or zero
177 if no file descriptor became ready during the requested
178 .I timeout
179 milliseconds.
180 On failure,
181 .BR epoll_wait ()
182 returns \-1 and
183 .I errno
184 is set to indicate the error.
185 .SH ERRORS
186 .TP
187 .B EBADF
188 .I epfd
189 is not a valid file descriptor.
190 .TP
191 .B EFAULT
192 The memory area pointed to by
193 .I events
194 is not accessible with write permissions.
195 .TP
196 .B EINTR
197 The call was interrupted by a signal handler before either (1) any of the
198 requested events occurred or (2) the
199 .I timeout
200 expired; see
201 .BR signal (7).
202 .TP
203 .B EINVAL
204 .I epfd
205 is not an
206 .B epoll
207 file descriptor, or
208 .I maxevents
209 is less than or equal to zero.
210 .SH VERSIONS
211 .BR epoll_wait ()
212 was added in Linux 2.6.
213 .\" To be precise: kernel 2.5.44.
214 .\" The interface should be finalized by Linux 2.5.66.
215 Library support is provided in glibc 2.3.2.
216 .PP
217 .BR epoll_pwait ()
218 was added in Linux 2.6.19.
219 Library support is provided in glibc 2.6.
220 .PP
221 .BR epoll_pwait2 ()
222 was added in Linux 5.11.
223 .SH STANDARDS
224 .BR epoll_wait (),
225 .BR epoll_pwait (),
226 and
227 .BR epoll_pwait2 ()
228 are Linux-specific.
229 .SH NOTES
230 While one thread is blocked in a call to
231 .BR epoll_wait (),
232 it is possible for another thread to add a file descriptor to the waited-upon
233 .B epoll
234 instance.
235 If the new file descriptor becomes ready,
236 it will cause the
237 .BR epoll_wait ()
238 call to unblock.
239 .PP
240 If more than
241 .I maxevents
242 file descriptors are ready when
243 .BR epoll_wait ()
244 is called, then successive
245 .BR epoll_wait ()
246 calls will round robin through the set of ready file descriptors.
247 This behavior helps avoid starvation scenarios,
248 where a process fails to notice that additional file descriptors
249 are ready because it focuses on a set of file descriptors that
250 are already known to be ready.
251 .PP
252 Note that it is possible to call
253 .BR epoll_wait ()
254 on an
255 .B epoll
256 instance whose interest list is currently empty
257 (or whose interest list becomes empty because file descriptors are closed
258 or removed from the interest in another thread).
259 The call will block until some file descriptor is later added to the
260 interest list (in another thread) and that file descriptor becomes ready.
261 .SS C library/kernel differences
262 The raw
263 .BR epoll_pwait ()
264 and
265 .BR epoll_pwait2 ()
266 system calls have a sixth argument,
267 .IR "size_t sigsetsize" ,
268 which specifies the size in bytes of the
269 .I sigmask
270 argument.
271 The glibc
272 .BR epoll_pwait ()
273 wrapper function specifies this argument as a fixed value
274 (equal to
275 .IR sizeof(sigset_t) ).
276 .SH BUGS
277 Before Linux 2.6.37, a
278 .I timeout
279 value larger than approximately
280 .I LONG_MAX / HZ
281 milliseconds is treated as \-1 (i.e., infinity).
282 Thus, for example, on a system where
283 .I sizeof(long)
284 is 4 and the kernel
285 .I HZ
286 value is 1000,
287 this means that timeouts greater than 35.79 minutes are treated as infinity.
288 .SH SEE ALSO
289 .BR epoll_create (2),
290 .BR epoll_ctl (2),
291 .BR epoll (7)