man2/epoll_wait.2

   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 2021-03-22 "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 *" sigmask );
  25 .BI "int epoll_pwait2(int " epfd ", struct epoll_event *" events ,
  26 .BI "               int " maxevents ", const struct timespec *" timeout ,
  27 .BI "               const sigset_t *" sigmask );
  28 .fi
  29 .SH DESCRIPTION
  30 The
  31 .BR epoll_wait ()
  32 system call waits for events on the
  33 .BR epoll (7)
  34 instance referred to by the file descriptor
  35 .IR epfd .
  36 The buffer pointed to by
  37 .I events
  38 is used to return information from the ready list
  39 about file descriptors in the interest list that
  40 have some events available.
  41 Up to
  42 .I maxevents
  43 are returned by
  44 .BR epoll_wait ().
  45 The
  46 .I maxevents
  47 argument must be greater than zero.
  48 .PP
  49 The
  50 .I timeout
  51 argument specifies the number of milliseconds that
  52 .BR epoll_wait ()
  53 will block.
  54 Time is measured against the
  55 .B CLOCK_MONOTONIC
  56 clock.
  57 .PP
  58 A call to
  59 .BR epoll_wait ()
  60 will block until either:
  61 .IP \(bu 2
  62 a file descriptor delivers an event;
  63 .IP \(bu
  64 the call is interrupted by a signal handler; or
  65 .IP \(bu
  66 the timeout expires.
  67 .PP
  68 Note that the
  69 .I timeout
  70 interval will be rounded up to the system clock granularity,
  71 and kernel scheduling delays mean that the blocking interval
  72 may overrun by a small amount.
  73 Specifying a
  74 .I timeout
  75 of \-1 causes
  76 .BR epoll_wait ()
  77 to block indefinitely, while specifying a
  78 .I timeout
  79 equal to zero cause
  80 .BR epoll_wait ()
  81 to return immediately, even if no events are available.
  82 .PP
  83 The
  84 .I struct epoll_event
  85 is described in
  86 .BR epoll_event (3type).
  87 .PP
  88 The
  89 .I data
  90 field of each returned
  91 .I epoll_event
  92 structure contains the same data as was specified
  93 in the most recent call to
  94 .BR epoll_ctl (2)
  95 .RB ( EPOLL_CTL_ADD ", " EPOLL_CTL_MOD )
  96 for the corresponding open file descriptor.
  97 .PP
  98 The
  99 .I events
 100 field is a bit mask that indicates the events that have occurred for the
 101 corresponding open file description.
 102 See
 103 .BR epoll_ctl (2)
 104 for a list of the bits that may appear in this mask.
 105 .\"
 106 .SS epoll_pwait()
 107 The relationship between
 108 .BR epoll_wait ()
 109 and
 110 .BR epoll_pwait ()
 111 is analogous to the relationship between
 112 .BR select (2)
 113 and
 114 .BR pselect (2):
 115 like
 116 .BR pselect (2),
 117 .BR epoll_pwait ()
 118 allows an application to safely wait until either a file descriptor
 119 becomes ready or until a signal is caught.
 120 .PP
 121 The following
 122 .BR epoll_pwait ()
 123 call:
 124 .PP
 125 .in +4n
 126 .EX
 127 ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
 128 .EE
 129 .in
 130 .PP
 131 is equivalent to
 132 .I atomically
 133 executing the following calls:
 134 .PP
 135 .in +4n
 136 .EX
 137 sigset_t origmask;
 138
 139 pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
 140 ready = epoll_wait(epfd, &events, maxevents, timeout);
 141 pthread_sigmask(SIG_SETMASK, &origmask, NULL);
 142 .EE
 143 .in
 144 .PP
 145 The
 146 .I sigmask
 147 argument may be specified as NULL, in which case
 148 .BR epoll_pwait ()
 149 is equivalent to
 150 .BR epoll_wait ().
 151 .\"
 152 .SS epoll_pwait2()
 153 The
 154 .BR epoll_pwait2 ()
 155 system call is equivalent to
 156 .BR epoll_pwait ()
 157 except for the
 158 .I timeout
 159 argument.
 160 It takes an argument of type
 161 .I timespec
 162 to be able to specify nanosecond resolution timeout.
 163 This argument functions the same as in
 164 .BR pselect (2)
 165 and
 166 .BR ppoll (2).
 167 If
 168 .I timeout
 169 is NULL, then
 170 .BR epoll_pwait2 ()
 171 can block indefinitely.
 172 .SH RETURN VALUE
 173 On success,
 174 .BR epoll_wait ()
 175 returns the number of file descriptors ready for the requested I/O, or zero
 176 if no file descriptor became ready during the requested
 177 .I timeout
 178 milliseconds.
 179 On failure,
 180 .BR epoll_wait ()
 181 returns \-1 and
 182 .I errno
 183 is set to indicate the error.
 184 .SH ERRORS
 185 .TP
 186 .B EBADF
 187 .I epfd
 188 is not a valid file descriptor.
 189 .TP
 190 .B EFAULT
 191 The memory area pointed to by
 192 .I events
 193 is not accessible with write permissions.
 194 .TP
 195 .B EINTR
 196 The call was interrupted by a signal handler before either (1) any of the
 197 requested events occurred or (2) the
 198 .I timeout
 199 expired; see
 200 .BR signal (7).
 201 .TP
 202 .B EINVAL
 203 .I epfd
 204 is not an
 205 .B epoll
 206 file descriptor, or
 207 .I maxevents
 208 is less than or equal to zero.
 209 .SH VERSIONS
 210 .BR epoll_wait ()
 211 was added to the kernel in version 2.6.
 212 .\" To be precise: kernel 2.5.44.
 213 .\" The interface should be finalized by Linux kernel 2.5.66.
 214 Library support is provided in glibc starting with version 2.3.2.
 215 .PP
 216 .BR epoll_pwait ()
 217 was added to Linux in kernel 2.6.19.
 218 Library support is provided in glibc starting with version 2.6.
 219 .PP
 220 .BR epoll_pwait2 ()
 221 was added to Linux in kernel 5.11.
 222 .SH STANDARDS
 223 .BR epoll_wait (),
 224 .BR epoll_pwait (),
 225 and
 226 .BR epoll_pwait2 ()
 227 are Linux-specific.
 228 .SH NOTES
 229 While one thread is blocked in a call to
 230 .BR epoll_wait (),
 231 it is possible for another thread to add a file descriptor to the waited-upon
 232 .B epoll
 233 instance.
 234 If the new file descriptor becomes ready,
 235 it will cause the
 236 .BR epoll_wait ()
 237 call to unblock.
 238 .PP
 239 If more than
 240 .I maxevents
 241 file descriptors are ready when
 242 .BR epoll_wait ()
 243 is called, then successive
 244 .BR epoll_wait ()
 245 calls will round robin through the set of ready file descriptors.
 246 This behavior helps avoid starvation scenarios,
 247 where a process fails to notice that additional file descriptors
 248 are ready because it focuses on a set of file descriptors that
 249 are already known to be ready.
 250 .PP
 251 Note that it is possible to call
 252 .BR epoll_wait ()
 253 on an
 254 .B epoll
 255 instance whose interest list is currently empty
 256 (or whose interest list becomes empty because file descriptors are closed
 257 or removed from the interest in another thread).
 258 The call will block until some file descriptor is later added to the
 259 interest list (in another thread) and that file descriptor becomes ready.
 260 .SS C library/kernel differences
 261 The raw
 262 .BR epoll_pwait ()
 263 and
 264 .BR epoll_pwait2 ()
 265 system calls have a sixth argument,
 266 .IR "size_t sigsetsize" ,
 267 which specifies the size in bytes of the
 268 .I sigmask
 269 argument.
 270 The glibc
 271 .BR epoll_pwait ()
 272 wrapper function specifies this argument as a fixed value
 273 (equal to
 274 .IR sizeof(sigset_t) ).
 275 .SH BUGS
 276 In kernels before 2.6.37, a
 277 .I timeout
 278 value larger than approximately
 279 .I LONG_MAX / HZ
 280 milliseconds is treated as \-1 (i.e., infinity).
 281 Thus, for example, on a system where
 282 .I sizeof(long)
 283 is 4 and the kernel
 284 .I HZ
 285 value is 1000,
 286 this means that timeouts greater than 35.79 minutes are treated as infinity.
 287 .SH SEE ALSO
 288 .BR epoll_create (2),
 289 .BR epoll_ctl (2),
 290 .BR epoll (7)