man2/epoll_ctl.2

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   2 .\"  Davide Libenzi <davidel@xmailserver.org>
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  19 .\"
  20 .TH EPOLL_CTL 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  21 .SH NAME
  22 epoll_ctl \- control interface for an epoll file descriptor
  23 .SH SYNOPSIS
  24 .B #include <sys/epoll.h>
  25 .sp
  26 .BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
  27 ", struct epoll_event *" event );
  28 .SH DESCRIPTION
  29 This system call performs control operations on the
  30 .BR epoll (7)
  31 instance
  32 referred to by the file descriptor
  33 .IR epfd .
  34 It requests that the operation
  35 .I op
  36 be performed for the target file descriptor,
  37 .IR fd .
  38
  39 Valid values for the
  40 .I op
  41 argument are:
  42 .TP
  43 .B EPOLL_CTL_ADD
  44 Register the target file descriptor
  45 .I fd
  46 on the
  47 .B epoll
  48 instance referred to by the file descriptor
  49 .I epfd
  50 and associate the event
  51 .I event
  52 with the internal file linked to
  53 .IR fd .
  54 .TP
  55 .B EPOLL_CTL_MOD
  56 Change the event
  57 .I event
  58 associated with the target file descriptor
  59 .IR fd .
  60 .TP
  61 .B EPOLL_CTL_DEL
  62 Remove (deregister) the target file descriptor
  63 .I fd
  64 from the
  65 .B epoll
  66 instance referred to by
  67 .IR epfd .
  68 The
  69 .I event
  70 is ignored and can be NULL (but see BUGS below).
  71 .PP
  72 The
  73 .I event
  74 argument describes the object linked to the file descriptor
  75 .IR fd .
  76 The
  77 .I struct epoll_event
  78 is defined as:
  79 .sp
  80 .in +4n
  81 .nf
  82 typedef union epoll_data {
  83     void        *ptr;
  84     int          fd;
  85     uint32_t     u32;
  86     uint64_t     u64;
  87 } epoll_data_t;
  88
  89 struct epoll_event {
  90     uint32_t     events;      /* Epoll events */
  91     epoll_data_t data;        /* User data variable */
  92 };
  93 .fi
  94 .in
  95
  96 The
  97 .I events
  98 member is a bit mask composed using the following available event
  99 types:
 100 .TP
 101 .B EPOLLIN
 102 The associated file is available for
 103 .BR read (2)
 104 operations.
 105 .TP
 106 .B EPOLLOUT
 107 The associated file is available for
 108 .BR write (2)
 109 operations.
 110 .TP
 111 .BR EPOLLRDHUP " (since Linux 2.6.17)"
 112 Stream socket peer closed connection,
 113 or shut down writing half of connection.
 114 (This flag is especially useful for writing simple code to detect
 115 peer shutdown when using Edge Triggered monitoring.)
 116 .TP
 117 .B EPOLLPRI
 118 There is urgent data available for
 119 .BR read (2)
 120 operations.
 121 .TP
 122 .B EPOLLERR
 123 Error condition happened on the associated file descriptor.
 124 .BR epoll_wait (2)
 125 will always wait for this event; it is not necessary to set it in
 126 .IR events .
 127 .TP
 128 .B EPOLLHUP
 129 Hang up happened on the associated file descriptor.
 130 .BR epoll_wait (2)
 131 will always wait for this event; it is not necessary to set it in
 132 .IR events .
 133 Note that when reading from a channel such as a pipe or a stream socket,
 134 this event merely indicates that the peer closed its end of the channel.
 135 Subsequent reads from the channel will return 0 (end of file)
 136 only after all outstanding data in the channel has been consumed.
 137 .TP
 138 .B EPOLLET
 139 Sets the Edge Triggered behavior for the associated file descriptor.
 140 The default behavior for
 141 .B epoll
 142 is Level Triggered.
 143 See
 144 .BR epoll (7)
 145 for more detailed information about Edge and Level Triggered event
 146 distribution architectures.
 147 .TP
 148 .BR EPOLLONESHOT " (since Linux 2.6.2)"
 149 Sets the one-shot behavior for the associated file descriptor.
 150 This means that after an event is pulled out with
 151 .BR epoll_wait (2)
 152 the associated file descriptor is internally disabled and no other events
 153 will be reported by the
 154 .B epoll
 155 interface.
 156 The user must call
 157 .BR epoll_ctl ()
 158 with
 159 .B EPOLL_CTL_MOD
 160 to rearm the file descriptor with a new event mask.
 161 .TP
 162 .BR EPOLLWAKEUP " (since Linux 3.5)"
 163 .\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
 164 If
 165 .B EPOLLONESHOT
 166 and
 167 .B EPOLLET
 168 are clear and the process has the
 169 .B CAP_BLOCK_SUSPEND
 170 capability,
 171 ensure that the system does not enter "suspend" or
 172 "hibernate" while this event is pending or being processed.
 173 The event is considered as being "processed" from the time
 174 when it is returned by a call to
 175 .BR epoll_wait (2)
 176 until the next call to
 177 .BR epoll_wait (2)
 178 on the same
 179 .BR epoll (7)
 180 file descriptor,
 181 the closure of that file descriptor,
 182 the removal of the event file descriptor with
 183 .BR EPOLL_CTL_DEL ,
 184 or the clearing of
 185 .B EPOLLWAKEUP
 186 for the event file descriptor with
 187 .BR EPOLL_CTL_MOD .
 188 See also BUGS.
 189 .TP
 190 .BR EPOLLEXCLUSIVE " (since Linux 4.5)"
 191 Sets an exclusive wakeup mode for the epoll file descriptor that is being
 192 attached to the target file descriptor,
 193 .IR fd .
 194 When a wakeup event occurs and multiple epoll file descriptors
 195 are attached to the same target file using
 196 .BR EPOLLEXCLUSIVE ,
 197 one or more of the epoll file descriptors will receive an event with
 198 .BR epoll_wait (2).
 199 The default in this scenario (when
 200 .BR EPOLLEXCLUSIVE
 201 is not set) is for all epoll file descriptors to receive an event.
 202 .BR EPOLLEXCLUSIVE
 203 is thus useful for avoiding thundering herd problems in certain scenarios.
 204
 205 If the same file descriptor is in multiple epoll instances,
 206 some with the
 207 .BR EPOLLEXCLUSIVE
 208 flag, and others without, then events will provided to all epoll
 209 instances that did not specify
 210 .BR EPOLLEXCLUSIVE ,
 211 and at least one of the epoll instances that did specify
 212 .BR EPOLLEXCLUSIVE .
 213
 214 The following values may be specified in conjunction with
 215 .BR EPOLLEXCLUSIVE :
 216 .BR EPOLLIN ,
 217 .BR EPOLLOUT ,
 218 .BR EPOLLWAKEUP,
 219 and
 220 .BR EPOLLET .
 221 .BR EPOLLHUP
 222 and
 223 .BR EPOLLERR
 224 can also be specified, but this is not required:
 225 as usual, these events are always reported if they occur,
 226 regardless of whether they are specified in
 227 .IR events .
 228 Attempts to specify other values in
 229 .I events
 230 yield an error.
 231 .B EPOLLEXCLUSIVE
 232 may be used only in an
 233 .B EPOLL_CTL_ADD
 234 operation; attempts to employ it with
 235 .B EPOLL_CTL_MOD
 236 yield an error.
 237 If
 238 .B EPOLLEXCLUSIVE
 239 has set using
 240 .BR epoll_ctl (),
 241 then a subsequent
 242 .B EPOLL_CTL_MOD
 243 on the same
 244 .IR epfd ",\ " fd
 245 pair yields an error.
 246 A call to
 247 .BR epoll_ctl ()
 248 that specifies
 249 .B EPOLLEXCLUSIVE
 250 in
 251 .I events
 252 and specifies the target file descriptor
 253 .I fd
 254 as an epoll instance will likewise fail.
 255 The error in all of these cases is
 256 .BR EINVAL .
 257 .SH RETURN VALUE
 258 When successful,
 259 .BR epoll_ctl ()
 260 returns zero.
 261 When an error occurs,
 262 .BR epoll_ctl ()
 263 returns \-1 and
 264 .I errno
 265 is set appropriately.
 266 .SH ERRORS
 267 .TP
 268 .B EBADF
 269 .I epfd
 270 or
 271 .I fd
 272 is not a valid file descriptor.
 273 .TP
 274 .B EEXIST
 275 .I op
 276 was
 277 .BR EPOLL_CTL_ADD ,
 278 and the supplied file descriptor
 279 .I fd
 280 is already registered with this epoll instance.
 281 .TP
 282 .B EINVAL
 283 .I epfd
 284 is not an
 285 .B epoll
 286 file descriptor,
 287 or
 288 .I fd
 289 is the same as
 290 .IR epfd ,
 291 or the requested operation
 292 .I op
 293 is not supported by this interface.
 294 .TP
 295 .B EINVAL
 296 An invalid event type was specified along with
 297 .B EPOLLEXCLUSIVE
 298 in
 299 .IR events .
 300 .TP
 301 .B EINVAL
 302 .I op
 303 was
 304 .B EPOLL_CTL_MOD
 305 and
 306 .IR events
 307 included
 308 .BR EPOLLEXCLUSIVE .
 309 .TP
 310 .B EINVAL
 311 .I op
 312 was
 313 .B EPOLL_CTL_MOD
 314 and the
 315 .BR EPOLLEXCLUSIVE
 316 flag has previously been applied to this
 317 .IR epfd ",\ " fd
 318 pair.
 319 .TP
 320 .B EINVAL
 321 .BR EPOLLEXCLUSIVE
 322 was specified in
 323 .IR event
 324 and
 325 .I fd
 326 refers to an epoll instance.
 327 .TP
 328 .B ELOOP
 329 .I fd
 330 refers to an epoll instance and this
 331 .B EPOLL_CTL_ADD
 332 operation would result in a circular loop of epoll instances
 333 monitoring one another.
 334 .TP
 335 .B ENOENT
 336 .I op
 337 was
 338 .B EPOLL_CTL_MOD
 339 or
 340 .BR EPOLL_CTL_DEL ,
 341 and
 342 .I fd
 343 is not registered with this epoll instance.
 344 .TP
 345 .B ENOMEM
 346 There was insufficient memory to handle the requested
 347 .I op
 348 control operation.
 349 .TP
 350 .B ENOSPC
 351 The limit imposed by
 352 .I /proc/sys/fs/epoll/max_user_watches
 353 was encountered while trying to register
 354 .RB ( EPOLL_CTL_ADD )
 355 a new file descriptor on an epoll instance.
 356 See
 357 .BR epoll (7)
 358 for further details.
 359 .TP
 360 .B EPERM
 361 The target file
 362 .I fd
 363 does not support
 364 .BR epoll .
 365 This error can occur if
 366 .I fd
 367 refers to, for example, a regular file or a directory.
 368 .SH VERSIONS
 369 .BR epoll_ctl ()
 370 was added to the kernel in version 2.6.
 371 .\" To be precise: kernel 2.5.44.
 372 .\" The interface should be finalized by Linux kernel 2.5.66.
 373 .SH CONFORMING TO
 374 .BR epoll_ctl ()
 375 is Linux-specific.
 376 Library support is provided in glibc starting with version 2.3.2.
 377 .SH NOTES
 378 The
 379 .B epoll
 380 interface supports all file descriptors that support
 381 .BR poll (2).
 382 .SH BUGS
 383 In kernel versions before 2.6.9, the
 384 .B EPOLL_CTL_DEL
 385 operation required a non-null pointer in
 386 .IR event ,
 387 even though this argument is ignored.
 388 Since Linux 2.6.9,
 389 .I event
 390 can be specified as NULL
 391 when using
 392 .BR EPOLL_CTL_DEL .
 393 Applications that need to be portable to kernels before 2.6.9
 394 should specify a non-null pointer in
 395 .IR event .
 396
 397 If
 398 .B EPOLLWAKEUP
 399 is specified in
 400 .IR flags ,
 401 but the caller does not have the
 402 .BR CAP_BLOCK_SUSPEND
 403 capability, then the
 404 .B EPOLLWAKEUP
 405 flag is
 406 .IR "silently ignored" .
 407 This unfortunate behavior is necessary because no validity
 408 checks were performed on the
 409 .IR flags
 410 argument in the original implementation, and the addition of the
 411 .B EPOLLWAKEUP
 412 with a check that caused the call to fail if the caller did not have the
 413 .B CAP_BLOCK_SUSPEND
 414 capability caused a breakage in at least one existing user-space
 415 application that happened to randomly (and uselessly) specify this bit.
 416 .\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
 417 .\" https://lwn.net/Articles/520198/
 418 A robust application should therefore double check that it has the
 419 .B CAP_BLOCK_SUSPEND
 420 capability if attempting to use the
 421 .B EPOLLWAKEUP
 422 flag.
 423 .SH SEE ALSO
 424 .BR epoll_create (2),
 425 .BR epoll_wait (2),
 426 .BR poll (2),
 427 .BR epoll (7)