man2/epoll_ctl.2

   1 .\"  Copyright (C) 2003  Davide Libenzi
   2 .\"  Davide Libenzi <davidel@xmailserver.org>
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  19 .\"
  20 .TH EPOLL_CTL 2 2012-04-15 "Linux" "Linux Programmer's Manual"
  21 .SH NAME
  22 epoll_ctl \- control interface for an epoll 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 set 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 .TP
 134 .B EPOLLET
 135 Sets the Edge Triggered behavior for the associated file descriptor.
 136 The default behavior for
 137 .B epoll
 138 is Level Triggered.
 139 See
 140 .BR epoll (7)
 141 for more detailed information about Edge and Level Triggered event
 142 distribution architectures.
 143 .TP
 144 .BR EPOLLONESHOT " (since Linux 2.6.2)"
 145 Sets the one-shot behavior for the associated file descriptor.
 146 This means that after an event is pulled out with
 147 .BR epoll_wait (2)
 148 the associated file descriptor is internally disabled and no other events
 149 will be reported by the
 150 .B epoll
 151 interface.
 152 The user must call
 153 .BR epoll_ctl ()
 154 with
 155 .B EPOLL_CTL_MOD
 156 to rearm the file descriptor with a new event mask.
 157 .TP
 158 .BR EPOLLWAKEUP " (since Linux 3.5)"
 159 .\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
 160 If
 161 .B EPOLLONESHOT
 162 and
 163 .B EPOLLET
 164 are clear and the process has the
 165 .B CAP_BLOCK_SUSPEND
 166 capability,
 167 ensure that the system does not enter "suspend" or
 168 "hibernate" while this event is pending or being processed.
 169 The event is considered as being "processed" from the time
 170 when it is returned by a call to
 171 .BR epoll_wait (2)
 172 until the next call to
 173 .BR epoll_wait (2)
 174 on the same
 175 .BR epoll (7)
 176 file descriptor.
 177 See also BUGS.
 178 .SH RETURN VALUE
 179 When successful,
 180 .BR epoll_ctl ()
 181 returns zero.
 182 When an error occurs,
 183 .BR epoll_ctl ()
 184 returns \-1 and
 185 .I errno
 186 is set appropriately.
 187 .SH ERRORS
 188 .TP
 189 .B EBADF
 190 .I epfd
 191 or
 192 .I fd
 193 is not a valid file descriptor.
 194 .TP
 195 .B EEXIST
 196 .I op
 197 was
 198 .BR EPOLL_CTL_ADD ,
 199 and the supplied file descriptor
 200 .I fd
 201 is already registered with this epoll instance.
 202 .TP
 203 .B EINVAL
 204 .I epfd
 205 is not an
 206 .B epoll
 207 file descriptor,
 208 or
 209 .I fd
 210 is the same as
 211 .IR epfd ,
 212 or the requested operation
 213 .I op
 214 is not supported by this interface.
 215 .TP
 216 .B ENOENT
 217 .I op
 218 was
 219 .B EPOLL_CTL_MOD
 220 or
 221 .BR EPOLL_CTL_DEL ,
 222 and
 223 .I fd
 224 is not registered with this epoll instance.
 225 .TP
 226 .B ENOMEM
 227 There was insufficient memory to handle the requested
 228 .I op
 229 control operation.
 230 .TP
 231 .B ENOSPC
 232 The limit imposed by
 233 .I /proc/sys/fs/epoll/max_user_watches
 234 was encountered while trying to register
 235 .RB ( EPOLL_CTL_ADD )
 236 a new file descriptor on an epoll instance.
 237 See
 238 .BR epoll (7)
 239 for further details.
 240 .TP
 241 .B EPERM
 242 The target file
 243 .I fd
 244 does not support
 245 .BR epoll .
 246 .SH VERSIONS
 247 .BR epoll_ctl ()
 248 was added to the kernel in version 2.6.
 249 .\" To be precise: kernel 2.5.44.
 250 .\" The interface should be finalized by Linux kernel 2.5.66.
 251 .SH CONFORMING TO
 252 .BR epoll_ctl ()
 253 is Linux-specific.
 254 Library support is provided in glibc starting with version 2.3.2.
 255 .SH NOTES
 256 The
 257 .B epoll
 258 interface supports all file descriptors that support
 259 .BR poll (2).
 260 .SH BUGS
 261 In kernel versions before 2.6.9, the
 262 .B EPOLL_CTL_DEL
 263 operation required a non-null pointer in
 264 .IR event ,
 265 even though this argument is ignored.
 266 Since Linux 2.6.9,
 267 .I event
 268 can be specified as NULL
 269 when using
 270 .BR EPOLL_CTL_DEL .
 271 Applications that need to be portable to kernels before 2.6.9
 272 should specify a non-null pointer in
 273 .IR event .
 274
 275 If
 276 .B EPOLLWAKEUP
 277 is specified in
 278 .IR flags ,
 279 but the caller does not have the
 280 .BR CAP_BLOCK_SUSPEND
 281 capability, then the
 282 .B EPOLLWAKEUP
 283 flag is
 284 .IR "silently ignored" .
 285 This unfortunate behavior is necessary because no validity
 286 checks were performed on the
 287 .IR flags
 288 argument in the original implementation, and the addition of the
 289 .B EPOLLWAKEUP
 290 with a check that caused the call to fail if the caller did not have the
 291 .B CAP_BLOCK_SUSPEND
 292 capability caused a breakage in at least one existing user-space
 293 application that happened to randomly (and uselessly) specify this bit.
 294 .\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
 295 .\" https://lwn.net/Articles/520198/
 296 A robust application should therefore double check that it has the
 297 .B CAP_BLOCK_SUSPEND
 298 capability if attempting to use the
 299 .B EPOLLWAKEUP
 300 flag.
 301 .SH SEE ALSO
 302 .BR epoll_create (2),
 303 .BR epoll_wait (2),
 304 .BR poll (2),
 305 .BR epoll (7)