man2/poll.2

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   3 .\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl)
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  26 .\" Additions from Richard Gooch <rgooch@atnf.CSIRO.AU> and aeb, 971207
  27 .\" 2006-03-13, mtk, Added ppoll() + various other rewordings
  28 .\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and
  29 .\"     formatting changes.
  30 .\"
  31 .TH POLL 2 2010-09-20 "Linux" "Linux Programmer's Manual"
  32 .SH NAME
  33 poll, ppoll \- wait for some event on a file descriptor
  34 .SH SYNOPSIS
  35 .nf
  36 .B #include <poll.h>
  37 .sp
  38 .BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout );
  39 .sp
  40 .B #define _GNU_SOURCE
  41 .B #include <poll.h>
  42 .sp
  43 .BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", "
  44 .BI "        const struct timespec *" timeout_ts ", const sigset_t *" sigmask );
  45 .fi
  46 .SH DESCRIPTION
  47 .BR poll ()
  48 performs a similar task to
  49 .BR select (2):
  50 it waits for one of a set of file descriptors to become ready
  51 to perform I/O.
  52
  53 The set of file descriptors to be monitored is specified in the
  54 .I fds
  55 argument, which is an array of structures of the following form:
  56 .in +4n
  57 .nf
  58
  59 struct pollfd {
  60     int   fd;         /* file descriptor */
  61     short events;     /* requested events */
  62     short revents;    /* returned events */
  63 };
  64 .in
  65 .fi
  66 .PP
  67 The caller should specify the number of items in the
  68 .I fds
  69 array in
  70 .IR nfds .
  71
  72 The field
  73 .I fd
  74 contains a file descriptor for an open file.
  75
  76 The field
  77 .I events
  78 is an input parameter, a bit mask specifying the events the application
  79 is interested in.
  80
  81 The field
  82 .I revents
  83 is an output parameter, filled by the kernel with the events that
  84 actually occurred.
  85 The bits returned in
  86 .I revents
  87 can include any of those specified in
  88 .IR events ,
  89 or one of the values
  90 .BR POLLERR ,
  91 .BR POLLHUP ,
  92 or
  93 .BR POLLNVAL .
  94 (These three bits are meaningless in the
  95 .I events
  96 field, and will be set in the
  97 .I revents
  98 field whenever the corresponding condition is true.)
  99
 100 If none of the events requested (and no error) has occurred for any
 101 of the file descriptors, then
 102 .BR poll ()
 103 blocks until one of the events occurs.
 104
 105 The
 106 .I timeout
 107 argument specifies an upper limit on the time for which
 108 .BR poll ()
 109 will block, in milliseconds.
 110 Specifying a negative value in
 111 .I timeout
 112 means an infinite timeout.
 113
 114 The bits that may be set/returned in
 115 .I events
 116 and
 117 .I revents
 118 are defined in \fI<poll.h>\fP:
 119 .RS
 120 .TP
 121 .B POLLIN
 122 There is data to read.
 123 .TP
 124 .B POLLPRI
 125 There is urgent data to read (e.g., out-of-band data on TCP socket;
 126 pseudoterminal master in packet mode has seen state change in slave).
 127 .TP
 128 .B POLLOUT
 129 Writing now will not block.
 130 .TP
 131 .BR POLLRDHUP " (since Linux 2.6.17)"
 132 Stream socket peer closed connection,
 133 or shut down writing half of connection.
 134 The
 135 .B _GNU_SOURCE
 136 feature test macro must be defined
 137 (before including
 138 .I any
 139 header files)
 140 in order to obtain this definition.
 141 .TP
 142 .B POLLERR
 143 Error condition (output only).
 144 .TP
 145 .B POLLHUP
 146 Hang up (output only).
 147 .TP
 148 .B POLLNVAL
 149 Invalid request:
 150 .I fd
 151 not open (output only).
 152 .RE
 153 .PP
 154 When compiling with
 155 .B _XOPEN_SOURCE
 156 defined, one also has the following,
 157 which convey no further information beyond the bits listed above:
 158 .RS
 159 .TP
 160 .B POLLRDNORM
 161 Equivalent to
 162 .BR POLLIN .
 163 .TP
 164 .B POLLRDBAND
 165 Priority band data can be read (generally unused on Linux).
 166 .\" POLLRDBAND is used in the DECnet protocol.
 167 .TP
 168 .B POLLWRNORM
 169 Equivalent to
 170 .BR POLLOUT .
 171 .TP
 172 .B POLLWRBAND
 173 Priority data may be written.
 174 .RE
 175 .PP
 176 Linux also knows about, but does not use
 177 .BR POLLMSG .
 178 .SS ppoll()
 179 The relationship between
 180 .BR poll ()
 181 and
 182 .BR ppoll ()
 183 is analogous to the relationship between
 184 .BR select (2)
 185 and
 186 .BR pselect (2):
 187 like
 188 .BR pselect (2),
 189 .BR ppoll ()
 190 allows an application to safely wait until either a file descriptor
 191 becomes ready or until a signal is caught.
 192 .PP
 193 Other than the difference in the precision of the
 194 timeout argument, the following
 195 .BR ppoll ()
 196 call:
 197 .nf
 198
 199     ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
 200
 201 .fi
 202 is equivalent to
 203 .I atomically
 204 executing the following calls:
 205 .nf
 206
 207     sigset_t origmask;
 208     int timeout;
 209
 210     timeout = (timeout_ts == NULL) ? -1 :
 211               (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
 212     sigprocmask(SIG_SETMASK, &sigmask, &origmask);
 213     ready = poll(&fds, nfds, timeout);
 214     sigprocmask(SIG_SETMASK, &origmask, NULL);
 215 .fi
 216 .PP
 217 See the description of
 218 .BR pselect (2)
 219 for an explanation of why
 220 .BR ppoll ()
 221 is necessary.
 222
 223 If the
 224 .I sigmask
 225 argument is specified as NULL, then
 226 no signal mask manipulation is performed
 227 (and thus
 228 .BR ppoll ()
 229 differs from
 230 .BR poll ()
 231 only in the precision of the timeout argument).
 232
 233 The
 234 .I timeout_ts
 235 argument specifies an upper limit on the amount of time that
 236 .BR ppoll ()
 237 will block.
 238 This argument is a pointer to a structure of the following form:
 239 .in +4n
 240 .nf
 241
 242 struct timespec {
 243     long    tv_sec;         /* seconds */
 244     long    tv_nsec;        /* nanoseconds */
 245 };
 246 .fi
 247 .in
 248
 249 If
 250 .I timeout_ts
 251 is specified as NULL, then
 252 .BR ppoll ()
 253 can block indefinitely.
 254 .SH "RETURN VALUE"
 255 On success, a positive number is returned; this is
 256 the number of structures which have nonzero
 257 .I revents
 258 fields (in other words, those descriptors with events or errors reported).
 259 A value of 0 indicates that the call timed out and no file
 260 descriptors were ready.
 261 On error, \-1 is returned, and
 262 .I errno
 263 is set appropriately.
 264 .SH ERRORS
 265 .TP
 266 .B EFAULT
 267 The array given as argument was not contained in the calling program's
 268 address space.
 269 .TP
 270 .B EINTR
 271 A signal occurred before any requested event; see
 272 .BR signal (7).
 273 .TP
 274 .B EINVAL
 275 The
 276 .I nfds
 277 value exceeds the
 278 .B RLIMIT_NOFILE
 279 value.
 280 .TP
 281 .B ENOMEM
 282 There was no space to allocate file descriptor tables.
 283 .SH VERSIONS
 284 The
 285 .BR poll ()
 286 system call was introduced in Linux 2.1.23.
 287 The
 288 .BR poll ()
 289 library call was introduced in libc 5.4.28
 290 (and provides emulation using select(2) if your kernel does not
 291 have a
 292 .BR poll ()
 293 system call).
 294
 295 The
 296 .BR ppoll ()
 297 system call was added to Linux in kernel 2.6.16.
 298 The
 299 .BR ppoll ()
 300 library call was added in glibc 2.4.
 301 .SH "CONFORMING TO"
 302 .BR poll ()
 303 conforms to POSIX.1-2001.
 304 .BR ppoll ()
 305 is Linux-specific.
 306 .\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
 307 .SH NOTES
 308 Some implementations define the nonstandard constant
 309 .B INFTIM
 310 with the value \-1 for use as a
 311 .IR timeout
 312 for
 313 .BR poll ().
 314 This constant is not provided in glibc.
 315 .SS "Linux Notes"
 316 The Linux
 317 .BR ppoll ()
 318 system call modifies its
 319 .I timeout_ts
 320 argument.
 321 However, the glibc wrapper function hides this behavior
 322 by using a local variable for the timeout argument that
 323 is passed to the system call.
 324 Thus, the glibc
 325 .BR ppoll ()
 326 function does not modify its
 327 .I timeout_ts
 328 argument.
 329 .SH BUGS
 330 See the discussion of spurious readiness notifications under the
 331 BUGS section of
 332 .BR select (2).
 333 .SH "SEE ALSO"
 334 .BR select (2),
 335 .BR select_tut (2),
 336 .BR feature_test_macros (7),
 337 .BR time (7)