man2/timerfd_create.2

   1 .\" Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
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  18 .\"
  19 .\" FIXME Linux 3.0: timerfd_settime() adds a TFD_TIMER_CANCEL_ON_SET flag;
  20 .\" This flag needs to documented.
  21 .\"
  22 .TH TIMERFD_CREATE 2 2019-03-06 Linux "Linux Programmer's Manual"
  23 .SH NAME
  24 timerfd_create, timerfd_settime, timerfd_gettime \-
  25 timers that notify via file descriptors
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <sys/timerfd.h>
  29 .PP
  30 .BI "int timerfd_create(int " clockid ", int " flags );
  31 .PP
  32 .BI "int timerfd_settime(int " fd ", int " flags ,
  33 .BI "                    const struct itimerspec *" new_value ,
  34 .BI "                    struct itimerspec *" old_value );
  35 .PP
  36 .BI "int timerfd_gettime(int " fd ", struct itimerspec *" curr_value );
  37 .fi
  38 .SH DESCRIPTION
  39 These system calls create and operate on a timer
  40 that delivers timer expiration notifications via a file descriptor.
  41 They provide an alternative to the use of
  42 .BR setitimer (2)
  43 or
  44 .BR timer_create (2),
  45 with the advantage that the file descriptor may be monitored by
  46 .BR select (2),
  47 .BR poll (2),
  48 and
  49 .BR epoll (7).
  50 .PP
  51 The use of these three system calls is analogous to the use of
  52 .BR timer_create (2),
  53 .BR timer_settime (2),
  54 and
  55 .BR timer_gettime (2).
  56 (There is no analog of
  57 .BR timer_getoverrun (2),
  58 since that functionality is provided by
  59 .BR read (2),
  60 as described below.)
  61 .\"
  62 .SS timerfd_create()
  63 .BR timerfd_create ()
  64 creates a new timer object,
  65 and returns a file descriptor that refers to that timer.
  66 The
  67 .I clockid
  68 argument specifies the clock that is used to mark the progress
  69 of the timer, and must be one of the following:
  70 .TP
  71 .B CLOCK_REALTIME
  72 A settable system-wide real-time clock.
  73 .TP
  74 .B CLOCK_MONOTONIC
  75 A nonsettable monotonically increasing clock that measures time
  76 from some unspecified point in the past that does not change
  77 after system startup.
  78 .TP
  79 .BR CLOCK_BOOTTIME " (Since Linux 3.15)"
  80 .\"    commit 4a2378a943f09907fb1ae35c15de917f60289c14
  81 Like
  82 .BR CLOCK_MONOTONIC ,
  83 this is a monotonically increasing clock.
  84 However, whereas the
  85 .BR CLOCK_MONOTONIC
  86 clock does not measure the time while a system is suspended, the
  87 .BR CLOCK_BOOTTIME
  88 clock does include the time during which the system is suspended.
  89 This is useful for applications that need to be suspend-aware.
  90 .BR CLOCK_REALTIME
  91 is not suitable for such applications, since that clock is affected
  92 by discontinuous changes to the system clock.
  93 .TP
  94 .BR CLOCK_REALTIME_ALARM " (since Linux 3.11)"
  95 .\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8
  96 This clock is like
  97 .BR CLOCK_REALTIME ,
  98 but will wake the system if it is suspended.
  99 The caller must have the
 100 .B CAP_WAKE_ALARM
 101 capability in order to set a timer against this clock.
 102 .TP
 103 .BR CLOCK_BOOTTIME_ALARM " (since Linux 3.11)"
 104 .\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8
 105 This clock is like
 106 .BR CLOCK_BOOTTIME ,
 107 but will wake the system if it is suspended.
 108 The caller must have the
 109 .B CAP_WAKE_ALARM
 110 capability in order to set a timer against this clock.
 111 .PP
 112 The current value of each of these clocks can be retrieved using
 113 .BR clock_gettime (2).
 114 .PP
 115 Starting with Linux 2.6.27, the following values may be bitwise ORed in
 116 .IR flags
 117 to change the behavior of
 118 .BR timerfd_create ():
 119 .TP 14
 120 .B TFD_NONBLOCK
 121 Set the
 122 .BR O_NONBLOCK
 123 file status flag on the open file description (see
 124 .BR open (2))
 125 referred to by the new file descriptor.
 126 Using this flag saves extra calls to
 127 .BR fcntl (2)
 128 to achieve the same result.
 129 .TP
 130 .B TFD_CLOEXEC
 131 Set the close-on-exec
 132 .RB ( FD_CLOEXEC )
 133 flag on the new file descriptor.
 134 See the description of the
 135 .B O_CLOEXEC
 136 flag in
 137 .BR open (2)
 138 for reasons why this may be useful.
 139 .PP
 140 In Linux versions up to and including 2.6.26,
 141 .I flags
 142 must be specified as zero.
 143 .SS timerfd_settime()
 144 .BR timerfd_settime ()
 145 arms (starts) or disarms (stops)
 146 the timer referred to by the file descriptor
 147 .IR fd .
 148 .PP
 149 The
 150 .I new_value
 151 argument specifies the initial expiration and interval for the timer.
 152 The
 153 .I itimerspec
 154 structure used for this argument contains two fields,
 155 each of which is in turn a structure of type
 156 .IR timespec :
 157 .PP
 158 .in +4n
 159 .EX
 160 struct timespec {
 161     time_t tv_sec;                /* Seconds */
 162     long   tv_nsec;               /* Nanoseconds */
 163 };
 164
 165 struct itimerspec {
 166     struct timespec it_interval;  /* Interval for periodic timer */
 167     struct timespec it_value;     /* Initial expiration */
 168 };
 169 .EE
 170 .in
 171 .PP
 172 .I new_value.it_value
 173 specifies the initial expiration of the timer,
 174 in seconds and nanoseconds.
 175 Setting either field of
 176 .I new_value.it_value
 177 to a nonzero value arms the timer.
 178 Setting both fields of
 179 .I new_value.it_value
 180 to zero disarms the timer.
 181 .PP
 182 Setting one or both fields of
 183 .I new_value.it_interval
 184 to nonzero values specifies the period, in seconds and nanoseconds,
 185 for repeated timer expirations after the initial expiration.
 186 If both fields of
 187 .I new_value.it_interval
 188 are zero, the timer expires just once, at the time specified by
 189 .IR new_value.it_value .
 190 .PP
 191 By default,
 192 the initial expiration time specified in
 193 .I new_value
 194 is interpreted relative to the current time
 195 on the timer's clock at the time of the call (i.e.,
 196 .I new_value.it_value
 197 specifies a time relative to the current value of the clock specified by
 198 .IR clockid ).
 199 An absolute timeout can be selected via the
 200 .I flags
 201 argument.
 202 .PP
 203 The
 204 .I flags
 205 argument is a bit mask that can include the following values:
 206 .TP
 207 .B TFD_TIMER_ABSTIME
 208 Interpret
 209 .I new_value.it_value
 210 as an absolute value on the timer's clock.
 211 The timer will expire when the value of the timer's
 212 clock reaches the value specified in
 213 .IR new_value.it_value .
 214 .TP
 215 .BR TFD_TIMER_CANCEL_ON_SET
 216 If this flag is specified along with
 217 .B TFD_TIMER_ABSTIME
 218 and the clock for this timer is
 219 .BR CLOCK_REALTIME
 220 or
 221 .BR CLOCK_REALTIME_ALARM ,
 222 then mark this timer as cancelable if the real-time clock
 223 undergoes a discontinuous change
 224 .RB ( settimeofday (2),
 225 .BR clock_settime (2),
 226 or similar).
 227 When such changes occur, a current or future
 228 .BR read (2)
 229 from the file descriptor will fail with the error
 230 .BR ECANCELED .
 231 .PP
 232 If the
 233 .I old_value
 234 argument is not NULL, then the
 235 .I itimerspec
 236 structure that it points to is used to return the setting of the timer
 237 that was current at the time of the call;
 238 see the description of
 239 .BR timerfd_gettime ()
 240 following.
 241 .\"
 242 .SS timerfd_gettime()
 243 .BR timerfd_gettime ()
 244 returns, in
 245 .IR curr_value ,
 246 an
 247 .IR itimerspec
 248 structure that contains the current setting of the timer
 249 referred to by the file descriptor
 250 .IR fd .
 251 .PP
 252 The
 253 .I it_value
 254 field returns the amount of time
 255 until the timer will next expire.
 256 If both fields of this structure are zero,
 257 then the timer is currently disarmed.
 258 This field always contains a relative value, regardless of whether the
 259 .BR TFD_TIMER_ABSTIME
 260 flag was specified when setting the timer.
 261 .PP
 262 The
 263 .I it_interval
 264 field returns the interval of the timer.
 265 If both fields of this structure are zero,
 266 then the timer is set to expire just once, at the time specified by
 267 .IR curr_value.it_value .
 268 .SS Operating on a timer file descriptor
 269 The file descriptor returned by
 270 .BR timerfd_create ()
 271 supports the following operations:
 272 .TP
 273 .BR read (2)
 274 If the timer has already expired one or more times since
 275 its settings were last modified using
 276 .BR timerfd_settime (),
 277 or since the last successful
 278 .BR read (2),
 279 then the buffer given to
 280 .BR read (2)
 281 returns an unsigned 8-byte integer
 282 .RI ( uint64_t )
 283 containing the number of expirations that have occurred.
 284 (The returned value is in host byte order\(emthat is,
 285 the native byte order for integers on the host machine.)
 286 .IP
 287 If no timer expirations have occurred at the time of the
 288 .BR read (2),
 289 then the call either blocks until the next timer expiration,
 290 or fails with the error
 291 .B EAGAIN
 292 if the file descriptor has been made nonblocking
 293 (via the use of the
 294 .BR fcntl (2)
 295 .B F_SETFL
 296 operation to set the
 297 .B O_NONBLOCK
 298 flag).
 299 .IP
 300 A
 301 .BR read (2)
 302 fails with the error
 303 .B EINVAL
 304 if the size of the supplied buffer is less than 8 bytes.
 305 .IP
 306 If the associated clock is either
 307 .BR CLOCK_REALTIME
 308 or
 309 .BR CLOCK_REALTIME_ALARM ,
 310 the timer is absolute
 311 .RB ( TFD_TIMER_ABSTIME ),
 312 and the flag
 313 .BR TFD_TIMER_CANCEL_ON_SET
 314 was specified when calling
 315 .BR timerfd_settime (),
 316 then
 317 .BR read (2)
 318 fails with the error
 319 .BR ECANCELED
 320 if the real-time clock undergoes a discontinuous change.
 321 (This allows the reading application to discover
 322 such discontinuous changes to the clock.)
 323 .TP
 324 .BR poll "(2), " select "(2) (and similar)"
 325 The file descriptor is readable
 326 (the
 327 .BR select (2)
 328 .I readfds
 329 argument; the
 330 .BR poll (2)
 331 .B POLLIN
 332 flag)
 333 if one or more timer expirations have occurred.
 334 .IP
 335 The file descriptor also supports the other file-descriptor
 336 multiplexing APIs:
 337 .BR pselect (2),
 338 .BR ppoll (2),
 339 and
 340 .BR epoll (7).
 341 .TP
 342 .BR ioctl (2)
 343 The following timerfd-specific command is supported:
 344 .RS
 345 .TP
 346 .BR TFD_IOC_SET_TICKS " (since Linux 3.17)"
 347 .\" commit 5442e9fbd7c23172a1c9bc736629cd123a9923f0
 348 Adjust the number of timer expirations that have occurred.
 349 The argument is a pointer to a nonzero 8-byte integer
 350 .RI ( uint64_t *)
 351 containing the new number of expirations.
 352 Once the number is set, any waiter on the timer is woken up.
 353 The only purpose of this command is to restore the expirations
 354 for the purpose of checkpoint/restore.
 355 This operation is available only if the kernel was configured with the
 356 .BR CONFIG_CHECKPOINT_RESTORE
 357 option.
 358 .RE
 359 .TP
 360 .BR close (2)
 361 When the file descriptor is no longer required it should be closed.
 362 When all file descriptors associated with the same timer object
 363 have been closed,
 364 the timer is disarmed and its resources are freed by the kernel.
 365 .\"
 366 .SS fork(2) semantics
 367 After a
 368 .BR fork (2),
 369 the child inherits a copy of the file descriptor created by
 370 .BR timerfd_create ().
 371 The file descriptor refers to the same underlying
 372 timer object as the corresponding file descriptor in the parent,
 373 and
 374 .BR read (2)s
 375 in the child will return information about
 376 expirations of the timer.
 377 .\"
 378 .SS execve(2) semantics
 379 A file descriptor created by
 380 .BR timerfd_create ()
 381 is preserved across
 382 .BR execve (2),
 383 and continues to generate timer expirations if the timer was armed.
 384 .SH RETURN VALUE
 385 On success,
 386 .BR timerfd_create ()
 387 returns a new file descriptor.
 388 On error, \-1 is returned and
 389 .I errno
 390 is set to indicate the error.
 391 .PP
 392 .BR timerfd_settime ()
 393 and
 394 .BR timerfd_gettime ()
 395 return 0 on success;
 396 on error they return \-1, and set
 397 .I errno
 398 to indicate the error.
 399 .SH ERRORS
 400 .BR timerfd_create ()
 401 can fail with the following errors:
 402 .TP
 403 .B EINVAL
 404 The
 405 .I clockid
 406 argument is neither
 407 .B CLOCK_MONOTONIC
 408 nor
 409 .BR CLOCK_REALTIME ;
 410 .TP
 411 .B EINVAL
 412 .I flags
 413 is invalid;
 414 or, in Linux 2.6.26 or earlier,
 415 .I flags
 416 is nonzero.
 417 .TP
 418 .B EMFILE
 419 The per-process limit on the number of open file descriptors has been reached.
 420 .TP
 421 .B ENFILE
 422 The system-wide limit on the total number of open files has been
 423 reached.
 424 .TP
 425 .B ENODEV
 426 Could not mount (internal) anonymous inode device.
 427 .TP
 428 .B ENOMEM
 429 There was insufficient kernel memory to create the timer.
 430 .PP
 431 .BR timerfd_settime ()
 432 and
 433 .BR timerfd_gettime ()
 434 can fail with the following errors:
 435 .TP
 436 .B EBADF
 437 .I fd
 438 is not a valid file descriptor.
 439 .TP
 440 .B EFAULT
 441 .IR new_value ,
 442 .IR old_value ,
 443 or
 444 .I curr_value
 445 is not valid a pointer.
 446 .TP
 447 .B EINVAL
 448 .I fd
 449 is not a valid timerfd file descriptor.
 450 .PP
 451 .BR timerfd_settime ()
 452 can also fail with the following errors:
 453 .TP
 454 .B EINVAL
 455 .I new_value
 456 is not properly initialized (one of the
 457 .I tv_nsec
 458 falls outside the range zero to 999,999,999).
 459 .TP
 460 .B EINVAL
 461 .\" This case only checked since 2.6.29, and 2.2.2[78].some-stable-version.
 462 .\" In older kernel versions, no check was made for invalid flags.
 463 .I flags
 464 is invalid.
 465 .SH VERSIONS
 466 These system calls are available on Linux since kernel 2.6.25.
 467 Library support is provided by glibc since version 2.8.
 468 .SH CONFORMING TO
 469 These system calls are Linux-specific.
 470 .SH BUGS
 471 Currently,
 472 .\" 2.6.29
 473 .BR timerfd_create ()
 474 supports fewer types of clock IDs than
 475 .BR timer_create (2).
 476 .SH EXAMPLE
 477 The following program creates a timer and then monitors its progress.
 478 The program accepts up to three command-line arguments.
 479 The first argument specifies the number of seconds for
 480 the initial expiration of the timer.
 481 The second argument specifies the interval for the timer, in seconds.
 482 The third argument specifies the number of times the program should
 483 allow the timer to expire before terminating.
 484 The second and third command-line arguments are optional.
 485 .PP
 486 The following shell session demonstrates the use of the program:
 487 .PP
 488 .in +4n
 489 .EX
 490 .RB "$" " a.out 3 1 100"
 491 0.000: timer started
 492 3.000: read: 1; total=1
 493 4.000: read: 1; total=2
 494 .BR "^Z " "                 # type control-Z to suspend the program"
 495 [1]+  Stopped                 ./timerfd3_demo 3 1 100
 496 .RB "$ " "fg" "                # Resume execution after a few seconds"
 497 a.out 3 1 100
 498 9.660: read: 5; total=7
 499 10.000: read: 1; total=8
 500 11.000: read: 1; total=9
 501 .BR "^C " "                 # type control-C to suspend the program"
 502 .EE
 503 .in
 504 .SS Program source
 505 \&
 506 .EX
 507 .\" The commented out code here is what we currently need until
 508 .\" the required stuff is in glibc
 509 .\"
 510 .\"
 511 .\"/* Link with -lrt */
 512 .\"#define _GNU_SOURCE
 513 .\"#include <sys/syscall.h>
 514 .\"#include <unistd.h>
 515 .\"#include <time.h>
 516 .\"#if defined(__i386__)
 517 .\"#define __NR_timerfd_create 322
 518 .\"#define __NR_timerfd_settime 325
 519 .\"#define __NR_timerfd_gettime 326
 520 .\"#endif
 521 .\"
 522 .\"static int
 523 .\"timerfd_create(int clockid, int flags)
 524 .\"{
 525 .\"    return syscall(__NR_timerfd_create, clockid, flags);
 526 .\"}
 527 .\"
 528 .\"static int
 529 .\"timerfd_settime(int fd, int flags, struct itimerspec *new_value,
 530 .\"        struct itimerspec *curr_value)
 531 .\"{
 532 .\"    return syscall(__NR_timerfd_settime, fd, flags, new_value,
 533 .\"                   curr_value);
 534 .\"}
 535 .\"
 536 .\"static int
 537 .\"timerfd_gettime(int fd, struct itimerspec *curr_value)
 538 .\"{
 539 .\"    return syscall(__NR_timerfd_gettime, fd, curr_value);
 540 .\"}
 541 .\"
 542 .\"#define TFD_TIMER_ABSTIME (1 << 0)
 543 .\"
 544 .\"////////////////////////////////////////////////////////////
 545 #include <sys/timerfd.h>
 546 #include <time.h>
 547 #include <unistd.h>
 548 #include <stdlib.h>
 549 #include <stdio.h>
 550 #include <stdint.h>        /* Definition of uint64_t */
 551
 552 #define handle_error(msg) \e
 553         do { perror(msg); exit(EXIT_FAILURE); } while (0)
 554
 555 static void
 556 print_elapsed_time(void)
 557 {
 558     static struct timespec start;
 559     struct timespec curr;
 560     static int first_call = 1;
 561     int secs, nsecs;
 562
 563     if (first_call) {
 564         first_call = 0;
 565         if (clock_gettime(CLOCK_MONOTONIC, &start) == \-1)
 566             handle_error("clock_gettime");
 567     }
 568
 569     if (clock_gettime(CLOCK_MONOTONIC, &curr) == \-1)
 570         handle_error("clock_gettime");
 571
 572     secs = curr.tv_sec \- start.tv_sec;
 573     nsecs = curr.tv_nsec \- start.tv_nsec;
 574     if (nsecs < 0) {
 575         secs\-\-;
 576         nsecs += 1000000000;
 577     }
 578     printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);
 579 }
 580
 581 int
 582 main(int argc, char *argv[])
 583 {
 584     struct itimerspec new_value;
 585     int max_exp, fd;
 586     struct timespec now;
 587     uint64_t exp, tot_exp;
 588     ssize_t s;
 589
 590     if ((argc != 2) && (argc != 4)) {
 591         fprintf(stderr, "%s init\-secs [interval\-secs max\-exp]\en",
 592                 argv[0]);
 593         exit(EXIT_FAILURE);
 594     }
 595
 596     if (clock_gettime(CLOCK_REALTIME, &now) == \-1)
 597         handle_error("clock_gettime");
 598
 599     /* Create a CLOCK_REALTIME absolute timer with initial
 600        expiration and interval as specified in command line */
 601
 602     new_value.it_value.tv_sec = now.tv_sec + atoi(argv[1]);
 603     new_value.it_value.tv_nsec = now.tv_nsec;
 604     if (argc == 2) {
 605         new_value.it_interval.tv_sec = 0;
 606         max_exp = 1;
 607     } else {
 608         new_value.it_interval.tv_sec = atoi(argv[2]);
 609         max_exp = atoi(argv[3]);
 610     }
 611     new_value.it_interval.tv_nsec = 0;
 612
 613     fd = timerfd_create(CLOCK_REALTIME, 0);
 614     if (fd == \-1)
 615         handle_error("timerfd_create");
 616
 617     if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == \-1)
 618         handle_error("timerfd_settime");
 619
 620     print_elapsed_time();
 621     printf("timer started\en");
 622
 623     for (tot_exp = 0; tot_exp < max_exp;) {
 624         s = read(fd, &exp, sizeof(uint64_t));
 625         if (s != sizeof(uint64_t))
 626             handle_error("read");
 627
 628         tot_exp += exp;
 629         print_elapsed_time();
 630         printf("read: %llu; total=%llu\en",
 631                 (unsigned long long) exp,
 632                 (unsigned long long) tot_exp);
 633     }
 634
 635     exit(EXIT_SUCCESS);
 636 }
 637 .EE
 638 .SH SEE ALSO
 639 .BR eventfd (2),
 640 .BR poll (2),
 641 .BR read (2),
 642 .BR select (2),
 643 .BR setitimer (2),
 644 .BR signalfd (2),
 645 .BR timer_create (2),
 646 .BR timer_gettime (2),
 647 .BR timer_settime (2),
 648 .BR epoll (7),
 649 .BR time (7)