man2/timer_create.2

   1 .\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk
   2 .\"     <mtk.manpages@gmail.com>
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  25 .\"
  26 .\" FIXME: Linux 2.6.39 adds CLOCK_BOOTTIME
  27 .\" Does this also affect timerfd_create()?
  28 .\" FIXME: Linux 2.3.0 adds CLOCK_BOOTTIME_ALARM and CLOCK_REALTIME_ALARM
  29 .\" Does this also affect timerfd_create()?
  30 .\"
  31 .TH TIMER_CREATE 2 2014-08-19 Linux "Linux Programmer's Manual"
  32 .SH NAME
  33 timer_create \- create a POSIX per-process timer
  34 .SH SYNOPSIS
  35 .nf
  36 .B  #include <signal.h>
  37 .B  #include <time.h>
  38
  39 .BI "int timer_create(clockid_t " clockid ", struct sigevent *" sevp ,
  40 .BI "                 timer_t *" timerid );
  41 .fi
  42
  43 Link with \fI\-lrt\fP.
  44 .sp
  45 .in -4n
  46 Feature Test Macro Requirements for glibc (see
  47 .BR feature_test_macros (7)):
  48 .in
  49 .sp
  50 .BR timer_create ():
  51 _POSIX_C_SOURCE\ >=\ 199309L
  52 .SH DESCRIPTION
  53 .BR timer_create ()
  54 creates a new per-process interval timer.
  55 The ID of the new timer is returned in the buffer pointed to by
  56 .IR timerid ,
  57 which must be a non-null pointer.
  58 This ID is unique within the process, until the timer is deleted.
  59 The new timer is initially disarmed.
  60
  61 The
  62 .I clockid
  63 argument specifies the clock that the new timer uses to measure time.
  64 It can be specified as one of the following values:
  65 .TP
  66 .B CLOCK_REALTIME
  67 A settable system-wide real-time clock.
  68 .TP
  69 .B CLOCK_MONOTONIC
  70 A nonsettable monotonically increasing clock that measures time
  71 from some unspecified point in the past that does not change
  72 after system startup.
  73 .\" Note: the CLOCK_MONOTONIC_RAW clock added for clock_gettime()
  74 .\" in 2.6.28 is not supported for POSIX timers -- mtk, Feb 2009
  75 .TP
  76 .BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
  77 A clock that measures (user and system) CPU time consumed by
  78 (all of the threads in) the calling process.
  79 .TP
  80 .BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
  81 A clock that measures (user and system) CPU time consumed by
  82 the calling thread.
  83 .\" The CLOCK_MONOTONIC_RAW that was added in 2.6.28 can't be used
  84 .\" to create a timer -- mtk, Feb 2009
  85 .PP
  86 As well as the above values,
  87 .I clockid
  88 can be specified as the
  89 .I clockid
  90 returned by a call to
  91 .BR clock_getcpuclockid (3)
  92 or
  93 .BR pthread_getcpuclockid (3).
  94
  95 The
  96 .I sevp
  97 argument points to a
  98 .I sigevent
  99 structure that specifies how the caller
 100 should be notified when the timer expires.
 101 For the definition and general details of this structure, see
 102 .BR sigevent (7).
 103
 104 The
 105 .I sevp.sigev_notify
 106 field can have the following values:
 107 .TP
 108 .BR SIGEV_NONE
 109 Don't asynchronously notify when the timer expires.
 110 Progress of the timer can be monitored using
 111 .BR timer_gettime (2).
 112 .TP
 113 .BR SIGEV_SIGNAL
 114 Upon timer expiration, generate the signal
 115 .I sigev_signo
 116 for the process.
 117 See
 118 .BR sigevent (7)
 119 for general details.
 120 The
 121 .I si_code
 122 field of the
 123 .I siginfo_t
 124 structure will be set to
 125 .BR SI_TIMER .
 126 At any point in time,
 127 at most one signal is queued to the process for a given timer; see
 128 .BR timer_getoverrun (2)
 129 for more details.
 130 .TP
 131 .BR SIGEV_THREAD
 132 Upon timer expiration, invoke
 133 .I sigev_notify_function
 134 as if it were the start function of a new thread.
 135 See
 136 .BR sigevent (7)
 137 for details.
 138 .TP
 139 .BR SIGEV_THREAD_ID " (Linux-specific)"
 140 As for
 141 .BR SIGEV_SIGNAL ,
 142 but the signal is targeted at the thread whose ID is given in
 143 .IR sigev_notify_thread_id ,
 144 which must be a thread in the same process as the caller.
 145 The
 146 .IR sigev_notify_thread_id
 147 field specifies a kernel thread ID, that is, the value returned by
 148 .BR clone (2)
 149 or
 150 .BR gettid (2).
 151 This flag is intended only for use by threading libraries.
 152 .PP
 153 Specifying
 154 .I sevp
 155 as NULL is equivalent to specifying a pointer to a
 156 .I sigevent
 157 structure in which
 158 .I sigev_notify
 159 is
 160 .BR SIGEV_SIGNAL ,
 161 .I sigev_signo
 162 is
 163 .BR SIGALRM ,
 164 and
 165 .I sigev_value.sival_int
 166 is the timer ID.
 167 .SH RETURN VALUE
 168 On success,
 169 .BR timer_create ()
 170 returns 0, and the ID of the new timer is placed in
 171 .IR *timerid .
 172 On failure, \-1 is returned, and
 173 .I errno
 174 is set to indicate the error.
 175 .SH ERRORS
 176 .TP
 177 .B EAGAIN
 178 Temporary error during kernel allocation of timer structures.
 179 .TP
 180 .B EINVAL
 181 Clock ID,
 182 .IR sigev_notify ,
 183 .IR sigev_signo ,
 184 or
 185 .IR sigev_notify_thread_id
 186 is invalid.
 187 .TP
 188 .B ENOMEM
 189 .\" glibc layer: malloc()
 190 Could not allocate memory.
 191 .SH VERSIONS
 192 This system call is available since Linux 2.6.
 193 .SH CONFORMING TO
 194 POSIX.1-2001.
 195 .SH NOTES
 196 A program may create multiple interval timers using
 197 .BR timer_create ().
 198
 199 Timers are not inherited by the child of a
 200 .BR fork (2),
 201 and are disarmed and deleted during an
 202 .BR execve (2).
 203
 204 The kernel preallocates a "queued real-time signal"
 205 for each timer created using
 206 .BR timer_create ().
 207 Consequently, the number of timers is limited by the
 208 .BR RLIMIT_SIGPENDING
 209 resource limit (see
 210 .BR setrlimit (2)).
 211
 212 The timers created by
 213 .BR timer_create ()
 214 are commonly known as "POSIX (interval) timers".
 215 The POSIX timers API consists of the following interfaces:
 216 .IP * 3
 217 .BR timer_create ():
 218 Create a timer.
 219 .IP *
 220 .BR timer_settime (2):
 221 Arm (start) or disarm (stop) a timer.
 222 .IP *
 223 .BR timer_gettime (2):
 224 Fetch the time remaining until the next expiration of a timer,
 225 along with the interval setting of the timer.
 226 .IP *
 227 .BR timer_getoverrun (2):
 228 Return the overrun count for the last timer expiration.
 229 .IP *
 230 .BR timer_delete (2):
 231 Disarm and delete a timer.
 232
 233 Since Linux 3.10, the
 234 .IR /proc/[pid]/timers
 235 file can be used to list the POSIX timers for the process with PID
 236 .IR pid .
 237 See
 238 .BR proc (5)
 239 for further information.
 240 .\"
 241 .SS C library/kernel ABI differences
 242 Part of the implementation of the POSIX timers API is provided by glibc.
 243 In particular:
 244 .IP * 3
 245 The functionality for
 246 .BR SIGEV_THREAD
 247 is implemented within glibc, rather than the kernel.
 248 .IP *
 249 The timer IDs presented at user level are maintained by glibc,
 250 which maps these IDs to the timer IDs employed by the kernel.
 251 .\" See the glibc source file kernel-posix-timers.h for the structure
 252 .\" that glibc uses to map user-space timer IDs to kernel timer IDs
 253 .\" The kernel-level timer ID is exposed via siginfo.si_tid.
 254 .PP
 255 The POSIX timers system calls first appeared in Linux 2.6.
 256 Prior to this,
 257 glibc provided an incomplete user-space implementation
 258 .RB ( CLOCK_REALTIME
 259 timers only) using POSIX threads,
 260 and current glibc falls back to this implementation on systems
 261 running pre-2.6 Linux kernels.
 262 .SH EXAMPLE
 263 The program below takes two arguments: a sleep period in seconds,
 264 and a timer frequency in nanoseconds.
 265 The program establishes a handler for the signal it uses for the timer,
 266 blocks that signal,
 267 creates and arms a timer that expires with the given frequency,
 268 sleeps for the specified number of seconds,
 269 and then unblocks the timer signal.
 270 Assuming that the timer expired at least once while the program slept,
 271 the signal handler will be invoked,
 272 and the handler displays some information about the timer notification.
 273 The program terminates after one invocation of the signal handler.
 274
 275 In the following example run, the program sleeps for 1 second,
 276 after creating a timer that has a frequency of 100 nanoseconds.
 277 By the time the signal is unblocked and delivered,
 278 there have been around ten million overruns.
 279 .in +4n
 280 .nf
 281
 282 $ \fB./a.out 1 100\fP
 283 Establishing handler for signal 34
 284 Blocking signal 34
 285 timer ID is 0x804c008
 286 Sleeping for 1 seconds
 287 Unblocking signal 34
 288 Caught signal 34
 289     sival_ptr = 0xbfb174f4;     *sival_ptr = 0x804c008
 290     overrun count = 10004886
 291 .fi
 292 .in
 293 .SS Program source
 294 \&
 295 .nf
 296 #include <stdlib.h>
 297 #include <unistd.h>
 298 #include <stdio.h>
 299 #include <signal.h>
 300 #include <time.h>
 301
 302 #define CLOCKID CLOCK_REALTIME
 303 #define SIG SIGRTMIN
 304
 305 #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \\
 306                         } while (0)
 307
 308 static void
 309 print_siginfo(siginfo_t *si)
 310 {
 311     timer_t *tidp;
 312     int or;
 313
 314     tidp = si\->si_value.sival_ptr;
 315
 316     printf("    sival_ptr = %p; ", si\->si_value.sival_ptr);
 317     printf("    *sival_ptr = 0x%lx\\n", (long) *tidp);
 318
 319     or = timer_getoverrun(*tidp);
 320     if (or == \-1)
 321         errExit("timer_getoverrun");
 322     else
 323         printf("    overrun count = %d\\n", or);
 324 }
 325
 326 static void
 327 handler(int sig, siginfo_t *si, void *uc)
 328 {
 329     /* Note: calling printf() from a signal handler is not
 330        strictly correct, since printf() is not async\-signal\-safe;
 331        see signal(7) */
 332
 333     printf("Caught signal %d\\n", sig);
 334     print_siginfo(si);
 335     signal(sig, SIG_IGN);
 336 }
 337
 338 int
 339 main(int argc, char *argv[])
 340 {
 341     timer_t timerid;
 342     struct sigevent sev;
 343     struct itimerspec its;
 344     long long freq_nanosecs;
 345     sigset_t mask;
 346     struct sigaction sa;
 347
 348     if (argc != 3) {
 349         fprintf(stderr, "Usage: %s <sleep\-secs> <freq\-nanosecs>\\n",
 350                 argv[0]);
 351         exit(EXIT_FAILURE);
 352     }
 353
 354     /* Establish handler for timer signal */
 355
 356     printf("Establishing handler for signal %d\\n", SIG);
 357     sa.sa_flags = SA_SIGINFO;
 358     sa.sa_sigaction = handler;
 359     sigemptyset(&sa.sa_mask);
 360     if (sigaction(SIG, &sa, NULL) == \-1)
 361         errExit("sigaction");
 362
 363     /* Block timer signal temporarily */
 364
 365     printf("Blocking signal %d\\n", SIG);
 366     sigemptyset(&mask);
 367     sigaddset(&mask, SIG);
 368     if (sigprocmask(SIG_SETMASK, &mask, NULL) == \-1)
 369         errExit("sigprocmask");
 370
 371     /* Create the timer */
 372
 373     sev.sigev_notify = SIGEV_SIGNAL;
 374     sev.sigev_signo = SIG;
 375     sev.sigev_value.sival_ptr = &timerid;
 376     if (timer_create(CLOCKID, &sev, &timerid) == \-1)
 377         errExit("timer_create");
 378
 379     printf("timer ID is 0x%lx\\n", (long) timerid);
 380
 381     /* Start the timer */
 382
 383     freq_nanosecs = atoll(argv[2]);
 384     its.it_value.tv_sec = freq_nanosecs / 1000000000;
 385     its.it_value.tv_nsec = freq_nanosecs % 1000000000;
 386     its.it_interval.tv_sec = its.it_value.tv_sec;
 387     its.it_interval.tv_nsec = its.it_value.tv_nsec;
 388
 389     if (timer_settime(timerid, 0, &its, NULL) == \-1)
 390          errExit("timer_settime");
 391
 392     /* Sleep for a while; meanwhile, the timer may expire
 393        multiple times */
 394
 395     printf("Sleeping for %d seconds\\n", atoi(argv[1]));
 396     sleep(atoi(argv[1]));
 397
 398     /* Unlock the timer signal, so that timer notification
 399        can be delivered */
 400
 401     printf("Unblocking signal %d\\n", SIG);
 402     if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == \-1)
 403         errExit("sigprocmask");
 404
 405     exit(EXIT_SUCCESS);
 406 }
 407 .fi
 408 .SH SEE ALSO
 409 .ad l
 410 .nh
 411 .BR clock_gettime (2),
 412 .BR setitimer (2),
 413 .BR timer_delete (2),
 414 .BR timer_getoverrun (2),
 415 .BR timer_settime (2),
 416 .BR timerfd_create (2),
 417 .BR clock_getcpuclockid (3),
 418 .BR pthread_getcpuclockid (3),
 419 .BR pthreads (7),
 420 .BR sigevent (7),
 421 .BR signal (7),
 422 .BR time (7)