man2/clock_getres.2

   1 .\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
   2 .\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl), Aug 24, 2003
   3 .\" Copyright (c) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
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  26 .\"
  27 .\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
  28 .\" 2003-08-24 aeb, large parts rewritten
  29 .\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
  30 .\"
  31 .TH CLOCK_GETRES 2 2020-04-11 "" "Linux Programmer's Manual"
  32 .SH NAME
  33 clock_getres, clock_gettime, clock_settime \- clock and time functions
  34 .SH SYNOPSIS
  35 .B #include <time.h>
  36 .PP
  37 .BI "int clock_getres(clockid_t " clockid ", struct timespec *" res );
  38 .PP
  39 .BI "int clock_gettime(clockid_t " clockid ", struct timespec *" tp );
  40 .PP
  41 .BI "int clock_settime(clockid_t " clockid ", const struct timespec *" tp );
  42 .PP
  43 Link with \fI\-lrt\fP (only for glibc versions before 2.17).
  44 .PP
  45 .in -4n
  46 Feature Test Macro Requirements for glibc (see
  47 .BR feature_test_macros (7)):
  48 .in
  49 .PP
  50 .ad l
  51 .BR clock_getres (),
  52 .BR clock_gettime (),
  53 .BR clock_settime ():
  54 .RS
  55 _POSIX_C_SOURCE\ >=\ 199309L
  56 .RE
  57 .ad b
  58 .SH DESCRIPTION
  59 The function
  60 .BR clock_getres ()
  61 finds the resolution (precision) of the specified clock
  62 .IR clockid ,
  63 and, if
  64 .I res
  65 is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
  66 .IR res .
  67 The resolution of clocks depends on the implementation and cannot be
  68 configured by a particular process.
  69 If the time value pointed to by the argument
  70 .I tp
  71 of
  72 .BR clock_settime ()
  73 is not a multiple of
  74 .IR res ,
  75 then it is truncated to a multiple of
  76 .IR res .
  77 .PP
  78 The functions
  79 .BR clock_gettime ()
  80 and
  81 .BR clock_settime ()
  82 retrieve and set the time of the specified clock
  83 .IR clockid .
  84 .PP
  85 The
  86 .I res
  87 and
  88 .I tp
  89 arguments are
  90 .I timespec
  91 structures, as specified in
  92 .IR <time.h> :
  93 .PP
  94 .in +4n
  95 .EX
  96 struct timespec {
  97     time_t   tv_sec;        /* seconds */
  98     long     tv_nsec;       /* nanoseconds */
  99 };
 100 .EE
 101 .in
 102 .PP
 103 The
 104 .I clockid
 105 argument is the identifier of the particular clock on which to act.
 106 A clock may be system-wide and hence visible for all processes, or
 107 per-process if it measures time only within a single process.
 108 .PP
 109 All implementations support the system-wide real-time clock,
 110 which is identified by
 111 .BR CLOCK_REALTIME .
 112 Its time represents seconds and nanoseconds since the Epoch.
 113 When its time is changed, timers for a relative interval are
 114 unaffected, but timers for an absolute point in time are affected.
 115 .PP
 116 More clocks may be implemented.
 117 The interpretation of the
 118 corresponding time values and the effect on timers is unspecified.
 119 .PP
 120 Sufficiently recent versions of glibc and the Linux kernel
 121 support the following clocks:
 122 .TP
 123 .B CLOCK_REALTIME
 124 A settable system-wide clock that measures real (i.e., wall-clock) time.
 125 Setting this clock requires appropriate privileges.
 126 This clock is affected by discontinuous jumps in the system time
 127 (e.g., if the system administrator manually changes the clock),
 128 and by the incremental adjustments performed by
 129 .BR adjtime (3)
 130 and NTP.
 131 .TP
 132 .BR CLOCK_REALTIME_ALARM " (since Linux 3.0; Linux-specific)"
 133 Like
 134 .BR CLOCK_REALTIME ,
 135 but not settable.
 136 See
 137 .BR timer_create (2)
 138 for further details.
 139 .TP
 140 .BR CLOCK_REALTIME_COARSE " (since Linux 2.6.32; Linux-specific)"
 141 .\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
 142 A faster but less precise version of
 143 .BR CLOCK_REALTIME .
 144 This clock is not settable.
 145 Use when you need very fast, but not fine-grained timestamps.
 146 Requires per-architecture support,
 147 and probably also architecture support for this flag in the
 148 .BR vdso (7).
 149 .TP
 150 .BR CLOCK_TAI " (since Linux 3.10; Linux-specific)"
 151 .\" commit 1ff3c9677bff7e468e0c487d0ffefe4e901d33f4
 152 A nonsettable system-wide clock derived from wall-clock time
 153 but ignoring leap seconds.
 154 This clock does
 155 not experience discontinuities and backwards jumps caused by NTP
 156 inserting leap seconds as
 157 .BR CLOCK_REALTIME
 158 does.
 159 .IP
 160 The acronym TAI refers to International Atomic Time.
 161 .TP
 162 .B CLOCK_MONOTONIC
 163 A nonsettable system-wide clock that
 164 represents monotonic time since\(emas described
 165 by POSIX\(em"some unspecified point in the past".
 166 On Linux, that point corresponds to the number of seconds that the system
 167 has been running since it was booted.
 168 .IP
 169 The
 170 .B CLOCK_MONOTONIC
 171 clock is not affected by discontinuous jumps in the system time
 172 (e.g., if the system administrator manually changes the clock),
 173 but is affected by the incremental adjustments performed by
 174 .BR adjtime (3)
 175 and NTP.
 176 This clock does not count time that the system is suspended.
 177 All
 178 .B CLOCK_MONOTONIC
 179 variants guarantee that the time returned by consecutive calls will not go
 180 backwards, but successive calls may\(emdepending on the architecture\(emreturn
 181 identical (not-increased) time values.
 182 .TP
 183 .BR CLOCK_MONOTONIC_COARSE " (since Linux 2.6.32; Linux-specific)"
 184 .\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
 185 A faster but less precise version of
 186 .BR CLOCK_MONOTONIC .
 187 Use when you need very fast, but not fine-grained timestamps.
 188 Requires per-architecture support,
 189 and probably also architecture support for this flag in the
 190 .BR vdso (7).
 191 .TP
 192 .BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
 193 .\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
 194 Similar to
 195 .BR CLOCK_MONOTONIC ,
 196 but provides access to a raw hardware-based time
 197 that is not subject to NTP adjustments or
 198 the incremental adjustments performed by
 199 .BR adjtime (3).
 200 This clock does not count time that the system is suspended.
 201 .TP
 202 .BR CLOCK_BOOTTIME " (since Linux 2.6.39; Linux-specific)"
 203 .\" commit 7fdd7f89006dd5a4c702fa0ce0c272345fa44ae0
 204 .\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
 205 A nonsettable system-wide clock that is identical to
 206 .BR CLOCK_MONOTONIC ,
 207 except that it also includes any time that the system is suspended.
 208 This allows applications to get a suspend-aware monotonic clock
 209 without having to deal with the complications of
 210 .BR CLOCK_REALTIME ,
 211 which may have discontinuities if the time is changed using
 212 .BR settimeofday (2)
 213 or similar.
 214 .TP
 215 .BR CLOCK_BOOTTIME_ALARM " (since Linux 3.0; Linux-specific)"
 216 Like
 217 .BR CLOCK_BOOTTIME .
 218 See
 219 .BR timer_create (2)
 220 for further details.
 221 .TP
 222 .BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
 223 This is a clock that measures CPU time consumed by this process
 224 (i.e., CPU time consumed by all threads in the process).
 225 On Linux, this clock is not settable.
 226 .TP
 227 .BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
 228 This is a clock that measures CPU time consumed by this thread.
 229 On Linux, this clock is not settable.
 230 .PP
 231 Linux also implements dynamic clock instances as described below.
 232 .SH DYNAMIC CLOCKS " (since Linux 2.6.39)"
 233 In addition to hard coded SYS-V style clock ids, Linux also supports
 234 POSIX clock operations on certain character devices.
 235 Such devices are
 236 called "dynamic" clocks.
 237 Using the appropriate macros, open file
 238 descriptors may be converted into clock ids and passed to
 239 .BR clock_gettime (),
 240 .BR clock_settime (),
 241 and
 242 .BR clock_adj (2).
 243 The follow example shows how to convert a file descriptor into a
 244 dynamic clock id.
 245 .PP
 246 .in +4n
 247 .EX
 248 #define CLOCKFD 3
 249 #define FD_TO_CLOCKID(fd)       ((~(clockid_t) (fd) << 3) | CLOCKFD)
 250 #define CLOCKID_TO_FD(clk)      ((unsigned int) ~((clk) >> 3))
 251
 252 struct timeval tv;
 253 clockid_t clkid;
 254 int fd;
 255
 256 fd = open("/dev/ptp0", O_RDWR);
 257 clkid = FD_TO_CLOCKID(fd);
 258 clock_gettime(clkid, &tv);
 259 .EE
 260 .in
 261 .SH RETURN VALUE
 262 .BR clock_gettime (),
 263 .BR clock_settime (),
 264 and
 265 .BR clock_getres ()
 266 return 0 for success, or \-1 for failure (in which case
 267 .I errno
 268 is set appropriately).
 269 .SH ERRORS
 270 .TP
 271 .B EFAULT
 272 .I tp
 273 points outside the accessible address space.
 274 .TP
 275 .B EINVAL
 276 The
 277 .I clockid
 278 specified is invalid for one of two reasons.
 279 Either the SYS-V style
 280 hard coded positive value is out of range, or the dynamic clock id
 281 does not refer to a valid instance of a clock object.
 282 .\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
 283 .\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
 284 .\" EPERM.
 285 .TP
 286 .B EINVAL
 287 .RB ( clock_settime ()):
 288 .I tp.tv_sec
 289 is negative or
 290 .I tp.tv_nsec
 291 is outside the range [0..999,999,999].
 292 .TP
 293 .B EINVAL
 294 The
 295 .I clockid
 296 specified in a call to
 297 .BR clock_settime ()
 298 is not a settable clock.
 299 .TP
 300 .B ENOTSUP
 301 The operation is not supported by the dynamic POSIX clock device
 302 specified.
 303 .TP
 304 .BR EINVAL " (since Linux 4.3)"
 305 .\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
 306 A call to
 307 .BR clock_settime ()
 308 with a
 309 .I clockid
 310 of
 311 .B CLOCK_REALTIME
 312 attempted to set the time to a value less than
 313 the current value of the
 314 .B CLOCK_MONOTONIC
 315 clock.
 316 .TP
 317 .B ENODEV
 318 The hot-plugable device (like USB for example) represented by a
 319 dynamic
 320 .I clk_id
 321 has disappeared after its character device was opened.
 322 .TP
 323 .B EPERM
 324 .BR clock_settime ()
 325 does not have permission to set the clock indicated.
 326 .TP
 327 .B EACCES
 328 .BR clock_settime ()
 329 does not have write permission for the dynamic POSIX
 330 clock device indicated.
 331 .SH VERSIONS
 332 These system calls first appeared in Linux 2.6.
 333 .SH ATTRIBUTES
 334 For an explanation of the terms used in this section, see
 335 .BR attributes (7).
 336 .TS
 337 allbox;
 338 lbw32 lb lb
 339 l l l.
 340 Interface       Attribute       Value
 341 T{
 342 .BR clock_getres (),
 343 .BR clock_gettime (),
 344 .BR clock_settime ()
 345 T}      Thread safety   MT-Safe
 346 .TE
 347 .sp 1
 348 .SH CONFORMING TO
 349 POSIX.1-2001, POSIX.1-2008, SUSv2.
 350 .SH AVAILABILITY
 351 On POSIX systems on which these functions are available, the symbol
 352 .B _POSIX_TIMERS
 353 is defined in \fI<unistd.h>\fP to a value greater than 0.
 354 The symbols
 355 .BR _POSIX_MONOTONIC_CLOCK ,
 356 .BR _POSIX_CPUTIME ,
 357 .B _POSIX_THREAD_CPUTIME
 358 indicate that
 359 .BR CLOCK_MONOTONIC ,
 360 .BR CLOCK_PROCESS_CPUTIME_ID ,
 361 .B CLOCK_THREAD_CPUTIME_ID
 362 are available.
 363 (See also
 364 .BR sysconf (3).)
 365 .SH NOTES
 366 POSIX.1 specifies the following:
 367 .RS
 368 .PP
 369 Setting the value of the
 370 .B CLOCK_REALTIME
 371 clock via
 372 .BR clock_settime ()
 373 shall have no effect on threads that are blocked waiting for a relative time
 374 service based upon this clock, including the
 375 .BR nanosleep ()
 376 function; nor on the expiration of relative timers based upon this clock.
 377 Consequently, these time services shall expire when the requested relative
 378 interval elapses, independently of the new or old value of the clock.
 379 .RE
 380 .PP
 381 According to POSIX.1-2001, a process with "appropriate privileges" may set the
 382 .B CLOCK_PROCESS_CPUTIME_ID
 383 and
 384 .B CLOCK_THREAD_CPUTIME_ID
 385 clocks using
 386 .BR clock_settime ().
 387 On Linux, these clocks are not settable
 388 (i.e., no process has "appropriate privileges").
 389 .\" See http://bugzilla.kernel.org/show_bug.cgi?id=11972
 390 .\"
 391 .SS C library/kernel differences
 392 On some architectures, an implementation of
 393 .BR clock_gettime ()
 394 is provided in the
 395 .BR vdso (7).
 396 .\"
 397 .SS Historical note for SMP systems
 398 Before Linux added kernel support for
 399 .B CLOCK_PROCESS_CPUTIME_ID
 400 and
 401 .BR CLOCK_THREAD_CPUTIME_ID ,
 402 glibc implemented these clocks on many platforms using timer
 403 registers from the CPUs
 404 (TSC on i386, AR.ITC on Itanium).
 405 These registers may differ between CPUs and as a consequence
 406 these clocks may return
 407 .B bogus results
 408 if a process is migrated to another CPU.
 409 .PP
 410 If the CPUs in an SMP system have different clock sources, then
 411 there is no way to maintain a correlation between the timer registers since
 412 each CPU will run at a slightly different frequency.
 413 If that is the case, then
 414 .I clock_getcpuclockid(0)
 415 will return
 416 .B ENOENT
 417 to signify this condition.
 418 The two clocks will then be useful only if it
 419 can be ensured that a process stays on a certain CPU.
 420 .PP
 421 The processors in an SMP system do not start all at exactly the same
 422 time and therefore the timer registers are typically running at an offset.
 423 Some architectures include code that attempts to limit these offsets on bootup.
 424 However, the code cannot guarantee to accurately tune the offsets.
 425 Glibc contains no provisions to deal with these offsets (unlike the Linux
 426 Kernel).
 427 Typically these offsets are small and therefore the effects may be
 428 negligible in most cases.
 429 .PP
 430 Since glibc 2.4,
 431 the wrapper functions for the system calls described in this page avoid
 432 the abovementioned problems by employing the kernel implementation of
 433 .B CLOCK_PROCESS_CPUTIME_ID
 434 and
 435 .BR CLOCK_THREAD_CPUTIME_ID ,
 436 on systems that provide such an implementation
 437 (i.e., Linux 2.6.12 and later).
 438 .SH EXAMPLE
 439 The program below demonstrates the use of
 440 .BR clock_gettime ()
 441 and
 442 .BR clock_getres ()
 443 with various clocks.
 444 This is an example of what we might see when running the program:
 445 .PP
 446 .in +4n
 447 .EX
 448 $ \fB./clock_times x\fP
 449 CLOCK_REALTIME : 1585985459.446 (18356 days +  7h 30m 59s)
 450      resolution:          0.000000001
 451 CLOCK_TAI      : 1585985496.447 (18356 days +  7h 31m 36s)
 452      resolution:          0.000000001
 453 CLOCK_MONOTONIC:      52395.722 (14h 33m 15s)
 454      resolution:          0.000000001
 455 CLOCK_BOOTTIME :      72691.019 (20h 11m 31s)
 456      resolution:          0.000000001
 457 .EE
 458 .in
 459 .SS Program source
 460 \&
 461 .EX
 462 /* clock_times.c
 463
 464    Licensed under GNU General Public License v2 or later.
 465 */
 466 #define _XOPEN_SOURCE 600
 467 #include <time.h>
 468 #include <stdio.h>
 469 #include <stdlib.h>
 470 #include <stdbool.h>
 471 #include <unistd.h>
 472
 473 #define SECS_IN_DAY (24 * 60 * 60)
 474
 475 static void
 476 displayClock(clockid_t clock, char *name, bool showRes)
 477 {
 478     struct timespec ts;
 479
 480     if (clock_gettime(clock, &ts) == \-1) {
 481         perror("clock_gettime");
 482         exit(EXIT_FAILURE);
 483     }
 484
 485     printf("%\-15s: %10ld.%03ld (", name,
 486             (long) ts.tv_sec, ts.tv_nsec / 1000000);
 487
 488     long days = ts.tv_sec / SECS_IN_DAY;
 489     if (days > 0)
 490         printf("%ld days + ", days);
 491
 492     printf("%2ldh %2ldm %2lds", (ts.tv_sec % SECS_IN_DAY) / 3600,
 493             (ts.tv_sec % 3600) / 60, ts.tv_sec % 60);
 494     printf(")\en");
 495
 496     if (clock_getres(clock, &ts) == \-1) {
 497         perror("clock_getres");
 498         exit(EXIT_FAILURE);
 499     }
 500
 501     if (showRes)
 502         printf("     resolution: %10ld.%09ld\en",
 503                 (long) ts.tv_sec, ts.tv_nsec);
 504 }
 505
 506 int
 507 main(int argc, char *argv[])
 508 {
 509     bool showRes = argc > 1;
 510
 511     displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
 512 #ifdef CLOCK_TAI
 513     displayClock(CLOCK_TAI, "CLOCK_TAI", showRes);
 514 #endif
 515     displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes);
 516 #ifdef CLOCK_BOOTTIME
 517     displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes);
 518 #endif
 519     exit(EXIT_SUCCESS);
 520 }
 521 .EE
 522 .SH SEE ALSO
 523 .BR date (1),
 524 .BR gettimeofday (2),
 525 .BR settimeofday (2),
 526 .BR time (2),
 527 .BR adjtime (3),
 528 .BR clock_getcpuclockid (3),
 529 .BR ctime (3),
 530 .BR ftime (3),
 531 .BR pthread_getcpuclockid (3),
 532 .BR sysconf (3),
 533 .BR time (7),
 534 .BR time_namespaces (7),
 535 .BR vdso (7),
 536 .BR hwclock (8)