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>
   4 .\"
   5 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   6 .\"
   7 .\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
   8 .\" 2003-08-24 aeb, large parts rewritten
   9 .\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
  10 .\"
  11 .TH CLOCK_GETRES 2 2021-03-22 "" "Linux Programmer's Manual"
  12 .SH NAME
  13 clock_getres, clock_gettime, clock_settime \- clock and time functions
  14 .SH LIBRARY
  15 Standard C library
  16 .RI ( libc ", " \-lc ),
  17 since glibc 2.17
  18 .PP
  19 Before glibc 2.17,
  20 Real-time library
  21 .RI ( librt ", " \-lrt )
  22 .SH SYNOPSIS
  23 .nf
  24 .B #include <time.h>
  25 .PP
  26 .BI "int clock_getres(clockid_t " clockid ", struct timespec *" res );
  27 .PP
  28 .BI "int clock_gettime(clockid_t " clockid ", struct timespec *" tp );
  29 .BI "int clock_settime(clockid_t " clockid ", const struct timespec *" tp );
  30 .fi
  31 .PP
  32 .RS -4
  33 Feature Test Macro Requirements for glibc (see
  34 .BR feature_test_macros (7)):
  35 .RE
  36 .PP
  37 .BR clock_getres (),
  38 .BR clock_gettime (),
  39 .BR clock_settime ():
  40 .nf
  41     _POSIX_C_SOURCE >= 199309L
  42 .fi
  43 .SH DESCRIPTION
  44 The function
  45 .BR clock_getres ()
  46 finds the resolution (precision) of the specified clock
  47 .IR clockid ,
  48 and, if
  49 .I res
  50 is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
  51 .IR res .
  52 The resolution of clocks depends on the implementation and cannot be
  53 configured by a particular process.
  54 If the time value pointed to by the argument
  55 .I tp
  56 of
  57 .BR clock_settime ()
  58 is not a multiple of
  59 .IR res ,
  60 then it is truncated to a multiple of
  61 .IR res .
  62 .PP
  63 The functions
  64 .BR clock_gettime ()
  65 and
  66 .BR clock_settime ()
  67 retrieve and set the time of the specified clock
  68 .IR clockid .
  69 .PP
  70 The
  71 .I res
  72 and
  73 .I tp
  74 arguments are
  75 .BR timespec (3)
  76 structures.
  77 .PP
  78 The
  79 .I clockid
  80 argument is the identifier of the particular clock on which to act.
  81 A clock may be system-wide and hence visible for all processes, or
  82 per-process if it measures time only within a single process.
  83 .PP
  84 All implementations support the system-wide real-time clock,
  85 which is identified by
  86 .BR CLOCK_REALTIME .
  87 Its time represents seconds and nanoseconds since the Epoch.
  88 When its time is changed, timers for a relative interval are
  89 unaffected, but timers for an absolute point in time are affected.
  90 .PP
  91 More clocks may be implemented.
  92 The interpretation of the
  93 corresponding time values and the effect on timers is unspecified.
  94 .PP
  95 Sufficiently recent versions of glibc and the Linux kernel
  96 support the following clocks:
  97 .TP
  98 .B CLOCK_REALTIME
  99 A settable system-wide clock that measures real (i.e., wall-clock) time.
 100 Setting this clock requires appropriate privileges.
 101 This clock is affected by discontinuous jumps in the system time
 102 (e.g., if the system administrator manually changes the clock),
 103 and by the incremental adjustments performed by
 104 .BR adjtime (3)
 105 and NTP.
 106 .TP
 107 .BR CLOCK_REALTIME_ALARM " (since Linux 3.0; Linux-specific)"
 108 Like
 109 .BR CLOCK_REALTIME ,
 110 but not settable.
 111 See
 112 .BR timer_create (2)
 113 for further details.
 114 .TP
 115 .BR CLOCK_REALTIME_COARSE " (since Linux 2.6.32; Linux-specific)"
 116 .\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
 117 A faster but less precise version of
 118 .BR CLOCK_REALTIME .
 119 This clock is not settable.
 120 Use when you need very fast, but not fine-grained timestamps.
 121 Requires per-architecture support,
 122 and probably also architecture support for this flag in the
 123 .BR vdso (7).
 124 .TP
 125 .BR CLOCK_TAI " (since Linux 3.10; Linux-specific)"
 126 .\" commit 1ff3c9677bff7e468e0c487d0ffefe4e901d33f4
 127 A nonsettable system-wide clock derived from wall-clock time
 128 but ignoring leap seconds.
 129 This clock does
 130 not experience discontinuities and backwards jumps caused by NTP
 131 inserting leap seconds as
 132 .BR CLOCK_REALTIME
 133 does.
 134 .IP
 135 The acronym TAI refers to International Atomic Time.
 136 .TP
 137 .B CLOCK_MONOTONIC
 138 A nonsettable system-wide clock that
 139 represents monotonic time since\(emas described
 140 by POSIX\(em"some unspecified point in the past".
 141 On Linux, that point corresponds to the number of seconds that the system
 142 has been running since it was booted.
 143 .IP
 144 The
 145 .B CLOCK_MONOTONIC
 146 clock is not affected by discontinuous jumps in the system time
 147 (e.g., if the system administrator manually changes the clock),
 148 but is affected by the incremental adjustments performed by
 149 .BR adjtime (3)
 150 and NTP.
 151 This clock does not count time that the system is suspended.
 152 All
 153 .B CLOCK_MONOTONIC
 154 variants guarantee that the time returned by consecutive calls will not go
 155 backwards, but successive calls may\(emdepending on the architecture\(emreturn
 156 identical (not-increased) time values.
 157 .TP
 158 .BR CLOCK_MONOTONIC_COARSE " (since Linux 2.6.32; Linux-specific)"
 159 .\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
 160 A faster but less precise version of
 161 .BR CLOCK_MONOTONIC .
 162 Use when you need very fast, but not fine-grained timestamps.
 163 Requires per-architecture support,
 164 and probably also architecture support for this flag in the
 165 .BR vdso (7).
 166 .TP
 167 .BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
 168 .\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
 169 Similar to
 170 .BR CLOCK_MONOTONIC ,
 171 but provides access to a raw hardware-based time
 172 that is not subject to NTP adjustments or
 173 the incremental adjustments performed by
 174 .BR adjtime (3).
 175 This clock does not count time that the system is suspended.
 176 .TP
 177 .BR CLOCK_BOOTTIME " (since Linux 2.6.39; Linux-specific)"
 178 .\" commit 7fdd7f89006dd5a4c702fa0ce0c272345fa44ae0
 179 .\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
 180 A nonsettable system-wide clock that is identical to
 181 .BR CLOCK_MONOTONIC ,
 182 except that it also includes any time that the system is suspended.
 183 This allows applications to get a suspend-aware monotonic clock
 184 without having to deal with the complications of
 185 .BR CLOCK_REALTIME ,
 186 which may have discontinuities if the time is changed using
 187 .BR settimeofday (2)
 188 or similar.
 189 .TP
 190 .BR CLOCK_BOOTTIME_ALARM " (since Linux 3.0; Linux-specific)"
 191 Like
 192 .BR CLOCK_BOOTTIME .
 193 See
 194 .BR timer_create (2)
 195 for further details.
 196 .TP
 197 .BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
 198 This is a clock that measures CPU time consumed by this process
 199 (i.e., CPU time consumed by all threads in the process).
 200 On Linux, this clock is not settable.
 201 .TP
 202 .BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
 203 This is a clock that measures CPU time consumed by this thread.
 204 On Linux, this clock is not settable.
 205 .PP
 206 Linux also implements dynamic clock instances as described below.
 207 .SS Dynamic clocks
 208 In addition to the hard-coded System-V style clock IDs described above,
 209 Linux also supports
 210 POSIX clock operations on certain character devices.
 211 Such devices are
 212 called "dynamic" clocks, and are supported since Linux 2.6.39.
 213 .PP
 214 Using the appropriate macros, open file
 215 descriptors may be converted into clock IDs and passed to
 216 .BR clock_gettime (),
 217 .BR clock_settime (),
 218 and
 219 .BR clock_adjtime (2).
 220 The following example shows how to convert a file descriptor into a
 221 dynamic clock ID.
 222 .PP
 223 .in +4n
 224 .EX
 225 #define CLOCKFD 3
 226 #define FD_TO_CLOCKID(fd)   ((\(ti(clockid_t) (fd) << 3) | CLOCKFD)
 227 #define CLOCKID_TO_FD(clk)  ((unsigned int) \(ti((clk) >> 3))
 228
 229 struct timespec ts;
 230 clockid_t clkid;
 231 int fd;
 232
 233 fd = open("/dev/ptp0", O_RDWR);
 234 clkid = FD_TO_CLOCKID(fd);
 235 clock_gettime(clkid, &ts);
 236 .EE
 237 .in
 238 .SH RETURN VALUE
 239 .BR clock_gettime (),
 240 .BR clock_settime (),
 241 and
 242 .BR clock_getres ()
 243 return 0 for success.
 244 On error, \-1 is returned and
 245 .I errno
 246 is set to indicate the error.
 247 .SH ERRORS
 248 .TP
 249 .B EACCES
 250 .BR clock_settime ()
 251 does not have write permission for the dynamic POSIX
 252 clock device indicated.
 253 .TP
 254 .B EFAULT
 255 .I tp
 256 points outside the accessible address space.
 257 .TP
 258 .B EINVAL
 259 The
 260 .I clockid
 261 specified is invalid for one of two reasons.
 262 Either the System-V style
 263 hard coded positive value is out of range, or the dynamic clock ID
 264 does not refer to a valid instance of a clock object.
 265 .\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
 266 .\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
 267 .\" EPERM.
 268 .TP
 269 .B EINVAL
 270 .RB ( clock_settime ()):
 271 .I tp.tv_sec
 272 is negative or
 273 .I tp.tv_nsec
 274 is outside the range [0..999,999,999].
 275 .TP
 276 .B EINVAL
 277 The
 278 .I clockid
 279 specified in a call to
 280 .BR clock_settime ()
 281 is not a settable clock.
 282 .TP
 283 .BR EINVAL " (since Linux 4.3)"
 284 .\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
 285 A call to
 286 .BR clock_settime ()
 287 with a
 288 .I clockid
 289 of
 290 .B CLOCK_REALTIME
 291 attempted to set the time to a value less than
 292 the current value of the
 293 .B CLOCK_MONOTONIC
 294 clock.
 295 .TP
 296 .B ENODEV
 297 The hot-pluggable device (like USB for example) represented by a
 298 dynamic
 299 .I clk_id
 300 has disappeared after its character device was opened.
 301 .TP
 302 .B ENOTSUP
 303 The operation is not supported by the dynamic POSIX clock device
 304 specified.
 305 .TP
 306 .B EPERM
 307 .BR clock_settime ()
 308 does not have permission to set the clock indicated.
 309 .SH VERSIONS
 310 These system calls first appeared in Linux 2.6.
 311 .SH ATTRIBUTES
 312 For an explanation of the terms used in this section, see
 313 .BR attributes (7).
 314 .ad l
 315 .nh
 316 .TS
 317 allbox;
 318 lbx lb lb
 319 l l l.
 320 Interface       Attribute       Value
 321 T{
 322 .BR clock_getres (),
 323 .BR clock_gettime (),
 324 .BR clock_settime ()
 325 T}      Thread safety   MT-Safe
 326 .TE
 327 .hy
 328 .ad
 329 .sp 1
 330 .SH CONFORMING TO
 331 POSIX.1-2001, POSIX.1-2008, SUSv2.
 332 .PP
 333 On POSIX systems on which these functions are available, the symbol
 334 .B _POSIX_TIMERS
 335 is defined in \fI<unistd.h>\fP to a value greater than 0.
 336 The symbols
 337 .BR _POSIX_MONOTONIC_CLOCK ,
 338 .BR _POSIX_CPUTIME ,
 339 .B _POSIX_THREAD_CPUTIME
 340 indicate that
 341 .BR CLOCK_MONOTONIC ,
 342 .BR CLOCK_PROCESS_CPUTIME_ID ,
 343 .B CLOCK_THREAD_CPUTIME_ID
 344 are available.
 345 (See also
 346 .BR sysconf (3).)
 347 .SH NOTES
 348 POSIX.1 specifies the following:
 349 .RS
 350 .PP
 351 Setting the value of the
 352 .B CLOCK_REALTIME
 353 clock via
 354 .BR clock_settime ()
 355 shall have no effect on threads that are blocked waiting for a relative time
 356 service based upon this clock, including the
 357 .BR nanosleep ()
 358 function; nor on the expiration of relative timers based upon this clock.
 359 Consequently, these time services shall expire when the requested relative
 360 interval elapses, independently of the new or old value of the clock.
 361 .RE
 362 .PP
 363 According to POSIX.1-2001, a process with "appropriate privileges" may set the
 364 .B CLOCK_PROCESS_CPUTIME_ID
 365 and
 366 .B CLOCK_THREAD_CPUTIME_ID
 367 clocks using
 368 .BR clock_settime ().
 369 On Linux, these clocks are not settable
 370 (i.e., no process has "appropriate privileges").
 371 .\" See http://bugzilla.kernel.org/show_bug.cgi?id=11972
 372 .\"
 373 .SS C library/kernel differences
 374 On some architectures, an implementation of
 375 .BR clock_gettime ()
 376 is provided in the
 377 .BR vdso (7).
 378 .\"
 379 .SS Historical note for SMP systems
 380 Before Linux added kernel support for
 381 .B CLOCK_PROCESS_CPUTIME_ID
 382 and
 383 .BR CLOCK_THREAD_CPUTIME_ID ,
 384 glibc implemented these clocks on many platforms using timer
 385 registers from the CPUs
 386 (TSC on i386, AR.ITC on Itanium).
 387 These registers may differ between CPUs and as a consequence
 388 these clocks may return
 389 .B bogus results
 390 if a process is migrated to another CPU.
 391 .PP
 392 If the CPUs in an SMP system have different clock sources, then
 393 there is no way to maintain a correlation between the timer registers since
 394 each CPU will run at a slightly different frequency.
 395 If that is the case, then
 396 .I clock_getcpuclockid(0)
 397 will return
 398 .B ENOENT
 399 to signify this condition.
 400 The two clocks will then be useful only if it
 401 can be ensured that a process stays on a certain CPU.
 402 .PP
 403 The processors in an SMP system do not start all at exactly the same
 404 time and therefore the timer registers are typically running at an offset.
 405 Some architectures include code that attempts to limit these offsets on bootup.
 406 However, the code cannot guarantee to accurately tune the offsets.
 407 Glibc contains no provisions to deal with these offsets (unlike the Linux
 408 Kernel).
 409 Typically these offsets are small and therefore the effects may be
 410 negligible in most cases.
 411 .PP
 412 Since glibc 2.4,
 413 the wrapper functions for the system calls described in this page avoid
 414 the abovementioned problems by employing the kernel implementation of
 415 .B CLOCK_PROCESS_CPUTIME_ID
 416 and
 417 .BR CLOCK_THREAD_CPUTIME_ID ,
 418 on systems that provide such an implementation
 419 (i.e., Linux 2.6.12 and later).
 420 .SH EXAMPLES
 421 The program below demonstrates the use of
 422 .BR clock_gettime ()
 423 and
 424 .BR clock_getres ()
 425 with various clocks.
 426 This is an example of what we might see when running the program:
 427 .PP
 428 .in +4n
 429 .EX
 430 $ \fB./clock_times x\fP
 431 CLOCK_REALTIME : 1585985459.446 (18356 days +  7h 30m 59s)
 432      resolution:          0.000000001
 433 CLOCK_TAI      : 1585985496.447 (18356 days +  7h 31m 36s)
 434      resolution:          0.000000001
 435 CLOCK_MONOTONIC:      52395.722 (14h 33m 15s)
 436      resolution:          0.000000001
 437 CLOCK_BOOTTIME :      72691.019 (20h 11m 31s)
 438      resolution:          0.000000001
 439 .EE
 440 .in
 441 .SS Program source
 442 \&
 443 .EX
 444 /* clock_times.c
 445
 446    Licensed under GNU General Public License v2 or later.
 447 */
 448 #define _XOPEN_SOURCE 600
 449 #include <time.h>
 450 #include <stdint.h>
 451 #include <stdio.h>
 452 #include <stdlib.h>
 453 #include <stdbool.h>
 454 #include <unistd.h>
 455
 456 #define SECS_IN_DAY (24 * 60 * 60)
 457
 458 static void
 459 displayClock(clockid_t clock, const char *name, bool showRes)
 460 {
 461     struct timespec ts;
 462
 463     if (clock_gettime(clock, &ts) == \-1) {
 464         perror("clock_gettime");
 465         exit(EXIT_FAILURE);
 466     }
 467
 468     printf("%\-15s: %10jd.%03ld (", name,
 469             (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);
 470
 471     long days = ts.tv_sec / SECS_IN_DAY;
 472     if (days > 0)
 473         printf("%ld days + ", days);
 474
 475     printf("%2dh %2dm %2ds",
 476             (int) (ts.tv_sec % SECS_IN_DAY) / 3600,
 477             (int) (ts.tv_sec % 3600) / 60,
 478             (int) ts.tv_sec % 60);
 479     printf(")\en");
 480
 481     if (clock_getres(clock, &ts) == \-1) {
 482         perror("clock_getres");
 483         exit(EXIT_FAILURE);
 484     }
 485
 486     if (showRes)
 487         printf("     resolution: %10jd.%09ld\en",
 488                 (intmax_t) ts.tv_sec, ts.tv_nsec);
 489 }
 490
 491 int
 492 main(int argc, char *argv[])
 493 {
 494     bool showRes = argc > 1;
 495
 496     displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
 497 #ifdef CLOCK_TAI
 498     displayClock(CLOCK_TAI, "CLOCK_TAI", showRes);
 499 #endif
 500     displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes);
 501 #ifdef CLOCK_BOOTTIME
 502     displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes);
 503 #endif
 504     exit(EXIT_SUCCESS);
 505 }
 506 .EE
 507 .SH SEE ALSO
 508 .BR date (1),
 509 .BR gettimeofday (2),
 510 .BR settimeofday (2),
 511 .BR time (2),
 512 .BR adjtime (3),
 513 .BR clock_getcpuclockid (3),
 514 .BR ctime (3),
 515 .BR ftime (3),
 516 .BR pthread_getcpuclockid (3),
 517 .BR sysconf (3),
 518 .BR timespec (3),
 519 .BR time (7),
 520 .BR time_namespaces (7),
 521 .BR vdso (7),
 522 .BR hwclock (8)