man2/clock_nanosleep.2

   1 .\" Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
   2 .\" <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH CLOCK_NANOSLEEP 2 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
   7 .SH NAME
   8 clock_nanosleep \- high-resolution sleep with specifiable clock
   9 .SH LIBRARY
  10 Standard C library
  11 .RI ( libc ", " \-lc ),
  12 since glibc 2.17
  13 .PP
  14 Before glibc 2.17,
  15 Real-time library
  16 .RI ( librt ", " \-lrt )
  17 .SH SYNOPSIS
  18 .B #include <time.h>
  19 .nf
  20 .PP
  21 .BI "int clock_nanosleep(clockid_t " clockid ", int " flags ,
  22 .BI "                    const struct timespec *" request ,
  23 .BI "                    struct timespec *" remain );
  24 .fi
  25 .PP
  26 .RS -4
  27 Feature Test Macro Requirements for glibc (see
  28 .BR feature_test_macros (7)):
  29 .RE
  30 .PP
  31 .BR clock_nanosleep ():
  32 .nf
  33     _POSIX_C_SOURCE >= 200112L
  34 .fi
  35 .SH DESCRIPTION
  36 Like
  37 .BR nanosleep (2),
  38 .BR clock_nanosleep ()
  39 allows the calling thread to sleep for an interval specified
  40 with nanosecond precision.
  41 It differs in allowing the caller to select the clock against
  42 which the sleep interval is to be measured,
  43 and in allowing the sleep interval to be specified as
  44 either an absolute or a relative value.
  45 .PP
  46 The time values passed to and returned by this call are specified using
  47 .BR timespec (3)
  48 structures.
  49 .PP
  50 The
  51 .I clockid
  52 argument specifies the clock against which the sleep interval
  53 is to be measured.
  54 This argument can have one of the following values:
  55 .\" Look in time/posix-timers.c (kernel 5.6 sources) for the
  56 .\" 'struct k_clock' structures that have an 'nsleep' method
  57 .TP
  58 .B CLOCK_REALTIME
  59 A settable system-wide real-time clock.
  60 .TP
  61 .BR CLOCK_TAI " (since Linux 3.10)"
  62 A system-wide clock derived from wall-clock time but ignoring leap seconds.
  63 .TP
  64 .B CLOCK_MONOTONIC
  65 A nonsettable, monotonically increasing clock that measures time
  66 since some unspecified point in the past that does not change after
  67 system startup.
  68 .\" On Linux this clock measures time since boot.
  69 .TP
  70 .BR CLOCK_BOOTTIME " (since Linux 2.6.39)"
  71 Identical to
  72 .BR CLOCK_MONOTONIC ,
  73 except that it also includes any time that the system is suspended.
  74 .TP
  75 .B CLOCK_PROCESS_CPUTIME_ID
  76 A settable per-process clock that measures CPU time consumed
  77 by all threads in the process.
  78 .\" There is some trickery between glibc and the kernel
  79 .\" to deal with the CLOCK_PROCESS_CPUTIME_ID case.
  80 .PP
  81 See
  82 .BR clock_getres (2)
  83 for further details on these clocks.
  84 In addition, the CPU clock IDs returned by
  85 .BR clock_getcpuclockid (3)
  86 and
  87 .BR pthread_getcpuclockid (3)
  88 can also be passed in
  89 .IR clockid .
  90 .\" Sleeping against CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM
  91 .\" is also possible (tested), with CAP_WAKE_ALARM, but I'm not
  92 .\" sure if this is useful or needs to be documented.
  93 .PP
  94 If
  95 .I flags
  96 is 0, then the value specified in
  97 .I request
  98 is interpreted as an interval relative to the current
  99 value of the clock specified by
 100 .IR clockid .
 101 .PP
 102 If
 103 .I flags
 104 is
 105 .BR TIMER_ABSTIME ,
 106 then
 107 .I request
 108 is interpreted as an absolute time as measured by the clock,
 109 .IR clockid .
 110 If
 111 .I request
 112 is less than or equal to the current value of the clock,
 113 then
 114 .BR clock_nanosleep ()
 115 returns immediately without suspending the calling thread.
 116 .PP
 117 .BR clock_nanosleep ()
 118 suspends the execution of the calling thread
 119 until either at least the time specified by
 120 .I request
 121 has elapsed,
 122 or a signal is delivered that causes a signal handler to be called or
 123 that terminates the process.
 124 .PP
 125 If the call is interrupted by a signal handler,
 126 .BR clock_nanosleep ()
 127 fails with the error
 128 .BR EINTR .
 129 In addition, if
 130 .I remain
 131 is not NULL, and
 132 .I flags
 133 was not
 134 .BR TIMER_ABSTIME ,
 135 it returns the remaining unslept time in
 136 .IR remain .
 137 This value can then be used to call
 138 .BR clock_nanosleep ()
 139 again and complete a (relative) sleep.
 140 .SH RETURN VALUE
 141 On successfully sleeping for the requested interval,
 142 .BR clock_nanosleep ()
 143 returns 0.
 144 If the call is interrupted by a signal handler or encounters an error,
 145 then it returns one of the positive error number listed in ERRORS.
 146 .SH ERRORS
 147 .TP
 148 .B EFAULT
 149 .I request
 150 or
 151 .I remain
 152 specified an invalid address.
 153 .TP
 154 .B EINTR
 155 The sleep was interrupted by a signal handler; see
 156 .BR signal (7).
 157 .TP
 158 .B EINVAL
 159 The value in the
 160 .I tv_nsec
 161 field was not in the range 0 to 999999999 or
 162 .I tv_sec
 163 was negative.
 164 .TP
 165 .B EINVAL
 166 .I clockid
 167 was invalid.
 168 .RB ( CLOCK_THREAD_CPUTIME_ID
 169 is not a permitted value for
 170 .IR clockid .)
 171 .TP
 172 .B ENOTSUP
 173 The kernel does not support sleeping against this
 174 .IR clockid .
 175 .SH VERSIONS
 176 The
 177 .BR clock_nanosleep ()
 178 system call first appeared in Linux 2.6.
 179 Support is available in glibc since version 2.1.
 180 .SH STANDARDS
 181 POSIX.1-2001, POSIX.1-2008.
 182 .SH NOTES
 183 If the interval specified in
 184 .I request
 185 is not an exact multiple of the granularity underlying clock (see
 186 .BR time (7)),
 187 then the interval will be rounded up to the next multiple.
 188 Furthermore, after the sleep completes, there may still be a delay before
 189 the CPU becomes free to once again execute the calling thread.
 190 .PP
 191 Using an absolute timer is useful for preventing
 192 timer drift problems of the type described in
 193 .BR nanosleep (2).
 194 (Such problems are exacerbated in programs that try to restart
 195 a relative sleep that is repeatedly interrupted by signals.)
 196 To perform a relative sleep that avoids these problems, call
 197 .BR clock_gettime (2)
 198 for the desired clock,
 199 add the desired interval to the returned time value,
 200 and then call
 201 .BR clock_nanosleep ()
 202 with the
 203 .B TIMER_ABSTIME
 204 flag.
 205 .PP
 206 .BR clock_nanosleep ()
 207 is never restarted after being interrupted by a signal handler,
 208 regardless of the use of the
 209 .BR sigaction (2)
 210 .B SA_RESTART
 211 flag.
 212 .PP
 213 The
 214 .I remain
 215 argument is unused, and unnecessary, when
 216 .I flags
 217 is
 218 .BR TIMER_ABSTIME .
 219 (An absolute sleep can be restarted using the same
 220 .I request
 221 argument.)
 222 .PP
 223 POSIX.1 specifies that
 224 .BR clock_nanosleep ()
 225 has no effect on signals dispositions or the signal mask.
 226 .PP
 227 POSIX.1 specifies that after changing the value of the
 228 .B CLOCK_REALTIME
 229 clock via
 230 .BR clock_settime (2),
 231 the new clock value shall be used to determine the time
 232 at which a thread blocked on an absolute
 233 .BR clock_nanosleep ()
 234 will wake up;
 235 if the new clock value falls past the end of the sleep interval, then the
 236 .BR clock_nanosleep ()
 237 call will return immediately.
 238 .PP
 239 POSIX.1 specifies that
 240 changing the value of the
 241 .B CLOCK_REALTIME
 242 clock via
 243 .BR clock_settime (2)
 244 shall have no effect on a thread that is blocked on a relative
 245 .BR clock_nanosleep ().
 246 .SH SEE ALSO
 247 .BR clock_getres (2),
 248 .BR nanosleep (2),
 249 .BR restart_syscall (2),
 250 .BR timer_create (2),
 251 .BR sleep (3),
 252 .BR timespec (3),
 253 .BR usleep (3),
 254 .BR time (7)