man2/clock_nanosleep.2

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  26 .TH CLOCK_NANOSLEEP 2 2020-04-11 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 clock_nanosleep \- high-resolution sleep with specifiable clock
  29 .SH SYNOPSIS
  30 .B #include <time.h>
  31 .nf
  32 .PP
  33 .BI "int clock_nanosleep(clockid_t " clockid ", int " flags ,
  34 .BI "                    const struct timespec *" request ,
  35 .BI "                    struct timespec *" remain );
  36 .fi
  37 .PP
  38 Link with \fI\-lrt\fP (only for glibc versions before 2.17).
  39 .PP
  40 .ad l
  41 .in -4n
  42 Feature Test Macro Requirements for glibc (see
  43 .BR feature_test_macros (7)):
  44 .in
  45 .PP
  46 .BR clock_nanosleep ():
  47 .RS 4
  48 _POSIX_C_SOURCE\ >=\ 200112L
  49 .RE
  50 .ad
  51 .SH DESCRIPTION
  52 Like
  53 .BR nanosleep (2),
  54 .BR clock_nanosleep ()
  55 allows the calling thread to sleep for an interval specified
  56 with nanosecond precision.
  57 It differs in allowing the caller to select the clock against
  58 which the sleep interval is to be measured,
  59 and in allowing the sleep interval to be specified as
  60 either an absolute or a relative value.
  61 .PP
  62 The time values passed to and returned by this call are specified using
  63 .I timespec
  64 structures, defined as follows:
  65 .PP
  66 .in +4n
  67 .EX
  68 struct timespec {
  69     time_t tv_sec;        /* seconds */
  70     long   tv_nsec;       /* nanoseconds [0 .. 999999999] */
  71 };
  72 .EE
  73 .in
  74 .PP
  75 The
  76 .I clockid
  77 argument specifies the clock against which the sleep interval
  78 is to be measured.
  79 This argument can have one of the following values:
  80 .\" Look in time/posix-timers.c (kernel 5.6 sources) for the
  81 .\" 'struct k_clock' structures that have an 'nsleep' method
  82 .TP
  83 .BR CLOCK_REALTIME
  84 A settable system-wide real-time clock.
  85 .TP
  86 .BR CLOCK_TAI " (since Linux 3.10)"
  87 A system-wide clock derived from wall-clock time but ignoring leap seconds.
  88 .TP
  89 .BR CLOCK_MONOTONIC
  90 A nonsettable, monotonically increasing clock that measures time
  91 since some unspecified point in the past that does not change after
  92 system startup.
  93 .\" On Linux this clock measures time since boot.
  94 .TP
  95 .BR CLOCK_BOOTIME " (since Linux 2.6.39)"
  96 Identical to
  97 .BR CLOCK_MONOTONIC ,
  98 except that it also includes any time that the system is suspended.
  99 .TP
 100 .BR CLOCK_PROCESS_CPUTIME_ID
 101 A settable per-process clock that measures CPU time consumed
 102 by all threads in the process.
 103 .\" There is some trickery between glibc and the kernel
 104 .\" to deal with the CLOCK_PROCESS_CPUTIME_ID case.
 105 .PP
 106 See
 107 .BR clock_getres (2)
 108 for further details on these clocks.
 109 In addition, the CPU clock IDs returned by
 110 .BR clock_getcpuclockid (3)
 111 and
 112 .BR pthread_getcpuclockid (3)
 113 can also be passed in
 114 .IR clockid .
 115 .\" Sleeping against CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM
 116 .\" is also possible (tested), with CAP_WAKE_ALARM, but I'm not
 117 .\" sure if this is useful or needs to be documented.
 118 .PP
 119 If
 120 .I flags
 121 is 0, then the value specified in
 122 .I request
 123 is interpreted as an interval relative to the current
 124 value of the clock specified by
 125 .IR clockid .
 126 .PP
 127 If
 128 .I flags
 129 is
 130 .BR TIMER_ABSTIME ,
 131 then
 132 .I request
 133 is interpreted as an absolute time as measured by the clock,
 134 .IR clockid .
 135 If
 136 .I request
 137 is less than or equal to the current value of the clock,
 138 then
 139 .BR clock_nanosleep ()
 140 returns immediately without suspending the calling thread.
 141 .PP
 142 .BR clock_nanosleep ()
 143 suspends the execution of the calling thread
 144 until either at least the time specified by
 145 .IR request
 146 has elapsed,
 147 or a signal is delivered that causes a signal handler to be called or
 148 that terminates the process.
 149 .PP
 150 If the call is interrupted by a signal handler,
 151 .BR clock_nanosleep ()
 152 fails with the error
 153 .BR EINTR .
 154 In addition, if
 155 .I remain
 156 is not NULL, and
 157 .I flags
 158 was not
 159 .BR TIMER_ABSTIME ,
 160 it returns the remaining unslept time in
 161 .IR remain .
 162 This value can then be used to call
 163 .BR clock_nanosleep ()
 164 again and complete a (relative) sleep.
 165 .SH RETURN VALUE
 166 On successfully sleeping for the requested interval,
 167 .BR clock_nanosleep ()
 168 returns 0.
 169 If the call is interrupted by a signal handler or encounters an error,
 170 then it returns one of the positive error number listed in ERRORS.
 171 .SH ERRORS
 172 .TP
 173 .B EFAULT
 174 .I request
 175 or
 176 .I remain
 177 specified an invalid address.
 178 .TP
 179 .B EINTR
 180 The sleep was interrupted by a signal handler; see
 181 .BR signal (7).
 182 .TP
 183 .B EINVAL
 184 The value in the
 185 .I tv_nsec
 186 field was not in the range 0 to 999999999 or
 187 .I tv_sec
 188 was negative.
 189 .TP
 190 .B EINVAL
 191 .I clockid
 192 was invalid.
 193 .RB ( CLOCK_THREAD_CPUTIME_ID
 194 is not a permitted value for
 195 .IR clockid .)
 196 .TP
 197 .B ENOTSUP
 198 The kernel does not support sleeping against this
 199 .IR clockid .
 200 .SH VERSIONS
 201 The
 202 .BR clock_nanosleep ()
 203 system call first appeared in Linux 2.6.
 204 Support is available in glibc since version 2.1.
 205 .SH CONFORMING TO
 206 POSIX.1-2001, POSIX.1-2008.
 207 .SH NOTES
 208 If the interval specified in
 209 .I request
 210 is not an exact multiple of the granularity underlying clock (see
 211 .BR time (7)),
 212 then the interval will be rounded up to the next multiple.
 213 Furthermore, after the sleep completes, there may still be a delay before
 214 the CPU becomes free to once again execute the calling thread.
 215 .PP
 216 Using an absolute timer is useful for preventing
 217 timer drift problems of the type described in
 218 .BR nanosleep (2).
 219 (Such problems are exacerbated in programs that try to restart
 220 a relative sleep that is repeatedly interrupted by signals.)
 221 To perform a relative sleep that avoids these problems, call
 222 .BR clock_gettime (2)
 223 for the desired clock,
 224 add the desired interval to the returned time value,
 225 and then call
 226 .BR clock_nanosleep ()
 227 with the
 228 .B TIMER_ABSTIME
 229 flag.
 230 .PP
 231 .BR clock_nanosleep ()
 232 is never restarted after being interrupted by a signal handler,
 233 regardless of the use of the
 234 .BR sigaction (2)
 235 .B SA_RESTART
 236 flag.
 237 .PP
 238 The
 239 .I remain
 240 argument is unused, and unnecessary, when
 241 .I flags
 242 is
 243 .BR TIMER_ABSTIME .
 244 (An absolute sleep can be restarted using the same
 245 .I request
 246 argument.)
 247 .PP
 248 POSIX.1 specifies that
 249 .BR clock_nanosleep ()
 250 has no effect on signals dispositions or the signal mask.
 251 .PP
 252 POSIX.1 specifies that after changing the value of the
 253 .B CLOCK_REALTIME
 254 clock via
 255 .BR clock_settime (2),
 256 the new clock value shall be used to determine the time
 257 at which a thread blocked on an absolute
 258 .BR clock_nanosleep ()
 259 will wake up;
 260 if the new clock value falls past the end of the sleep interval, then the
 261 .BR clock_nanosleep ()
 262 call will return immediately.
 263 .PP
 264 POSIX.1 specifies that
 265 changing the value of the
 266 .B CLOCK_REALTIME
 267 clock via
 268 .BR clock_settime (2)
 269 shall have no effect on a thread that is blocked on a relative
 270 .BR clock_nanosleep ().
 271 .SH SEE ALSO
 272 .BR clock_getres (2),
 273 .BR nanosleep (2),
 274 .BR restart_syscall (2),
 275 .BR timer_create (2),
 276 .BR sleep (3),
 277 .BR usleep (3),
 278 .BR time (7)