man2/nanosleep.2

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  25 .\" 1996-04-10  Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
  26 .\"             First version written
  27 .\" Modified, 2004-10-24, aeb
  28 .TH NANOSLEEP 2 2004-10-24 "Linux 2.6.9" "Linux Programmer's Manual"
  29 .SH NAME
  30 nanosleep \- pause execution for a specified time
  31 .SH SYNOPSIS
  32 .B #define _POSIX_C_SOURCE 199309
  33 .B #include <time.h>
  34 .sp
  35 \fBint nanosleep(const struct timespec *\fIreq\fB, struct timespec *\fIrem\fB);
  36 .fi
  37 .SH DESCRIPTION
  38 .BR nanosleep ()
  39 delays the execution of the program for at least the time specified in
  40 .IR *req .
  41 The function can return earlier if a signal has been delivered to the
  42 process.
  43 In this case, it returns \-1, sets \fIerrno\fR to
  44 .BR EINTR ,
  45 and writes the
  46 remaining time into the structure pointed to by
  47 .IR rem
  48 unless
  49 .I rem
  50 is NULL.
  51 The value of
  52 .I *rem
  53 can then be used to call
  54 .BR nanosleep ()
  55 again and complete the specified pause.
  56
  57 The structure
  58 .I timespec
  59 is used to specify intervals of time with nanosecond precision.
  60 It is
  61 specified in
  62 .I <time.h>
  63 and has the form
  64 .sp
  65 .RS
  66 .nf
  67 struct timespec {
  68     time_t tv_sec;        /* seconds */
  69     long   tv_nsec;       /* nanoseconds */
  70 };
  71 .fi
  72 .RE
  73 .PP
  74 The value of the nanoseconds field must be in the range 0 to 999999999.
  75
  76 Compared to
  77 .BR sleep  (3)
  78 and
  79 .BR usleep (3),
  80 .BR nanosleep ()
  81 has the advantage of not affecting any signals, it is standardized by
  82 POSIX, it provides higher timing resolution, and it allows to continue
  83 a sleep that has been interrupted by a signal more easily.
  84 .SH "RETURN VALUE"
  85 On successfully sleeping for the requested interval,
  86 .BR nanosleep ()
  87 returns 0.
  88 If the call is interrupted by a signal handler or encounters an error,
  89 then it returns \-1, with
  90 .I errno
  91 set to indicate the error.
  92 .SH ERRORS
  93 .TP
  94 .B EFAULT
  95 Problem with copying information from user space.
  96 .TP
  97 .B EINTR
  98 The pause has been interrupted by a non-blocked signal that was
  99 delivered to the process.
 100 The remaining sleep time has been written
 101 into *\fIrem\fR so that the process can easily call
 102 .BR nanosleep ()
 103 again and continue with the pause.
 104 .TP
 105 .B EINVAL
 106 The value in the
 107 .I tv_nsec
 108 field was not in the range 0 to 999999999 or
 109 .I tv_sec
 110 was negative.
 111 .SH "CONFORMING TO"
 112 POSIX.1-2001.
 113 .SH BUGS
 114 The current implementation of
 115 .BR nanosleep ()
 116 is based on the normal kernel timer mechanism, which has a resolution
 117 of 1/\fIHZ\fR\ s (see
 118 .BR time (7)).
 119 Therefore,
 120 .BR nanosleep ()
 121 pauses always for at least the specified time, however it can take up
 122 to 10 ms longer than specified until the process becomes runnable
 123 again.
 124 For the same reason, the value returned in case of a delivered
 125 signal in *\fIrem\fR is usually rounded to the next larger multiple of
 126 1/\fIHZ\fR\ s.
 127 .SS "Old behaviour"
 128 In order to support applications requiring much more precise pauses
 129 (e.g., in order to control some time-critical hardware),
 130 .BR nanosleep ()
 131 would handle pauses of up to 2\ ms by busy waiting with microsecond
 132 precision when called from a process scheduled under a real-time policy
 133 like
 134 .I SCHED_FIFO
 135 or
 136 .IR SCHED_RR .
 137 This special extension was removed in kernel 2.5.39,
 138 hence is still present in
 139 current 2.4 kernels, but not in 2.6 kernels.
 140 .PP
 141 In Linux 2.4, if
 142 .BR nanosleep ()
 143 is stopped by a signal (e.g., SIGTSTP),
 144 then the call fails with the error
 145 .BR EINTR
 146 after the process is resumed by a SIGCONT signal.
 147 If the system call is subsequently restarted,
 148 then the time that the process spent in the stopped state is
 149 \fInot\fP counted against the sleep interval.
 150 .SH "SEE ALSO"
 151 .BR sched_setscheduler (2),
 152 .BR timer_create (2),
 153 .BR sleep (3),
 154 .BR usleep (3),
 155 .BR feature_test_macros (7),
 156 .BR time (7)