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. In this case, it returns \-1, sets \fIerrno\fR to
  43 .BR EINTR ,
  44 and writes the
  45 remaining time into the structure pointed to by
  46 .IR rem
  47 unless
  48 .I rem
  49 is NULL.
  50 The value of
  51 .I *rem
  52 can then be used to call
  53 .BR nanosleep ()
  54 again and complete the specified pause.
  55
  56 The structure
  57 .I timespec
  58 is used to specify intervals of time with nanosecond precision. It is
  59 specified in
  60 .I <time.h>
  61 and has the form
  62 .sp
  63 .RS
  64 .nf
  65 struct timespec {
  66     time_t tv_sec;        /* seconds */
  67     long   tv_nsec;       /* nanoseconds */
  68 };
  69 .fi
  70 .RE
  71 .PP
  72 The value of the nanoseconds field must be in the range 0 to 999999999.
  73
  74 Compared to
  75 .BR sleep  (3)
  76 and
  77 .BR usleep (3),
  78 .BR nanosleep ()
  79 has the advantage of not affecting any signals, it is standardized by
  80 POSIX, it provides higher timing resolution, and it allows to continue
  81 a sleep that has been interrupted by a signal more easily.
  82 .SH ERRORS
  83 In case of an error or exception, the
  84 .BR nanosleep ()
  85 system call returns \-1 instead of 0 and sets
  86 .I errno
  87 to one of the following values:
  88 .TP
  89 .B EFAULT
  90 Problem with copying information from user space.
  91 .TP
  92 .B EINTR
  93 The pause has been interrupted by a non-blocked signal that was
  94 delivered to the process. The remaining sleep time has been written
  95 into *\fIrem\fR so that the process can easily call
  96 .BR nanosleep ()
  97 again and continue with the pause.
  98 .TP
  99 .B EINVAL
 100 The value in the
 101 .I tv_nsec
 102 field was not in the range 0 to 999999999 or
 103 .I tv_sec
 104 was negative.
 105 .SH BUGS
 106 The current implementation of
 107 .BR nanosleep ()
 108 is based on the normal kernel timer mechanism, which has a resolution
 109 of 1/\fIHZ\fR\ s (i.e, 10\ ms on Linux/i386 and 1\ ms on Linux/Alpha).
 110 Therefore,
 111 .BR nanosleep ()
 112 pauses always for at least the specified time, however it can take up
 113 to 10 ms longer than specified until the process becomes runnable
 114 again. For the same reason, the value returned in case of a delivered
 115 signal in *\fIrem\fR is usually rounded to the next larger multiple of
 116 1/\fIHZ\fR\ s.
 117
 118 .SS "Old behaviour"
 119 In order to support applications requiring much more precise pauses
 120 (e.g., in order to control some time-critical hardware),
 121 .BR nanosleep ()
 122 would handle pauses of up to 2\ ms by busy waiting with microsecond
 123 precision when called from a process scheduled under a real-time policy
 124 like
 125 .I SCHED_FIFO
 126 or
 127 .IR SCHED_RR .
 128 This special extension was removed in kernel 2.5.39, hence is still present in
 129 current 2.4 kernels, but not in 2.6 kernels.
 130 .SH "CONFORMING TO"
 131 POSIX.1b (formerly POSIX.4).
 132 .SH "SEE ALSO"
 133 .BR sched_setscheduler (2),
 134 .BR timer_create (2),
 135 .BR sleep (3),
 136 .BR usleep (3)