man3/clock_getres.3

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   3 .\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
   4 .\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl), Aug 24, 2003
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  26 .\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
  27 .\" 2003-08-24 aeb, large parts rewritten
  28 .\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
  29 .\"
  30 .TH CLOCK_GETRES 3 2003-08-24 "" "Linux Programmer's Manual"
  31 .SH NAME
  32 clock_getres, clock_gettime, clock_settime \- clock and time functions
  33 .SH SYNOPSIS
  34 .B #include <time.h>
  35 .sp
  36 .BI "int clock_getres(clockid_t " clk_id ", struct timespec *" res );
  37 .br
  38 .BI "int clock_gettime(clockid_t " clk_id ", struct timespec *" tp );
  39 .br
  40 .BI "int clock_settime(clockid_t " clk_id ", const struct timespec *" tp );
  41 .SH DESCRIPTION
  42 The function
  43 .BR clock_getres ()
  44 finds the resolution (precision) of the specified clock
  45 .IR clk_id ,
  46 and, if
  47 .I res
  48 is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
  49 .IR res .
  50 The resolution of clocks depends on the implementation and cannot be
  51 configured by a particular process.
  52 If the time value pointed to by the argument
  53 .I tp
  54 of
  55 .BR clock_settime ()
  56 is not a multiple of
  57 .IR res ,
  58 then it is truncated to a multiple of
  59 .IR res .
  60 .PP
  61 The functions
  62 .BR clock_gettime ()
  63 and
  64 .BR clock_settime ()
  65 retrieve and set the time of the specified clock
  66 .IR clk_id .
  67 .PP
  68 The
  69 .I res
  70 and
  71 .I tp
  72 arguments are
  73 .B timespec
  74 structs, as specified  in
  75 .IR <time.h> :
  76 .sp
  77 .nf
  78 struct timespec {
  79 .in +8
  80 time_t   tv_sec;        /* seconds */
  81 long     tv_nsec;       /* nanoseconds */
  82 .in -8
  83 };
  84 .fi
  85 .PP
  86 The
  87 .I clk_id
  88 argument is the identifier of the particular clock on which to act.
  89 A clock may be system-wide and hence visible for all processes, or
  90 per-process if it measures time only within a single process.
  91 .LP
  92 All implementations support the system-wide realtime clock,
  93 which is identified by
  94 .BR CLOCK_REALTIME .
  95 Its time represents seconds and nanoseconds since the Epoch.
  96 When its time is changed, timers for a relative interval are
  97 unaffected, but timers for an absolute point in time are affected.
  98 .LP
  99 More clocks may be implemented.
 100 The interpretation of the
 101 corresponding time values and the effect on timers is unspecified.
 102 .LP
 103 Sufficiently recent versions of GNU libc and the Linux kernel
 104 support the following clocks:
 105 .TP
 106 .B CLOCK_REALTIME
 107 System-wide realtime clock.
 108 Setting this clock requires appropriate privileges.
 109 .TP
 110 .B CLOCK_MONOTONIC
 111 Clock that cannot be set and represents monotonic time since
 112 some unspecified starting point.
 113 .TP
 114 .B CLOCK_PROCESS_CPUTIME_ID
 115 High-resolution per-process timer from the CPU.
 116 .TP
 117 .B CLOCK_THREAD_CPUTIME_ID
 118 Thread-specific CPU-time clock.
 119 .SH "RETURN VALUE"
 120 .BR clock_gettime (),
 121 .BR clock_settime ()
 122 and
 123 .BR clock_getres ()
 124 return 0 for success, or \-1 for failure (in which case
 125 .I errno
 126 is set appropriately).
 127 .SH ERRORS
 128 .TP
 129 .B EFAULT
 130 .I tp
 131 points outside the accessible address space.
 132 .TP
 133 .B EINVAL
 134 The
 135 .I clk_id
 136 specified is not supported on this system.
 137 .TP
 138 .B EPERM
 139 .BR clock_settime ()
 140 does not have permission to set the clock indicated.
 141 .SH "CONFORMING TO"
 142 SUSv2, POSIX.1-2001.
 143 .SH AVAILABILITY
 144 On POSIX systems on which these functions are available, the symbol
 145 .B _POSIX_TIMERS
 146 is defined in \fI<unistd.h>\fP to a value greater than 0.
 147 The symbols
 148 .BR _POSIX_MONOTONIC_CLOCK ,
 149 .BR _POSIX_CPUTIME ,
 150 .B _POSIX_THREAD_CPUTIME
 151 indicate that
 152 .BR CLOCK_MONOTONIC ,
 153 .BR CLOCK_PROCESS_CPUTIME_ID ,
 154 .B CLOCK_THREAD_CPUTIME_ID
 155 are available.
 156 (See also
 157 .BR sysconf (3).)
 158 .SH NOTES
 159 Most systems require the program be linked with the librt
 160 library to use these functions.
 161 .SS Note for SMP systems
 162 The
 163 .B CLOCK_PROCESS_CPUTIME_ID
 164 and
 165 .B CLOCK_THREAD_CPUTIME_ID
 166 clocks are realized on many platforms using timers from the CPUs
 167 (TSC on i386, AR.ITC on Itanium).
 168 These registers may differ between CPUs and as a consequence
 169 these clocks may return
 170 .B bogus results
 171 if a process is migrated to another CPU.
 172 .PP
 173 If the CPUs in an SMP system have different clock sources then
 174 there is no way to maintain a correlation between the timer registers since
 175 each CPU will run at a slightly different frequency.
 176 If that is the case then
 177 .I clock_getcpuclockid(0)
 178 will return
 179 .B ENOENT
 180 to signify this condition.
 181 The two clocks will then only be useful if it
 182 can be ensured that a process stays on a certain CPU.
 183 .PP
 184 The processors in an SMP system do not start all at exactly the same
 185 time and therefore the timer registers are typically running at an offset.
 186 Some architectures include code that attempts to limit these offsets on bootup.
 187 However, the code cannot guarantee to accurately tune the offsets.
 188 Glibc contains no provisions to deal with these offsets (unlike the Linux
 189 Kernel).
 190 Typically these offsets are small and therefore the effects may be
 191 negligible in most cases.
 192 .SH "SEE ALSO"
 193 .BR date (1),
 194 .BR adjtimex (2),
 195 .BR gettimeofday (2),
 196 .BR settimeofday (2),
 197 .BR time (2),
 198 .BR ctime (3),
 199 .BR ftime (3),
 200 .BR sysconf (3)