clock_getres(3)

1CLOCK_GETRES(3)            Linux Programmer's Manual           CLOCK_GETRES(3)
2
3
4

NAME

6       clock_getres, clock_gettime, clock_settime - clock and time functions
7

SYNOPSIS

9       #include <time.h>
10
11       int clock_getres(clockid_t clk_id, struct timespec *res);
12       int clock_gettime(clockid_t clk_id, struct timespec *tp);
13       int clock_settime(clockid_t clk_id, const struct timespec *tp);
14

DESCRIPTION

16       The  function  clock_getres()  finds  the resolution (precision) of the
17       specified clock clk_id, and, if res  is  non-NULL,  stores  it  in  the
18       struct timespec pointed to by res.  The resolution of clocks depends on
19       the implementation and cannot be configured by  a  particular  process.
20       If  the  time value pointed to by the argument tp of clock_settime() is
21       not a multiple of res, then it is truncated to a multiple of res.
22
23       The functions clock_gettime() and clock_settime() retrieve and set  the
24       time of the specified clock clk_id.
25
26       The  res  and  tp  arguments  are  timespec  structs,  as specified  in
27       <time.h>:
28
29       struct timespec {
30               time_t   tv_sec;        /* seconds */
31               long     tv_nsec;       /* nanoseconds */
32       };
33
34       The clk_id argument is the identifier of the particular clock on  which
35       to  act.   A  clock  may  be system-wide and hence visible for all pro‐
36       cesses, or per-process  if  it  measures  time  only  within  a  single
37       process.
38
39       All  implementations  support  the system-wide realtime clock, which is
40       identified by CLOCK_REALTIME.  Its time represents seconds and nanosec‐
41       onds  since the Epoch.  When its time is changed, timers for a relative
42       interval are unaffected, but timers for an absolute point in  time  are
43       affected.
44
45       More clocks may be implemented. The interpretation of the corresponding
46       time values and the effect on timers is unspecified.
47
48       Sufficiently recent versions of GNU libc and the Linux  kernel  support
49       the following clocks:
50
51       CLOCK_REALTIME
52              System-wide  realtime clock.  Setting this clock requires appro‐
53              priate privileges.
54
55       CLOCK_MONOTONIC
56              Clock that cannot be set and  represents  monotonic  time  since
57              some unspecified starting point.
58
59       CLOCK_PROCESS_CPUTIME_ID
60              High-resolution per-process timer from the CPU.
61
62       CLOCK_THREAD_CPUTIME_ID
63              Thread-specific CPU-time clock.
64

RETURN VALUE

66       clock_gettime(),  clock_settime()  and clock_getres() return 0 for suc‐
67       cess, or -1 for failure (in which case errno is set appropriately).
68

ERRORS

70       EFAULT tp points outside the accessible address space.
71
72       EINVAL The clk_id specified is not supported on this system.
73
74       EPERM  clock_settime() does not have permission to set the clock  indi‐
75              cated.
76

NOTE

78       Most  systems  require  the program be linked with the librt library to
79       use these functions.
80

NOTE for SMP systems

82       The CLOCK_PROCESS_CPUTIME_ID  and  CLOCK_THREAD_CPUTIME_ID  clocks  are
83       realized  on  many  platforms  using timers from the CPUs (TSC on i386,
84       AR.ITC on Itanium).  These registers may differ between CPUs and  as  a
85       consequence  these  clocks  may  return  bogus  results if a process is
86       migrated to another CPU.
87
88       If the CPUs in an SMP system have different clock sources then there is
89       no way to maintain a correlation between the timer registers since each
90       CPU will run at a slightly different frequency. If  that  is  the  case
91       then  clock_getcpuclockid(0)  will return ENOENT to signify this condi‐
92       tion. The two clocks will then only be useful if it can be ensured that
93       a process stays on a certain CPU.
94
95       The  processors  in  an SMP system do not start all at exactly the same
96       time and therefore the timer registers are typically running at an off‐
97       set.  Some architectures include code that attempts to limit these off‐
98       sets on bootup.  However, the code cannot guarantee to accurately  tune
99       the  offsets.   Glibc contains no provisions to deal with these offsets
100       (unlike the Linux Kernel). Typically these offsets are small and there‐
101       fore the effects may be negligible in most cases.
102

AVAILABILITY

104       On  POSIX  systems  on  which these functions are available, the symbol
105       _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0.   The
106       symbols  _POSIX_MONOTONIC_CLOCK,  _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME
107       indicate      that      CLOCK_MONOTONIC,      CLOCK_PROCESS_CPUTIME_ID,
108       CLOCK_THREAD_CPUTIME_ID are available.  (See also sysconf(3).)
109

CONFORMING TO

111       SUSv2, POSIX.1-2001.
112