1CLOCK_GETRES(2)            Linux Programmer's Manual           CLOCK_GETRES(2)
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
13       int clock_gettime(clockid_t clk_id, struct timespec *tp);
14
15       int clock_settime(clockid_t clk_id, const struct timespec *tp);
16
17       Link with -lrt (only for glibc versions before 2.17).
18
19   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20
21       clock_getres(), clock_gettime(), clock_settime():
22              _POSIX_C_SOURCE >= 199309L
23

DESCRIPTION

25       The  function  clock_getres()  finds  the resolution (precision) of the
26       specified clock clk_id, and, if res  is  non-NULL,  stores  it  in  the
27       struct timespec pointed to by res.  The resolution of clocks depends on
28       the implementation and cannot be configured by  a  particular  process.
29       If  the  time value pointed to by the argument tp of clock_settime() is
30       not a multiple of res, then it is truncated to a multiple of res.
31
32       The functions clock_gettime() and clock_settime() retrieve and set  the
33       time of the specified clock clk_id.
34
35       The  res  and  tp  arguments  are  timespec structures, as specified in
36       <time.h>:
37
38           struct timespec {
39               time_t   tv_sec;        /* seconds */
40               long     tv_nsec;       /* nanoseconds */
41           };
42
43       The clk_id argument is the identifier of the particular clock on  which
44       to  act.   A  clock  may  be system-wide and hence visible for all pro‐
45       cesses, or per-process  if  it  measures  time  only  within  a  single
46       process.
47
48       All  implementations  support the system-wide real-time clock, which is
49       identified by CLOCK_REALTIME.  Its time represents seconds and nanosec‐
50       onds  since the Epoch.  When its time is changed, timers for a relative
51       interval are unaffected, but timers for an absolute point in  time  are
52       affected.
53
54       More  clocks may be implemented.  The interpretation of the correspond‐
55       ing time values and the effect on timers is unspecified.
56
57       Sufficiently recent versions of glibc and the Linux kernel support  the
58       following clocks:
59
60       CLOCK_REALTIME
61              System-wide  clock  that  measures real (i.e., wall-clock) time.
62              Setting this clock requires appropriate privileges.  This  clock
63              is  affected by discontinuous jumps in the system time (e.g., if
64              the system administrator manually changes the clock), and by the
65              incremental adjustments performed by adjtime(3) and NTP.
66
67       CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
68              A  faster  but less precise version of CLOCK_REALTIME.  Use when
69              you need very fast, but not fine-grained  timestamps.   Requires
70              per-architecture support, and probably also architecture support
71              for this flag in the vdso(7).
72
73       CLOCK_MONOTONIC
74              Clock that cannot be set and  represents  monotonic  time  since
75              some  unspecified starting point.  This clock is not affected by
76              discontinuous jumps in the system  time  (e.g.,  if  the  system
77              administrator  manually  changes  the clock), but is affected by
78              the incremental adjustments performed by adjtime(3) and NTP.
79
80       CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific)
81              A faster but less precise version of CLOCK_MONOTONIC.  Use  when
82              you  need  very fast, but not fine-grained timestamps.  Requires
83              per-architecture support, and probably also architecture support
84              for this flag in the vdso(7).
85
86       CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
87              Similar  to  CLOCK_MONOTONIC, but provides access to a raw hard‐
88              ware-based time that is not subject to NTP  adjustments  or  the
89              incremental adjustments performed by adjtime(3).
90
91       CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
92              Identical  to  CLOCK_MONOTONIC, except it also includes any time
93              that the system is suspended.  This allows applications to get a
94              suspend-aware  monotonic  clock  without having to deal with the
95              complications of CLOCK_REALTIME, which may have  discontinuities
96              if the time is changed using settimeofday(2) or similar.
97
98       CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
99              Per-process  CPU-time  clock  (measures CPU time consumed by all
100              threads in the process).
101
102       CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
103              Thread-specific CPU-time clock.
104

RETURN VALUE

106       clock_gettime(), clock_settime(), and clock_getres() return 0 for  suc‐
107       cess, or -1 for failure (in which case errno is set appropriately).
108

ERRORS

110       EFAULT tp points outside the accessible address space.
111
112       EINVAL The clk_id specified is not supported on this system.
113
114       EPERM  clock_settime()  does not have permission to set the clock indi‐
115              cated.
116

VERSIONS

118       These system calls first appeared in Linux 2.6.
119

ATTRIBUTES

121       For  an  explanation  of  the  terms  used   in   this   section,   see
122       attributes(7).
123
124       ┌─────────────────────────────────┬───────────────┬─────────┐
125Interface                        Attribute     Value   
126       ├─────────────────────────────────┼───────────────┼─────────┤
127clock_getres(), clock_gettime(), │ Thread safety │ MT-Safe │
128clock_settime()                  │               │         │
129       └─────────────────────────────────┴───────────────┴─────────┘
130

CONFORMING TO

132       POSIX.1-2001, POSIX.1-2008, SUSv2.
133

AVAILABILITY

135       On POSIX systems on which these functions  are  available,  the  symbol
136       _POSIX_TIMERS  is defined in <unistd.h> to a value greater than 0.  The
137       symbols _POSIX_MONOTONIC_CLOCK,  _POSIX_CPUTIME,  _POSIX_THREAD_CPUTIME
138       indicate      that      CLOCK_MONOTONIC,      CLOCK_PROCESS_CPUTIME_ID,
139       CLOCK_THREAD_CPUTIME_ID are available.  (See also sysconf(3).)
140

NOTES

142       POSIX.1 specifies the following:
143
144              Setting the value of the  CLOCK_REALTIME  clock  via  clock_set‐
145              time()  shall have no effect on threads that are blocked waiting
146              for a relative time service based upon this clock, including the
147              nanosleep()  function;  nor on the expiration of relative timers
148              based upon this clock.  Consequently, these time services  shall
149              expire  when  the  requested relative interval elapses, indepen‐
150              dently of the new or old value of the clock.
151
152   C library/kernel differences
153       On some architectures, an implementation of clock_gettime() is provided
154       in the vdso(7).
155
156   Historical note for SMP systems
157       Before  Linux  added  kernel  support  for CLOCK_PROCESS_CPUTIME_ID and
158       CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on  many  plat‐
159       forms  using timer registers from the CPUs (TSC on i386, AR.ITC on Ita‐
160       nium).  These registers may differ between CPUs and  as  a  consequence
161       these  clocks  may  return  bogus  results  if a process is migrated to
162       another CPU.
163
164       If the CPUs in an SMP system have different clock sources,  then  there
165       is  no  way to maintain a correlation between the timer registers since
166       each CPU will run at a slightly different frequency.  If  that  is  the
167       case,  then  clock_getcpuclockid(0)  will return ENOENT to signify this
168       condition.  The two clocks will then  be  useful  only  if  it  can  be
169       ensured that a process stays on a certain CPU.
170
171       The  processors  in  an SMP system do not start all at exactly the same
172       time and therefore the timer registers are typically running at an off‐
173       set.  Some architectures include code that attempts to limit these off‐
174       sets on bootup.  However, the code cannot guarantee to accurately  tune
175       the  offsets.   Glibc contains no provisions to deal with these offsets
176       (unlike the Linux Kernel).   Typically  these  offsets  are  small  and
177       therefore the effects may be negligible in most cases.
178
179       Since  glibc  2.4, the wrapper functions for the system calls described
180       in this page avoid the abovementioned problems by employing the  kernel
181       implementation of CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID,
182       on systems that provide such an implementation (i.e., Linux 2.6.12  and
183       later).
184

BUGS

186       According  to POSIX.1-2001, a process with "appropriate privileges" may
187       set the  CLOCK_PROCESS_CPUTIME_ID  and  CLOCK_THREAD_CPUTIME_ID  clocks
188       using  clock_settime().  On Linux, these clocks are not settable (i.e.,
189       no process has "appropriate privileges").
190

SEE ALSO

192       date(1),   gettimeofday(2),   settimeofday(2),   time(2),   adjtime(3),
193       clock_getcpuclockid(3),  ctime(3),  ftime(3), pthread_getcpuclockid(3),
194       sysconf(3), time(7), vdso(7), hwclock(8)
195

COLOPHON

197       This page is part of release 4.15 of the Linux  man-pages  project.   A
198       description  of  the project, information about reporting bugs, and the
199       latest    version    of    this    page,    can     be     found     at
200       https://www.kernel.org/doc/man-pages/.
201
202
203
204                                  2017-09-15                   CLOCK_GETRES(2)
Impressum