gettimeofday.2: gettimeofday() is affected by discontinuous jumps in the system time

[thirdparty/man-pages.git] / man2 / clock_getres.2

.\" Hey Emacs! This file is -*- nroff -*- source.
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.\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
.\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl), Aug 24, 2003
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.\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
.\" 2003-08-24 aeb, large parts rewritten
.\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
.\"
.\" FIXME: Linux 2.6.39 adds CLOCK_BOOTTIME
.\"
.TH CLOCK_GETRES 2 2010-02-03 "" "Linux Programmer's Manual"
.SH NAME
clock_getres, clock_gettime, clock_settime \- clock and time functions
.SH SYNOPSIS
.B #include <time.h>
.sp
.BI "int clock_getres(clockid_t " clk_id ", struct timespec *" res );

.BI "int clock_gettime(clockid_t " clk_id ", struct timespec *" tp );

.BI "int clock_settime(clockid_t " clk_id ", const struct timespec *" tp );
.sp
Link with \fI\-lrt\fP.
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR clock_getres (),
.BR clock_gettime (),
.BR clock_settime ():
.RS
_POSIX_C_SOURCE\ >=\ 199309L
.RE
.ad b
.SH DESCRIPTION
The function
.BR clock_getres ()
finds the resolution (precision) of the specified clock
.IR clk_id ,
and, if
.I res
is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
.IR res .
The resolution of clocks depends on the implementation and cannot be
configured by a particular process.
If the time value pointed to by the argument
.I tp
of
.BR clock_settime ()
is not a multiple of
.IR res ,
then it is truncated to a multiple of
.IR res .
.PP
The functions
.BR clock_gettime ()
and
.BR clock_settime ()
retrieve and set the time of the specified clock
.IR clk_id .
.PP
The
.I res
and
.I tp
arguments are
.I timespec
structures, as specified in
.IR <time.h> :
.sp
.in +4n
.nf
struct timespec {
    time_t   tv_sec;        /* seconds */
    long     tv_nsec;       /* nanoseconds */
};
.fi
.in
.PP
The
.I clk_id
argument is the identifier of the particular clock on which to act.
A clock may be system-wide and hence visible for all processes, or
per-process if it measures time only within a single process.
.LP
All implementations support the system-wide real-time clock,
which is identified by
.BR CLOCK_REALTIME .
Its time represents seconds and nanoseconds since the Epoch.
When its time is changed, timers for a relative interval are
unaffected, but timers for an absolute point in time are affected.
.LP
More clocks may be implemented.
The interpretation of the
corresponding time values and the effect on timers is unspecified.
.LP
Sufficiently recent versions of glibc and the Linux kernel
support the following clocks:
.TP
.B CLOCK_REALTIME
System-wide real-time clock.
Setting this clock requires appropriate privileges.
.TP
.B CLOCK_MONOTONIC
Clock that cannot be set and represents monotonic time since
some unspecified starting point.
.TP
.BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
.\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
Similar to
.BR CLOCK_MONOTONIC ,
but provides access to a raw hardware-based time
that is not subject to NTP adjustments.
.TP
.B CLOCK_PROCESS_CPUTIME_ID
High-resolution per-process timer from the CPU.
.TP
.B CLOCK_THREAD_CPUTIME_ID
Thread-specific CPU-time clock.
.SH "RETURN VALUE"
.BR clock_gettime (),
.BR clock_settime ()
and
.BR clock_getres ()
return 0 for success, or \-1 for failure (in which case
.I errno
is set appropriately).
.SH ERRORS
.TP
.B EFAULT
.I tp
points outside the accessible address space.
.TP
.B EINVAL
The
.I clk_id
specified is not supported on this system.
.\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
.\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
.\" EPERM.
.TP
.B EPERM
.BR clock_settime ()
does not have permission to set the clock indicated.
.SH "CONFORMING TO"
SUSv2, POSIX.1-2001.
.SH AVAILABILITY
On POSIX systems on which these functions are available, the symbol
.B _POSIX_TIMERS
is defined in \fI<unistd.h>\fP to a value greater than 0.
The symbols
.BR _POSIX_MONOTONIC_CLOCK ,
.BR _POSIX_CPUTIME ,
.B _POSIX_THREAD_CPUTIME
indicate that
.BR CLOCK_MONOTONIC ,
.BR CLOCK_PROCESS_CPUTIME_ID ,
.B CLOCK_THREAD_CPUTIME_ID
are available.
(See also
.BR sysconf (3).)
.SH NOTES
.SS Note for SMP systems
The
.B CLOCK_PROCESS_CPUTIME_ID
and
.B CLOCK_THREAD_CPUTIME_ID
clocks are realized on many platforms using timers from the CPUs
(TSC on i386, AR.ITC on Itanium).
These registers may differ between CPUs and as a consequence
these clocks may return
.B bogus results
if a process is migrated to another CPU.
.PP
If the CPUs in an SMP system have different clock sources then
there is no way to maintain a correlation between the timer registers since
each CPU will run at a slightly different frequency.
If that is the case then
.I clock_getcpuclockid(0)
will return
.B ENOENT
to signify this condition.
The two clocks will then only be useful if it
can be ensured that a process stays on a certain CPU.
.PP
The processors in an SMP system do not start all at exactly the same
time and therefore the timer registers are typically running at an offset.
Some architectures include code that attempts to limit these offsets on bootup.
However, the code cannot guarantee to accurately tune the offsets.
Glibc contains no provisions to deal with these offsets (unlike the Linux
Kernel).
Typically these offsets are small and therefore the effects may be
negligible in most cases.
.SH BUGS
According to POSIX.1-2001, a process with "appropriate privileges" may set the
.B CLOCK_PROCESS_CPUTIME_ID
and
.B CLOCK_THREAD_CPUTIME_ID
clocks using
.BR clock_settime ().
On Linux, these clocks are not settable
(i.e., no process has "appropriate privileges").
.\" See http://bugzilla.kernel.org/show_bug.cgi?id=11972
.SH "SEE ALSO"
.BR date (1),
.BR adjtimex (2),
.BR gettimeofday (2),
.BR settimeofday (2),
.BR time (2),
.BR clock_getcpuclockid (3),
.BR ctime (3),
.BR ftime (3),
.BR pthread_getcpuclockid (3),
.BR sysconf (3),
.BR time (7)