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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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clock(3) Library Functions Manual clock(3)
clock - determine processor time
Standard C library (libc, -lc)
#include <time.h>
clock_t clock(void);
The clock() function returns an approximation of processor time
used by the program.
The value returned is the CPU time used so far as a clock_t; to
get the number of seconds used, divide by CLOCKS_PER_SEC. If the
processor time used is not available or its value cannot be
represented, the function returns the value (clock_t) -1.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ clock() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
XSI requires that CLOCKS_PER_SEC equals 1000000 independent of the
actual resolution.
On several other implementations, the value returned by clock()
also includes the times of any children whose status has been
collected via wait(2) (or another wait-type call). Linux does not
include the times of waited-for children in the value returned by
clock(). The times(2) function, which explicitly returns
(separate) information about the caller and its children, may be
preferable.
C11, POSIX.1-2008.
POSIX.1-2001, C89.
In glibc 2.17 and earlier, clock() was implemented on top of
times(2). For improved accuracy, since glibc 2.18, it is
implemented on top of clock_gettime(2) (using the
CLOCK_PROCESS_CPUTIME_ID clock).
The C standard allows for arbitrary values at the start of the
program; subtract the value returned from a call to clock() at the
start of the program to get maximum portability.
Note that the time can wrap around. On a 32-bit system where
CLOCKS_PER_SEC equals 1000000 this function will return the same
value approximately every 72 minutes.
clock_gettime(2), getrusage(2), times(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.10 2024-07-23 clock(3)
Pages that refer to this page: getrusage(2), times(2), clock_t(3type), ctime(3), time(7)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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