clock(3) — Linux manual page

clock(3)                Library Functions Manual                clock(3)

NAME

       clock - determine processor time

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <time.h>

       clock_t clock(void);

DESCRIPTION

       The clock() function returns an approximation of processor time
       used by the program.

RETURN VALUE

       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.

ATTRIBUTES

       For an explanation of the terms used in this section, see
       attributes(7).
       ┌─────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                           │ Attribute     │ Value   │
       ├─────────────────────────────────────┼───────────────┼─────────┤
       │ clock()                             │ Thread safety │ MT-Safe │
       └─────────────────────────────────────┴───────────────┴─────────┘

VERSIONS

       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.

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       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).

NOTES

       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.

SEE ALSO

       clock_gettime(2), getrusage(2), times(2)

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                       clock(3)

Pages that refer to this page: getrusage(2), times(2), clock_t(3type), ctime(3), time(7)

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