clock(3) — Linux manual page


CLOCK(3)                  Linux Programmer's Manual                 CLOCK(3)

NAME         top

       clock - determine processor time

SYNOPSIS         top

       #include <time.h>

       clock_t clock(void);

DESCRIPTION         top

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

RETURN VALUE         top

       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         top

       For an explanation of the terms used in this section, see

       │Interface Attribute     Value   │
       │clock()   │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, C89, C99.  XSI requires that
       CLOCKS_PER_SEC equals 1000000 independent of the actual resolution.

NOTES         top

       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.

       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.

       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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

GNU                              2017-09-15                         CLOCK(3)

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