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

NAME

     clock - determine processor time

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 |
     +----------+---------------+---------+

CONFORMING TO

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

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.

     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 clock).