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Linux 2.6 - man page for clock (linux section 3)

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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.

CONFORMING TO
       C89,  C99, POSIX.1-2001.  POSIX 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  pre-
       cision,	since  glibc  2.18,  it  is  implemented  on  top  of clock_gettime(2) (using the
       CLOCK_PROCESS_CPUTIME_ID clock).

SEE ALSO
       clock_gettime(2), getrusage(2), times(2)

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

GNU					    2013-08-19					 CLOCK(3)
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