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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).

SEE ALSO

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

COLOPHON

   This page is part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.



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