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

       clock - determine processor time

       #include <time.h>

       clock_t clock(void);

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

       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.

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

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

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

       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).

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

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GNU                               2017-09-15                          CLOCK(3)
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