clock(3)		    Library Functions Manual			clock(3)

NAME
     clock - determine processor time

LIBRARY
     Standard C library (libc, -lc)

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 re-
     turns the value (clock_t) -1.

ATTRIBUTES
     For an explanation of the terms used in this  section,  see  attributes(7).
     allbox; lbx lb lb l l l.  Interface Attribute Value T{ clock() T}	 Thread
     safety  MT-Safe

VERSIONS
     XSI  requires  that CLOCKS_PER_SEC equals 1000000 independent of the actual
     resolution.

     On several other implementations, the value returned by  clock()  also  in-
     cludes  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) func-
     tion, which explicitly returns (separate) information about the caller  and
     its children, may be preferable.

STANDARDS
     C11, POSIX.1-2008.

HISTORY
     POSIX.1-2001, C89.

     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_get-
     time(2) (using the CLOCK_PROCESS_CPUTIME_ID clock).

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 pro-
     gram 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 ap-
     proximately every 72 minutes.

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

Linux man-pages 6.18		   2026-02-08				clock(3)