freebsd man page for clock

CLOCK(3)						   BSD Library Functions Manual 						  CLOCK(3)

NAME
     clock -- determine processor time used

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     clock_t
     clock(void);

DESCRIPTION
     The clock() function determines the amount of processor time used since the invocation of the calling process, measured in CLOCKS_PER_SECs of
     a second.

RETURN VALUES
     The clock() function returns the amount of time used unless an error occurs, in which case the return value is -1.

SEE ALSO
     getrusage(2), clocks(7)

STANDARDS
     The clock() function conforms to ISO/IEC 9899:1990 (``ISO C90'').	However, Version 2 of the Single UNIX Specification (``SUSv2'') requires
     CLOCKS_PER_SEC to be defined as one million.  FreeBSD does not conform to this requirement; changing the value would introduce binary incom-
     patibility and one million is still inadequate on modern processors.

BSD
								   June 4, 1993 							       BSD