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FreeBSD 11.0 - man page for clock (freebsd section 3)

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