Defined in header <time.h>
#define CLOCKS_PER_SEC /*implementation defined*/

Expands to an expression (not necessarily a compile-time constant) of type clock_t equal to the number of clock ticks per second, as returned by clock().


POSIX defines CLOCKS_PER_SEC as one million, regardless of the actual precision of clock.

Until standardized as CLOCKS_PER_SEC in C89, this macro was sometimes known by its IEEE std 1003.1-1988 name CLK_TCK: that name was not included in C89 and was removed from POSIX itself in 1996 over ambiguity with _SC_CLK_TCK, which gives number of clocks per second for the function times()).


  • C11 standard (ISO/IEC 9899:2011):
    • 7.27.1/2 Components of time (p: 388)
  • C99 standard (ISO/IEC 9899:1999):
    • 7.23.1/2 Components of time (p: 338)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • 4.12.1 Components of time

See also

returns raw processor clock time since the program is started
processor time since era type

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