why C clock() returns 0

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clock function does not measure CPU clock cycles.

C says clock “returns the implementation’s best approximation to the processor
time used by the program since the beginning of an implementation-defined era related
only to the program invocation.”

If between two successive clock calls you program takes less time than one unity of the clock function, you could get 0.

POSIX clock defines the unity with CLOCKS_PER_SEC as 1000000 (unity is then 1 microsecond).

http://pubs.opengroup.org/onlinepubs/009604499/functions/clock.html

To measure clock cycles in x86/x64 you can use inline assembly to retreive the clock count of the CPU Time Stamp Counter register rdtsc.

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