clock()
is not an accurate timer, and it just don't work well for timing short intervals.
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 assembly to retreive the clock count of the CPU Time Stamp Counter register rdtsc
. (which can be achieved by inline assembling?) Note that it returns the time stamp, not the number of seconds elapsed. So you need to retrieve the cpu frequency as well.
However, the best way to get accurate time in seconds depends on your platform.
To sum up, it's virtually impossible to achieve calculating and printing clock_t
time in seconds accurately. You might want to see this on Stackoverflow to find a better approach (if accuracy is top priority).