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The clock() function measures the CPU ticks (operations) needed to do the processing and divides it with the processor frequency.

It is the same for parallel and serial operations - but it doesn't measure the real elapsed time in multi-core systems.

Use the standard chrono library to measure precisely the processing time (you should enable C++11).

#include <chrono>

//....
std::chrono::time_point<std::chrono::system_clock> start, end;
std::chrono::duration<double> elapsed;
start = std::chrono::system_clock::now();
//...do the processing here...
end = std::chrono::system_clock::now();
elapsed=end-start;
cout<<"Processing time: "<<elapsed.count();