C程序的计算执行时间错误？

Question

can anyone tell me what will happen if I miss double in below lines?

time_spent = (double)(end - begin) / CLOCKS_PER_SEC;

I was trying to calculate running time of my sorting problem, and forgot to typecast to double. The code ran for almost 90 mins, but the output time printed was "270.000006". Can anyone please help me in figuring out what this 270 signifies??

Also it is sometimes showing "-ve" values. Any solution for this problem.

EDIT - I am sorting 10^9 and 10^10 numbers, so code will be running for hours.

Answer 1

If the code runs for almost 90 minutes, you will be getting an overflow of clock_t type on 32-bit architecture in under 72 minutes already. I do believe that this is your case.

Answer 2

Correct usage is:

    clock_t begin, end;
    double time_spent;

    begin = clock();
    /* actual task that needs to be monitored */
    end = clock();
    time_spent = (double)(end - begin) / CLOCKS_PER_SEC; // in seconds

You are measuring CPU time here, not the elapsed time which includes I/O time as well. CLOCKS_PER_SEC is a constant which is declared in . The computation needs to be done in floating point arithmetic. Another alternative to compute timing is to use the time command.

Read How to log the time taken for a unix command?