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    Two common errors are made when performing this operation. They are illustrated below.

    Non

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Compliant Code

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:

int run_big_program() {
    clock_t start, finish;
    int seconds;
    start = clock();
    run_long_program()
    finish = clock();
    seconds = ((finish-start)/CLOCKS_PER_SEC);
    return seconds;
}

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C99 Section 7.23.1 states that CLOCKS_PER_SEC expands to a constant expression with type clock_t that is the number per second of the value returned by the clock() function, so dividing by CLOCKS_PER_SEC will net you the number of seconds. What is not specified, however, is the type that will be yielded by dividing CLOCKS_PER_SEC. The best recommendation is to look at the types defined in time.h and use a compatible (and large, to prevent overflow) type. The convention for linux and most x86 architectures seems to be to use a double.

    Compliant

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Code:

double run_big_program ()
{
    clock_t start, finish;
    double seconds;
    start = clock();
    if (start = (clock_t)(-1)){
        return -1.0;
    }
    run_long_program();
    finish = clock();
    if (finish = (clock_t)(-1)){
        return -1.0;
    }
    seconds = (double) (finish-start)/CLOCKS_PER_SEC;
    return seconds;
}

   When appropriate one should also check for overflow in seconds. The key, though, is that because the proper function is being called and a reasonable value will result. This function will correctly return the time in seconds.
 

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