Question

I am running a C program using GCC and a proprietary DSP cross-compiler to simulate some functioality. I am using the following code to measure the execution time of particular part of my program:

clock_t start,end;
printf("DECODING DATA:\n");
start=clock();
conv3_dec(encoded, decoded,3*length,0);
end=clock();
duration = (double)(end - start) / CLOCKS_PER_SEC;
printf("DECODING TIME = %f\n",duration);

where conv3_dec() is a function defined in my program and I want to find the run-time of this function.

Now the thing is when my program runs, the conv3_dec() functions runs for almost 2 hours but the output from the printf("DECODING TIME = %f\n",duration) says that the execution of the function finished in just half a second (DECODING TIME = 0.455443) . This is very confusing for me.

I have used the clock_t technique to measure the runtimes of programs previously but the difference has never been so huge. Is this being caused by the cross-compiler. Just as a side note, the simulator simulates a DSP processor running at just 500MHz, so is the difference in the clock speeds of the DSP processor and my CPU causing the error is measuring the CLOCKS_PER_SEC.

Answer 1

For durations like two hours, I wouldn’t be too concerned about clock(), it’s far more useful for measuring sub-second durations.

Just use time() if you want the actual elapsed time, something like (dummy stuff supplied for what was missing):

#include <stdio.h>
#include <time.h>

// Dummy stuff starts here
#include <unistd.h>
#define encoded 0
#define decoded 0
#define length 0
static void conv3_dec (int a, int b, int c, int d) {
    sleep (20);
}
// Dummy stuff ends here

int main (void) {
    time_t start, end, duration;
    puts ("DECODING DATA:");
    start = time (0);
    conv3_dec (encoded, decoded, 3 * length, 0);
    end = time (0);
    duration = end - start;
    printf ("DECODING TIME = %d\n", duration);
    return 0;
}

which generates:

DECODING DATA:
DECODING TIME = 20