查找代码花费的时间

Question

I am trying to find the time taken by memmove function in c using time.h library. However, When i execute the code, I get the value as zero. Any possible solution to find the time taken by the memmove function?

void main(){
uint64_t start,end;
uint8_t a,b;
char source[5000];
char dest[5000];
uint64_t j=0;

for(j=0;j<5000;j++){
source[j]=j;
}

start=clock();
memmove(dest,source,5000);
end=clock();
printf("%f",((double)end-start));
}

Answer 1

As I write in my comment, memmoving 5000 bytes is far too fast to be mesurable with clock . If you do your memmove 100000 times, then it will get mesurable.

This code below gives an output of 12 on my computer. But this is platform dependent, the number you get on your computer might be quite different.

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

int main(void) {
  uint64_t start, end;
  char source[5000];
  char dest[5000];
  uint64_t j = 0;

  for (j = 0; j < 5000; j++) {
    source[j] = j;
  }

  start = clock();

  for (int i = 0; i < 100000; i++)
  {
    memmove(dest, source, 5000);
  }

  end = clock();
  printf("%lld", (end - start));  // no need to convert to double, (end - start)
                                  // is an uint64_t.
 }

Answer 2

If you want to know the time it takes on a beagle bone or another device with a GIPO you can toggle a GPIO before and after your routine. You will have to hook up an oscilloscope or something similar that can sample voltage quickly.

I don't know much about beagle bones but it seems the library libpruio allows fast gpio toggling.

Also, what is your exact goal here? Compare speed on different hardware? As someone suggests, you could increase the number of loops so it becomes more easily measurable with time.h.