wrong results for clock() on 64 bit

Question

Hi all,

I've got a simple benchmarking function using the C clock() function.

start[timerId]=clock();
clock_t end;
float dif_sec;
end=clock();
dif_sec=((float)end-start[timerId])/CLOCKS_PER_SEC;
printf("%s: %f seconds\n", msg, dif_sec);

It's running fine in 32 bits on Mac OS X, but when I compile in 64 bits, the results are all wrong. Why?!

Answer 1

Here's what I got from a pure C version of your code (MacOS X 10.5.8 - Leopard; MacBook Pro):

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

int main(void)
{
    clock_t start = clock();
    clock_t end   = clock();
    float dif_sec = ((float)end-start)/CLOCKS_PER_SEC;
    printf("%s: %f seconds\n", "difference", dif_sec);
    printf("%s: %d\n", "CLOCKS_PER_SEC", CLOCKS_PER_SEC);
    return(0);
}

This is how I compiled it, and ran it, and the results I got:

Osiris JL: gcc -m32 -o xxx-32 xxx.c
Osiris JL: gcc -m64 -o xxx-64 xxx.c
Osiris JL: ./xxx-32
difference: 0.000006 seconds
CLOCKS_PER_SEC: 1000000
Osiris JL: ./xxx-64
difference: 0.000009 seconds
CLOCKS_PER_SEC: 1000000 
Osiris JL: ./xxx-64
difference: 0.000003 seconds
CLOCKS_PER_SEC: 1000000
Osiris JL: ./xxx-32
difference: 0.000003 seconds
CLOCKS_PER_SEC: 1000000
Osiris JL:

Looks sort of OK - rather fast, perhaps, but that's OK; it is a nice new machine, and it doesn't take long to make two system calls in a row.