Why doesn't this small C program work as expected?

Question

I've got a small C program that makes use of difftime. its really strange, it doesn't print out the line after 10 seconds.

If however I uncomment out the sleep line then it works.

Any ideas why this is happening?

/* difftime example */
#include <stdio.h>      /* printf */
#include <time.h>       /* time_t, struct tm, difftime, time, mktime */

int main ()
{
  time_t start, stop;

start = time(NULL);
        for(; /* some condition that takes forever to meet */;) {
        // do stuff that apparently takes forever.
                stop = time(NULL);
                double diff = difftime(stop, start);
                //sleep (1);
                if (diff >= 10) {
                        printf("10 seconds passed...");
                        start = time(NULL);
                }
        }
}

BTW: Code compiles fine and I'm running it on the Raspberry Pi. 3.6.11+ #474 PREEMPT Thu Jun 13 17:14:42 BST 2013 armv6l GNU/Linux

Answer 1

Console IO might be line buffered. Try flushing

printf("10 seconds passed...");
fflush(stdout)

or adding a newline \\n

printf("10 seconds passed...\n");

I can't reproduce any change in behaviour when the call to sleep is uncommented.

Answer 2

printf("10 seconds passed...\n");

Answer 3

For sleep(), you need to include unistd.h, as well as adding a newline as others have pointed out.

#include <stdio.h>      /* printf */
#include <time.h>       /* time_t, struct tm, difftime, time, mktime */
#include <unistd.h>     /* sleep() */

If your code compiled without warnings and without including unistd.h, you need to crank your warnings up.