C- while loop un-explained behaviour

Question

I am trying to run the following code :

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

int main()
{
unsigned int ms, oldms = 0,dif;
struct timeval tv;
while(1)
{
 gettimeofday(&tv, NULL);
 ms=tv.tv_sec; 
 //printf("%d\n",ms-oldms );
 dif=ms-oldms;
 if(dif>3)
   {    
        printf("3 seconds up");
        oldms=ms;
   }
 }
}

I am expecting it to print "3 seconds up" after every 3 seconds, but it doesn't display that message. I tried to debug it using gdb but nothing seems wrong and still no output. While trying to debug, I added a printf statement and magically the output can be seen.

If I run the program after removing the //printf("%d\n",ms-oldms ); statement , there is no output again.I am not sure what's happening and whether its dependant on anything.

$gcc --version gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2

Answer 1

Output buffering is the reason.

stdout is line buffered by default when attached to a terminal device. You can flush this out using fflush(stdout); or using \n in printf() i.e. printf("3 seconds up\n");. or disabling it with setbuf(stdout, 0);

I/O is slow in general. So implementations use a fixed size buffer and printf once it gets full. In practice, calling fflush(stdout); too often can affect performance.