¿Cómo mostrar por pantalla el tiempo transcurrido?

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

#define BILLION  1000000000L;

int main()
{
    struct timespec start, stop;
    double accum;

    if( clock_gettime( CLOCK_REALTIME, &start) == -1 ) {
      perror( "clock gettime" );
      exit( EXIT_FAILURE );
    }

    /* Do busy work. */
    accum = 0;
    for( int i=0; i<100000000; ++i )
        accum += i*i;

    if( clock_gettime( CLOCK_REALTIME, &stop) == -1 ) {
      perror( "clock gettime" );
      exit( EXIT_FAILURE );
    }

    printf( "%lf\n", ( stop.tv_sec - start.tv_sec ) +
             (double)( stop.tv_nsec - start.tv_nsec )/(double)BILLION );

    return 0;
}