clock() behavior in ncurses

This is the reduced code to demonstrate the behavior.

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

#define DELAY  2.0

int main ( void) {
    static clock_t start;
    static double delay = 0.0;
    double elapsed = 0.0;
    clock_t stop;

    initscr ( );
    timeout ( 20); // milliseconds getch waits for a character

    if ( delay < .0001) {
        start = clock();
        delay = DELAY;
    }
    do {
        // getch ( );
        stop = clock();
        elapsed = (double)( stop - start) / CLOCKS_PER_SEC;
        move ( 22, 0);
        printw ( "cps %ld elapsed %f\n", CLOCKS_PER_SEC, elapsed);
        refresh ( );
    } while ( elapsed < delay);
    endwin ( );
    return 0;
}

Compiled and linked with ncurses it runs for about two seconds.
Uncomment // getch ( ); recompile and link and it runs for about two hundred seconds.
The problem has been solved by using clock_gettime().
I am curious why clock() is behaving strangely.

>Solution :

On POSIX systems (like Linux or macOS) the clock function reports the CPU time used for the process. If it doesn’t use any CPU time, like when it’s waiting a lot of time, then you will get very small differences.

If you use clock_gettime with either CLOCK_MONOTONIC or CLOCK_REALTIME those are based on the wall clock of the system.

Note that the behavior of clock is different on Windows where it’s a representation of the wall clock.