clock() doesn't work correctly - c

Question

My code looks like this:
#include <time.h>
#include <stdio.h>
#include <unistd.h>
int main(void) {
time_t loop_begin, loop_end, scanf_begin, scanf_end;
double loop_time, scanf_time;
int q;
loop_begin = clock();
//some big loop
loop_end = clock();
loop_time = (double)(loop_end - loop_begin) / CLOCKS_PER_SEC;
printf("%.6f\n", loop_time);
scanf_begin = clock();
printf("qwe> ");scanf("%d", &q);
scanf_end = clock();
scanf_time = (double)(scanf_end - scanf_begin) / CLOCKS_PER_SEC;
printf("%.6f\n", scanf_time);
return 0;
}
But the output is like this:
20.087593
qwe> 543
0.000073
Am I missing something or is this normal? If this is normal, what would you suggest?

Answer 1

clock measures CPU time. When your program is waiting for your keypresses, it is blocked in a system call and does not use any processor time.
From Linux Programmer's Manual:
NAME
clock - determine processor time
SYNOPSIS
#include <time.h>
clock_t clock(void);
DESCRIPTION
The clock() function returns an approximation of processor time used by the program.
RETURN VALUE
The value returned is the CPU time used so far as a clock_t; to get the number
of seconds used, divide by CLOCKS_PER_SEC. If the processor time used is not
available or its value cannot be represented, the function returns the value (clock_t) -1.