Program not responding to linux signals sent from terminal without continuous printing on terminal

Question

I have a C program that goes like:

while(1){
    if(flag1){
         ...
    }
    if(flag2){
         ...
    }
}

Now these flags will be raised in handlers for SIGINT AND SIGTSTP signals. The terminal just shows ^C and ^Z when Ctrl+C or Ctrl+Z is entered, but the conditional code blocks are not executed.
However the exact same program, but if I give a printf like below:

while(1){
    printf("  ");
    if(flag1){
         ...
    }
    if(flag2){
         ...
    }
}

The program responds to the Ctrl+Z or Ctrl+C signals, and the respective conditional code blocks are executed. Could someone explain this behavior? I am not sure why keeping the terminal busy with something being printed continuously, makes my program respond to the signals, while not otherwise.

Answer 1

The conditional code blocks could be optimized out, because compiler doesn't know these variables can change out of the loop. Define flags as volatile, i.e.

volatile int flag1;
...

This way compiler knows not to make any assumptions about the values of these variables.