C: SIGALRM - alarm to display message every second

Question

So I'm trying to call an alarm to display a message "still working.." every second. I included signal.h.

Outside of my main I have my function: (I never declare/define s for int s)

void display_message(int s);   //Function for alarm set up
void display_message(int s) {
     printf("copyit: Still working...\n" );
     alarm(1);    //for every second
     signal(SIGALRM, display_message);
}

Then, in my main

while(1)
{
    signal(SIGALRM, display_message);
    alarm(1);     //Alarm signal every second.

That's in there as soon as the loop begins. But the program never outputs the 'still working...' message. What am I doing incorrectly? Thank you, ver much appreciated.

Answer 1

Signal handlers are not supposed to contain "business logic" or make library calls such as printf. See C11 §7.1.4/4 and its footnote:

Thus, a signal handler cannot, in general, call standard library functions.

All the signal handler should do is set a flag to be acted upon by non-interrupt code, and unblock a waiting system call. This program runs correctly and does not risk crashing, even if some I/O or other functionality were added:

#include <signal.h>
#include <stdio.h>
#include <stdbool.h>
#include <unistd.h>

volatile sig_atomic_t print_flag = false;

void handle_alarm( int sig ) {
    print_flag = true;
}

int main() {
    signal( SIGALRM, handle_alarm ); // Install handler first,
    alarm( 1 ); // before scheduling it to be called.
    for (;;) {
        sleep( 5 ); // Pretend to do something. Could also be read() or select().
        if ( print_flag ) {
            printf( "Hello\n" );
            print_flag = false;
            alarm( 1 ); // Reschedule.
        }
    }
}