Question

Is there an example of a working timer that executes some function every x amount seconds using C.

I’d appreciate an example working code.

Answer 1

You could spawn a new thread:

void *threadproc(void *arg)
{
    while(!done)
    {
        sleep(delay_in_seconds);
        call_function();
    }
    return 0;
}
...
pthread_t tid;
pthread_create(&tid, NULL, &threadproc, NULL);

Or, you could set an alarm with alarm(2) or setitimer(2):

void on_alarm(int signum)
{
    call_function();
    if(!done)
        alarm(delay_in_seconds);  // Reschedule alarm
}
...
// Setup on_alarm as a signal handler for the SIGALRM signal
struct sigaction act;
act.sa_handler = &on_alarm;
act.sa_mask = 0;
act.sa_flags = SA_RESTART;  // Restart interrupted system calls
sigaction(SIGALRM, &act, NULL);

alarm(delay_in_seconds);  // Setup initial alarm

Of course, both of these methods have the problem that the function you’re calling periodically needs to be thread-safe.

The signal method is particularly dangerous because it must also be async-safe, which is very hard to do — even something as simple as printf is unsafe because printf might allocate memory, and if the SIGALRM interrupted a call to malloc, you’re in trouble because malloc is not reentrant. So I wouldn’t recommend the signal method, unless all you do is set a flag in the signal handler which later gets checked by some other function, which puts you back in the same place as the threaded version.