Question

I am aware that there are available functions and libraries that allow concurrency in c.

(pthread.h, fork() etc.)

However, I was wondering if there is any way to simulate concurrency in C, without actually having to use more than one thread?

Example scenario:

There is a main program loop running and outputting calculations. Somehow another function notifies the main function that the time is now 12:00 p.m. and the program should stop outputting calculations as the user is going off to lunch. At 12:30 this function notifies the main function to start outputting calculations again.

Can someone point me in the right direction on how to do this?

EDIT:

In essence I believe there should be 2 ways to do this:

The first would be the main program constantly checking the alternate function so it knows when the clock hits 12:00. (this is very simple and I know how to do this)

The second would be having the alternate function contact the main program whenever required (i.e. at 12:00 and 12:30).

Answer 1

It seems to me like you just need a simple timer you could use alarm() for that:

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

volatile unsigned stop_working = 0;

void alarm_handler(int signum)
{
    stop_working = 1;
    //alarm(1); //reschedule alarm
}

int main()
{        
    signal(SIGALRM, alarm_handler);
    alarm(1); //schedule alarm in seconds

    while (!stop_working) {
      //do some work
    }
}

If however, you want to have something like coroutines you should look into user-level threads (or fibers) using functions like makecontext() and swapcontext()

Note: this example is UNIX/Linux specific, for MinGW you will need to use winapi, using functions like CreateWaitableTimer(), SetWaitableTimer(), and so on. See MSDN for details.