C Signal timer for task switching

Question

Im building a simple task switcher that runs two functions in a loop. The idea is that it runs a f1 for an amount of time and then gives control to f2 for the same amout, then f1, f2, in an endless loop.

The problem is that whenever i run the program, the first switch goes well but the following switches never happen. Getting stuck in f2.

Ive tried other implementations archieving 3 switches at most (with the program getting frozen after that).

This is my current implementation:

#include <stdio.h>
#include <signal.h>
#include <sys/time.h>
#include <unistd.h>

int count = 0;
int flag  = 0;

void f1() {
    for (;;) printf("A");
}

void f2() {
    for (;;) printf("B");
}

void sched() {
    flag = !flag;
    if (flag)
        f1();
    else
        f2();
}

void sighandler(int signo)
{
 printf("signal %d occurred %d times\n",signo, ++count);
 sched();
}

int main(void)
{
 struct itimerval it;
 struct sigaction act, oact;
 act.sa_handler = sighandler;
 sigemptyset(&act.sa_mask);
 act.sa_flags = 0;

 sigaction(SIGPROF, &act, &oact); 

 it.it_interval.tv_sec = 0;
 it.it_interval.tv_usec = 10000;
 it.it_value.tv_sec = 0;
 it.it_value.tv_usec = 10000;
 setitimer(ITIMER_PROF, &it, NULL);

 sched();
}

Any suggestion would be greatly appreciated.

Answer 1

Your signal handler calls sched(), which never returns (but ends up in either of the for (;;) loops). So after the first switch, you are always inside the signal handler, and further signals are masked.