Is there a way to change the signal mask of a thread from another thread? I am supposed to write a multithreaded C application that doesn't use mutex, semaphores and condition variables, only signals.

So it would look like something like this: The main Thread sends SIGUSR1 to its process and and one of the 2 threads (not including the main thread), will respond to the signal and block SIGUSR1 from the sigmask and sleep. Then the main thread sends SIGUSR1 again, the other thread will respond, block SIGUSR1 from its sigmask, unblock SIGUSR1 from the other threads sigmask, so it will respond to SIGUSR1 again.

So essentially whenever the main thread sends SIGUSR1 the two other threads swap between each other.

Can somebody help?