Multi-Thread Programming

Question

Is there a function call that can associate with a specific thread to make the thread run or wait? I have 4 threads in my program and I am trying to figure out a way to tell any one of the threads to wait or run when I want them to.

Answer 1

Your question is quite general. It really comes down to: Review the pthreads documentation.

If what you want is to have thread A wait for thread B to finish, check out pthread_join() .

If what you want is to have thread A wait until thread B says it's okay to continue, you will need a mutex and a conditional variable. Check out pthread_cond_wait() and associated functions.

Answer 2

A common mechanism for handling multiple threads is a master/slave approach where one thread hands off tasks for others to perform. The crux to this style, which I think is what you're getting at, is to realize that even in the slave threads, each thread inherently has total control over its own execution . The master doesn't really "force" the other threads to do anything, the master can make a request but the slave thread must voluntarily accept direction from the master... hrm... so maybe master/slave is a misnomer here... Anyway, The common solution for the slave threads is in pseudocode

while (!should_exit):
    while (!have_something_to_do):
        wait_for_something_to_do()
    do_the_thing_i_was_waiting_for()

You can implement this strategy quite easily in C using a structure like the following (I'm resorting to pseudo-c-ish code for the example's sake though)

struct Slave {
    Mutex              thread_lock;
    ConditionVariable  thread_cv;
    int                exit_flag;
    void             (*thread_operation)();
};

void slave_thread( Slave * slave ) {
    while( !exit_flag )
    {
        lock( thread_lock );
        while( slave->thread_operation == NULL )
            slave->thread_cv.wait( thread_lock );
        unlock( thread_lock );
        (*slave->thread_operation)(); // do the master's bidding...
    }
}

void master_thread()
{
   Slave slave1;
   Slave slave2;

   slave1.thread_operation = NULL;
   slave2.thread_operation = NULL;

   // create the threads. They'll immediately block on the condition variable

   slave1.thread_operation = some_function_pointer;
   slave2.thread_operation = some_other_function_pointer;

   notify_one( slave1.thread_cv ) // Now that the function pointers are set, wake
   notify_one( slave2.thread_cv ) // the slave threads

}

It's an overly simplistic example, of course, but hopefully it'll give you the general idea.