Semaphores

edk · 11-25-2001, 01:57 PM

OK, after my last thread on this subject failed miserably, I am trying again.

How do I go about creating and unlocking a set of 5 System V semaphores? Once this is done, how do I wait for them, and signal when I'm finished using it?

I'm trying to fork 5 children and execute their functions in a user-specified order. There are 5 children, so there should be a set of 5 semaphores. The user enters the order of execution (integer between 1 and 5, inclusive) using the keyboard. I fork the 5 children all at once, but I need to synchronize their execution with semaphores. How do I do this?

Also, please post code examples, not links to other pages. I have tried many other pages but they confuse me more than they help.

Thanks in advance.

edk · 11-25-2001, 03:55 PM

Here's the code segment for what i am doing:

Code:

	for (int i(0); i < theOrder.size(); i++) {
		pid = fork();
		if (pid < 0) {
			cerr<<"ERROR:  fork error\n";
			exit(0);
		}
		else if (pid == 0) {
			// wait for semaphore here (code not written yet
			doChildStuff(i);
			// signal semaphore here (code not written yet

			exit(0);
		}
		childPid[children] = pid;
		children++;
	}

...and here's my creation of the semaphore set:

Code:

	sem_key = 0x2222;
	if ((sem_id = semget(sem_key, theOrder.size(), 0666 | IPC_CREAT)) == -1) {
		cerr<<"ERROR:  semget error\n";
	}
	
	arg.val = 1;
	for (int i(0); i < theOrder.size(); i++) {
		if (semctl(sem_id, i, SETVAL, arg) == -1) {
			cerr<<"semctl error\n";
		}
	}

you can see, all o the semaphores are initialized to 1. now that you know this, how do i make it so that the first semaphore is signalled? also, how do i wait for a semaphore?

11-25-2001, 01:57 PM	#1
edk Registered User Join Date: Sep 2001 Posts: 20	Semaphores OK, after my last thread on this subject failed miserably, I am trying again. How do I go about creating and unlocking a set of 5 System V semaphores? Once this is done, how do I wait for them, and signal when I'm finished using it? I'm trying to fork 5 children and execute their functions in a user-specified order. There are 5 children, so there should be a set of 5 semaphores. The user enters the order of execution (integer between 1 and 5, inclusive) using the keyboard. I fork the 5 children all at once, but I need to synchronize their execution with semaphores. How do I do this? Also, please post code examples, not links to other pages. I have tried many other pages but they confuse me more than they help. Thanks in advance. __________________ liberate tutamet ex inferis
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