Semaphores, need advice on implementation.

Hi,

I am trying to control the movement of a graphics image across the screen, and was hoping of some advice on the matter.

Code:

for(;;)
	{
		Sleep(1);

		switch(train1.move())
		{
		case  BRIDGE_APPROACH:

			msgsem.wait();
			display_string("Train 1 arrived at Bridge\n",2);
			msgsem.signal();
			onbridge.signal();

			break;

		case  LEAVING_BRIDGE:

			msgsem.wait();
			display_string("Train 1 left the Bridge\n",2);
			msgsem.signal();
			onbridge.wait();

			break;

As the image crosses the bridge, I use the first line in bold to increase the semaphores value by one, and when the image leaves the bridge, I use the second line in bold to decrease the semaphores value by one.

Currently this is my only use of the semaphore, but when i run the code I recieve the following error:

Semaphore::signal
train 1 thread failed in signal semaphore (onbridge sem)
tried to increment semaphore that already was at it's maximum value.

I am trying to limit the number of images on the bridge to 2, and I understood that semaphores can hold a value of > than 1, so am confused why it is saying the semaphore was already at it's maximum value.

I wanted to solve the problem by reading the semaphores value, if IF > 2, stop other images from crosing the bridge.

If anyone can offer some advice on this, I would be most greateful.

Thanks!

Dear Swerve,

I have reviewed your code carefully and cannot recognize precisely which API you are using for your sample code. I have done some googling but realize it is probably much more beneficial to us both if you simply tell me what software you are incorporating into your project. I appreciate all auxilary information you can give.

-sphynx

You're using the semaphore the wrong way around. You need to wait on it when you enter the bridge and signal it when you leave.

That said, unless you use a separate thread for every train (which is a bad idea), waiting on a semaphore is the wrong way to control bridge access.