Thread: 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