Contexts and Command Queues

I briefly mentioned command queues in the first post, but I don't think I said anything about contexts (mostly because I don't think I truly understood them).

OpenCL contexts seem to be a way to group together a host system with devices in such a way as to enable giving commands to them (basically the programmer saying "these are the pieces of hardware which I will be using through this particular command queue"). I say this because, when constructing a command queue (refer to the first post for some resources on what a command queue is as well as a brief description), the constructor must be passed a context. In order to create a context we use the clCreateContext function, which basically associates a collection of devices in a certain platform with a single OpenCL context. Then, we can create a command queue which applies to a single device in that context using the clCreateCommandQueue function.

Inserting the following code after the code from the previous post results in full initialization of a context and command queue:

cl_context context = clCreateContext(NULL, 1, &device, NULL, NULL, &err); checkErr(err,"clCreateContext"); std::cout<<"Successfully created a context"<<std::endl; cl_command_queue queue = clCreateCommandQueue(context, device, 0, &err); checkErr(err,"clCreateCommandQueue"); std::cout<<"Successfully created command queue"<<std::endl;

This code creates a single OpenCL context associated with a single OpenCL device, and then attaches a command queue to it. I haven't tried this yet, but I imagine if you passed a device to clCreateCommandQueue which was not associated with the context passed, some error would result.