OpenCL C++ Bindings: How to implement a callback for enqueueWriteBuffer competition - c++

Question

I'm just getting started with OpenCL 1.2 and the C++ Bindings. I want to enqueue a write buffer asynchronous and get a callback once the operation has been completed. Here is a stripped down version of the relevant lines of code:
cl::Event enqueuingBufferReady;
auto error = enqueuingBufferReady.setCallback (CL_COMPLETE, [] (cl_event, cl_int, void*) { std::cout << "Enqueueing complete\n"; });
std::cout << "SetCallback return value: " << MyOpenCLHelpers::getErrorString (error) << std::endl;
// source is a std::vector<int>, buffer is a cl::Buffer of a matching size
commandQueue.enqueueWriteBuffer (buffer, CL_FALSE, 0, sizeof (int) * source.size(), source.data(), NULL, &enqueuingBufferReady);
//... execute the kernel - works successfully!
cl_int info;
enqueuingBufferAReady.getInfo (CL_EVENT_COMMAND_EXECUTION_STATUS, &info);
std::cout << "State of enqueuing " << MyOpenCLHelpers::getEventCommandExecutionStatusString (info) << std::endl;
What works:
The kernel is executed successfully and produces the right results. Enqueuing of the buffer should have worked. The program terminates with a print
State of enqueuing CL_COMPLETE
What does not work:
The setCallback call returns
SetCallback return value: CL_INVALID_EVENT
The callback is never called.
So what's wrong with this piece of code and how could it be changed to work as intended?

Answer 1

In the meanwhile I found it out by myself. My fault was to set the callback before enqueueing the write buffer. The right order is:
cl::Event enqueuingBufferReady;
// source is a std::vector<int>, buffer is a cl::Buffer of a matching size
commandQueue.enqueueWriteBuffer (buffer, CL_FALSE, 0, sizeof (int) * source.size(), source.data(), NULL, &enqueuingBufferReady);
auto error = enqueuingBufferReady.setCallback (CL_COMPLETE, [] (cl_event, cl_int, void*) { std::cout << "Enqueueing complete\n"; });
std::cout << "SetCallback return value: " << MyOpenCLHelpers::getErrorString (error) << std::endl;
Only after the call to enqueueWriteBuffer the passed in cl::Event becomes valid and the subsequent setCallback call works. I was a bit confused on this because I wasn't sure how it was guaranteed that enqueueing the buffer won't have finished before the callback was set, however my test showed that this doesn't matter as the callback is even called if it is set long after the operation was already completed.