Question

In OpenCL, can I set kernel argument as following?

cl_uint a = 0;
kernel.setArg(0, sizeof(a), &a);

I want to read&write one value from/to a kernel function, not only write to.

Answer 1

Setting a kernel argument in this manner can only be used for inputs to the kernel. Any output you want to read (either in a subsequent kernel or from the host program) must be written to a buffer or an image. In your case, that means you need to create a single-element buffer and pass the buffer to the kernel.

One way to think about this is that when you call setArg with the parameter &a, the OpenCL kernel is using the value of a, not the location of a. If the kernel were to write to kernel argument zero, your host program would have no way of recovering the value that was written.