how to correctly read unmapped buffer

OpenCL
1

Hi:

I am having trouble using clenqueueunmapmemobject. I want to know how to actually use the output from an unmapped buffer.
Say for example I wished to print out the contents of the buffer to the screen. The way I have been doing it up to this point is using clenqueuereadbuffer to get the results I want.

Is there a simple example that shows how this is done?
I realize that reading and writing buffers works better/is easier in many situations but I still wish to know.

Thanks for the help!

2

You don’t use the contents after clEnqueueUnmapMemObject.

Instead of clEnqueueReadBuffer, do a clEnqueueMapBuffer (with blocking), use the pointer returned to access the buffer, then clEnqueueUnmapMemObject when done.

On some hardware it will be slightly faster than clEnqueueReadBuffer because it uses pinned memory.

3

Let me see if I understand. If I want to print out the data the GPU changed I:

  1. map the clbuffer
  2. run the kernel
  3. use the pointer retuned from maping to go through the buffer and print each item
    4: Unmap the buffer

Does this make sense?

Thanks for your help!

4

No, swap steps 1 and 2 so it becomes:

  1. run the kernel
  2. map the clbuffer
  3. use the pointer retuned from maping to go through the buffer and print each item
    4: Unmap the buffer

Mapping is what makes the buffer visible on the host side. Unmapping makes it usable by the GPU again.