OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Accessing the Content of Buffers

The issue with all of the functions covered in this section so far (glBufferData(), glBufferSubData(), glCopyBufferSubData(), and glGetBufferSubData()) is that they all cause OpenGL to make a copy of your data. glBufferData() and glBufferSubData() both copy data from your application’s memory into memory owned by OpenGL. Obviously, glCopyBufferSubData() causes a copy of previously buffered data to be made. glGetBufferSubData() copies data from memory owned by OpenGL into memory provided by your application. Depending on the hardware configuration, it’s very possible that the memory owned by OpenGL would be accessible to your application if only you had a pointer to it. Well, you can get that pointer using glMapBuffer() ...