does glMapBuffer copy data?

Question

i am new to OpenGL. My question is: what does glMapBuffer do behind the scenes? does it allocate a new host memory, copies the GL object data to it and and returns the pointer?

is it gauranteed to receive the same pointer for subsequent calls to this method? ofcourse with releasing in between.

Answer 1

Like so often, the answer is "It depends". In some situations glMapBuffer will indeed allocate memory through malloc, copy the data there for your use and glUnmapBuffer releases it.

However the common way to implement glMapBuffer, is through memory mapping. If you don't know what this is, take a look at the documentation of the syscalls mmap (*nix systems like Linux, MacOS X) or CreateFileMap. What happens there is kind of interesting: Modern operating systems manage the running processes' address space in virtual memory. Everytime some "memory" is accessed the OS' memory management uses the accessed address as an index into a translation table, to redirect the operation to system RAM, swap space, etc. (of course the details are quite involved, the memory management is one of the more difficult things in a kernel to understand). A driver can install its very own access handler. So a process can mmap something managed by a driver into its address space and everytime is performs an access on this, the driver's mmap handler gets called. This allows the driver to map some GPU memory (through DMA) into the process' address space and do the neccessary bookkeeping. In this situation glMapBuffer will create such a memory mapping and the pointer you recieve will point to some address space in your process, that has been mapped into DMA memory reserved for the GPU.

Answer 2

It could do any of those. The specific memory behavior of glMapBuffer is implementation-defined.

If you map a buffer for reading, it might give you a pointer to that buffer object's memory. Or it might allocate a pointer and copy it. If you map a buffer for writing, it might give you a pointer to that buffer object's memory. Or it might allocate a pointer and hand that to you.

There is no way to know; you can't rely on it doing either one. It ought to give you better performance than glBufferSubData, assuming you can generate your data (from a file or from elsewhere) directly into the memory you get back from glMapPointer. The worst-case would be equal performance to glBufferSubData.

is it gauranteed to receive the same pointer for subsequent calls to this method?

Absolutely not.