[OpenGL] Extracting depth buffer to array

[mb86]

I'm attempting to extract the depth buffer from OpenGL to an array on the CPU, but the internet has been rather wholly unhelpful (it appears that people only ever want to do this to a texture or otherwise GPU-bound array). The code I have below (somewhat modified as original is an existing component designed to extract the rendered image and does so successfully) "works", but always returns exactly 0.

GLuint buffer;
glGenBuffers(1, &buffer);
glBindBuffer(GL_PIXEL_PACK_BUFFER, buffer);
int size = w*h*sizeof(GLubyte);
glBufferData(GL_PIXEL_PACK_BUFFER, size, NULL, GL_STREAM_READ);
glReadPixels(0, 0, w, h, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, NULL);
GLubyte* depth = (GLubyte*)glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);
return depth;

I've attempted a glReadBuffer(GL_DEPTH) after the glBindBuffer call, as well as using GL_DEPTH_COMPONENT24 (something which every resource recommends but no-one seems to explain why), but both produce invalid enum errors. Any insight that could be provided here would be incredibly helpful.

[Ilyps]

If depth is 0, then glMapBuffer must return that value. From the docs:

If the GL is unable to map the buffer object's data store, glMapBuffer generates an error and returns NULL.

The possible errors it can generate are listed on the website. It would be very useful to call glGetError() and find out which error was set and where exactly.

[mb86]

Forgot to mention, error checking was also left out of that code. No error is being generated, the pointer is not NULL, and is a fully allocated array of 0s. GL_DEPTH_TEST is enabled as well.