Getting OpenGL API functions

Question

I'm looking at the OpenGL wiki page, and I was curious about the following line:

For reasons that are ultimately irrelevant to this discussion, you must manually load functions via a platform-specific API call. This boilerplate work is done with various OpenGL loading libraries; these make this process smooth. You are strongly advised to use one. —OpenGL Wiki

Intuitively, you would think they would just provide a header for you to include. So, what are the historic reasons for this?

EDIT :
Thanks for the answers, I see now that that OpenGL supports multiple implementations of its functions, so there is no one single DLL/SO that everyone links to. I also found these quotes helpful:

• The OpenGL library supports multiple implementations of its functions. From MSDN

• To accommodate this variety, we specify ideal behavior instead of actual behavior for certain GL operations. From GL Spec

• When you run your program, opengl32.dll gets loaded and it checks in the Windows registry if there is a true GL driver. If there is, it will load it. For example, ATI's GL driver name starts with atioglxx.dll and NVIDIA's GL driver is nvoglv32.dll. The actual names change from release versions GL FAQ

I also found that Intel doesn't provide up-to-date implementation for OpenGL, so even though I have an i7-2500, I only have OpenGL 3.0 :(.

Answer 1

It has nothing to do with history. That's just how it is.

OpenGL implementations, by and large, come from some form of shared library (DLL/SO). However, this is complicated by the fact that you don't own the OpenGL "library"; it's part of the system infrastructure of whatever platform your code is running on. So you don't even know what specific DLL you might be using on someone else's computer.

Therefore, in order to get OpenGL function pointers, you must use the existing infrastructure to load them.

Answer 2

One of the main reasons is extensions: On every platform, OpenGL has always supported platform-specific extensions. These couldn't be part of the official headers, as they were usually added by the vendors in-between spec updates. In addition, those vendor-specific extensions may live in completely weird DLL/SO, for instance, deep inside the driver. There is also no guarantee that the driver DLL exports them under their "canonical" name, so OpenGL relied very early on platform-specific stuff to load a function pointer. This makes the whole extension thing feasible.

On all platforms, you usually do get some OpenGL without using extensions (OpenGL 1.4 or so), but as the extension method was successful and is easy to implement, everyone uses it now (similar for OpenCL!)