Learning to read GNU code?

[Elidor]

Currently I know C++ and am learning the discrepancies of C to C++. I've always wanted to understand C and C++ headers files at least a bit more. How can I go about learning GNU source. There is a flood of macros and typedefs but I'd like to understand there code.

One of the reasons for this is because I was making a pow function but I wanted to compare it to the actual math.h pow function. I want to know the design, structure, and style of how GNU goes about to making headers. So I want something a little more concrete than O(n) efficiency if possible.

[Cubbi]

The actual GNU math.h pow function is not in the header at all, it is here in its general form (written in C)
http://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/ieee754/dbl-64/e_pow.c
and here in its Intel-specific form (written in assembly)
http://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/x86_64/fpu/e_powl.S

GNU libraries have been evolving since the 1980s, so there is a lot of patches, kludges, and ancient compatibility macros. If you want a library that makes sense, look at clang's libc++ here: http://llvm.org/svn/llvm-project/libcxx/trunk/include/ -- but even that is not educational material, it is a production-grade library code.