The question is pretty easy. I've download a version of the GSL on the official ftp (the last one to be more precise).
I can easily built the library and find the header file. But when I look for the implementation of gsl_fft_complex_forward for example using grep, I can only find the documentation, the header file and an use of it in an example.
So where can I find the .c that contain the function implementation?
This might help you in the right direction...
Inside fft folder, look in c_main.c
It includes c_pass.h and defines int FUNCTION(gsl_fft_complex,forward) which might just be what you are looking for.
This function basically passes its arguments plus one additional argument sign to another function defined later on in the same file int FUNCTION(gsl_fft_complex,transform) which in turn does some other stuff and then calls FUNCTION(fft_complex,pass_[2,3,4,5,6,7,n]) which are defined in their own files c_pass_[2,3,4,5,6,7,n]
I'm know very little about the Fast Fourier Transform to help you analyse the code any further though...
Also this way of defining and calling functions seems very unusual to me, but I'm not very experienced in c++