Externally Define Preprocessor Macros in GLSL
GLSL has a full C-style preprocessor. The only thing that does not work is #include. One of the great features is that that you can used #ifdef to comment out functions and thus create one shader that can be thinned out if certain features are not used.
My Question is:
Is there a way to define a macro from C code?
There seems no way to do that with the openGL interface. The quick hack is to prepend a few lines with #define FOO before the code loaded form file. But it seems kind of backwards.
Answer
You don't really need to prepend it to the code you loaded. That's why there are multiple strings in the glShaderSourceARB API.
Something like the following does what you are looking for:
char *sources[2] = { "#define FOO\n", sourceFromFile };
glShaderSourceARB(shader, 2, sources, NULL);