Parsing a string with inline C

I can't help you with the inline C but if your string is null terminated you do not need the len variable at all. Just use

while (*data)

and the if statement could be avoided by writing

s = s + (*(data++)=='\n');

This exploits that the boolean expression returns 0 or 1.
Whether this is faster is to be benchmarked as it will depend on compiler and how the code is actually compiled.