How to “multithread” C code

Question

I have a number crunching application written in C. It is kind of a main loop that for each value calls, for increasing values of \"i\", a function that performs some calcul

Answer 1

If you are hoping to provide concurrency for a single loop for some kind of scientific computing or similar, OpenMP as @Novikov says really is your best bet; this is what it was designed for.

If you're looking to learn the more classical approach that you would more typically see in an application written in C... On POSIX you want pthread_create() et al. I'm not sure what your background might be with concurrency in other languages, but before going too deeply into that, you will want to know your synchronization primitives (mutexes, semaphores, etc.) fairly well, as well as understanding when you will need to use them. That topic could be a whole book or set of SO questions unto itself.