We're really just telling the compiler to find a file called stdio.h and act as though its contents were included at that part in our program. Whether the full

We're really just telling the compiler to find a file called stdio.h and act as though its contents were included at that part in our program. Whether the full function is there or linked later is a matter of implementation (some headers have full functions, typically trivial "inline" functions). I can't remember what the standard says via the stdlib, but in general a library you load later on may not have all/some of those functions, but as long as you didn't use them you'll be fine (just as with a redundant forward declaration you yourself did). And finally the whole "linking" part of C is mostly just a widespread convention, not something that the standard really dictates. See section 6.2.2 [1] for the brief words on it mainly to do with "static"/"extern". But the whole "compile" -> "link" (to some existing .so/.a) -> "program" is all implementation defined. 1. http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf