C doesn't have namespaces so the compiler is certainly within its right to deduce that the sin() function is the one from the standard library. Actually even i

C doesn't have namespaces so the compiler is certainly within its right to deduce that the sin() function is the one from the standard library. Actually even in C++ after the compiler performs the unqualified name lookup if the result of the lookup is the standard sin() function it will make use of its internal knowledge about the function to do optimization. Remember that the C or C++ standard doesn't deal with compilers and linkers; the standard deals with the "implementation" as a whole.