The behavior of a C compiler with old-styled functions without prototypes

Question

When my program consists of two files:

main.c

#include 

int main(void) { 
     printf(\"%lf\\n\",f());   
     return 0;
 }
         


        

Answer 1

C will assume a function has the prototype int func(); unless you have told it otherwise.(Note that in C int func(); and int func(void); are different things)

In your second case, you do a call to f() for which the compiler hasn't seen any prototype, so it assumes it is int f(); . Later on it sees your definition for f() which has a different prototype - and issues an error.

This doesn't happen in the 1. case, as they're in different compilation units.