Questions about C Function Prototypes and Compilation

Question

With the following code:

int main(){
    printf(\"%f\\n\",multiply(2));
    return 0;
}

float multiply(float n){
    return n * 2;
}

When

Answer 1

Question1:

So the invented prototype would be "int multiply(int)", and hence the errors. Is this correct?

Not exactelly yes because it it depends of your Cx (C89, C90, C99,...)

for function return values, prior to C99 it was explicitly specified that if no function declaration was visible the translator provided one. These implicit declarations defaulted to a return type of int

Justification from C Standard (6.2.5 page 506)

Prior to C90 there were no function prototypes. Developers expected to be able to interchange argu-ments that had signed and unsigned versions of the same integer type. Having to cast an argument, if the parameter type in the function definition had a different signedness, was seen as counter to C’s easy-going type-checking system and a little intrusive. The introduction of prototypes did not completely do away with the issue of interchangeability of arguments. The ellipsis notation specifies that nothing is known about the 1590 ellipsis supplies no information expected type of arguments. Similarly, for function return values, prior to C99 it was explicitly specified that if no function declaration was visible the translator provided one. These implicit declarations defaulted to a return type of int . If the actual function happened to return the type unsigned int , such a default declaration might have returned an unexpected result. A lot of developers had a casual attitude toward function declarations. The rest of us have to live with the consequences of the Committee not wanting to break all the source code they wrote. The interchangeability of function return values is now a moot point, because C99 requires that a function declaration be visible at the point of call (a default declaration is no longer provided)

Question 2:

How come when I break the code into 2 files it compiles?

it will compile and it will be treated like indicated in the first question exactelly the same