C function declaration

Question

Is it necessary to declare function before using it? Does the compiler give an error if we don't use a function declaration and call it directly. Please answer according to standard C.

If yes then what does it mean that argument of type are converted to int and float to double if arguments to function are not defined?

Answer 1

In ANSI C, you do not have to declare a function prototype; however, it is a best practice to use them. The only reason the standard allows you to not use them is for backward compatibility with very old code.

If you do not have a prototype, and you call a function, the compiler will infer a prototype from the parameters you pass to the function. If you declare the function later in the same compilation unit, you'll get a compile error if the function's signature is different from what the compiler guessed.

Worse, if the function is in another compilation unit, there's no way to get a compilation error, since without aa prototype there's no way to check. In that case, if the compiler gets it wrong, you could get undefined behavior if the function call pushes different types on the stack than the function expects.

Convention is to always declare a prototype in a header file that has the same name as the source file containing the function.

With prototypes, the compiler can verify you are calling the function correctly (using the right number and type of parameters).

Without prototypes, it's possible to have this:

// file1.c
void doit(double d)
{
    ....
}

int sum(int a, int b, int c)
{
    return a + b + c;
}

and this:

// file2.c

// In C, this is just a declaration and not a prototype
void doit();
int sum();

int main(int argc, char *argv[])
{
    char idea[] = "use prototypes!";

    // without the prototype, the compiler will pass a char *
    // to a function that expects a double
    doit(idea);

    // and here without a prototype the compiler allows you to
    // call a function that is expecting three argument with just
    // one argument (in the calling function, args b and c will be
    // random junk)
    return sum(argc);
}

Answer 2

In C, you need to define everything before you use it. I think the idea is that it can compile everything in one pass, since it always has all of the information it needs by the time it gets to it.

I'm not sure what you're asking with the second part. If the function isn't defined, it just won't work.

Answer 3

The short answer: in C89/90 it is not necessary, in C99 it is necessary.

In C89/90 the function does not have to be declared in order to be called. For undeclared function the compiler will make an assumption about function return type (assumes int ) and about function parameter types (will derive them from the argument types at the point of the call).

In C99 the function has to be previously declared in order to be called. Note, that even in C99 there's no requirement to provide a prototype for the function, only a declaration is needed, ie a non-prototype declaration is OK. This means that the compiler no longer has to make any assumptions about the function return type (since the declaration always specifies it explicitly), but it still might have to make assumptions about the function parameters (if the declaration is not a prototype).

Answer 4

The C99 standard in 6.5.2.2 "Function call" describes what happens in a function call expression, including if no prototype has been seen by the compiler for the function:

If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument

Later, it describes what happens if the function call expression doesn't match what how the function is actually defined:

If the function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function, the behavior is undefined.

Clearly, to help ensure correctness, using function prototypes is the right thing to do. Enough such that C++ made requiring function prototypes one of the 'breaking' changes with c.