eolas/zk/Functions_in_C.md

tags
C

What type is a function?

A function does not have to a dedicated primitive type. It is a derived type, meaning that its type-signature comes from the primitive types that it uses.

On this basis, a function type is derived from its return type and the number and types of its parameters.

E.g. for the function:

int mimic_number(int num) {
    return num;
}

It's type is a product of:

• its return type (int)
• the number of parameters (1)
• the type of its parameter (int)

Function declarators and function definitions

In C, it is possible to declare a function without specifying the body, e.g:

int f(void);
void g(int i, int j);

Are all examples of function declarators.

Later then you can go back and provide the definition. Note that this is just the same as a normal full function declaration, it's just you've previously declared its existence:

int f(void);

// Some other code

int f(void) {
    printf("Here's the definition.")
}

What's the point of this?

The only real utility in distinguishing the declaration from the definition is when creating reusable libraries.

These follow this sequence:

• define a .h header file with function declaration
• define a .c file with the function definition
• import the function into main.c and call function

Applied example:

// math_utils.h

int add (int a, int b);

// math_utils.c
int add (int a, int b) {
    return a + b;
}

And then in main.c:

#include "math_utils.h"

int main(void):
    int sum = add(5, 3)

The compiler compiles each .c file separately. The header file tells main.c that add and multiply exist somewhere, even though they're defined in a different file. The linker connects everything together at the end.

In C, the distinction between arguments and parameters is more acute than other languages: