Functions and scope

In plain English

A function is a named block of code that takes inputs and can give back a result. A prototype declares the function signature, which is its return type, name and parameters, so the compiler knows how it is called before it sees the body. Parameters are the inputs a caller passes in, and a return value is what the function hands back. Scope is the region of the program where a name is visible: a local variable declared inside a function exists only while that function runs, while a global variable is visible everywhere in the file. A header file is a text file that holds declarations, usually prototypes, so many source files can share the same interface.

Step by step

1. 1Write a prototype above main and the definition below it.
2. 2Call the function and print its return value.
3. 3Add a local variable and watch it disappear when the function returns.
4. 4Add a global variable and read it from a second function.
5. 5Move the prototype into a header and include it.

#include <stdio.h>

int add(int a, int b);

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

int global = 100;

void show(void) {
    int local = 5;
    printf("local = %d, global = %d\n", local, global);
}

int main(void) {
    printf("%d\n", add(2, 3));
    show();
    return 0;
}

Common mistakes

• Calling a function before the compiler has seen its prototype makes the compiler guess the signature and often warn.
• Changing a parameter inside a function does not change the variable the caller passed in, because C passes by value.
• Declaring a variable inside one function and using it in another fails, because local scope ends when the function returns.
• Putting a definition instead of a prototype in a header duplicates the body across files and causes linker errors.