Function Prototypes and Scope

Function prototypes help declare functions before they are used.

Variable scope determines where a variable can be accessed.

In this lesson, you will learn:

• How to use function prototypes.
• The difference between local and global variables.
• How variable scope affects program behavior.

A function prototype is a declaration of a function before it is defined.

Syntax:

returnType functionName(parameterType1, parameterType2);

Example:

int add(int, int);

This program demonstrates the use of a function prototype.

#include <stdio.h>

int add(int, int); // Function prototype

int main() {
    int sum = add(5, 10);
    printf("Sum: %d\n", sum);
    return 0;
}

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

Variables in C have different scopes:

• Local variables: Declared inside a function, accessible only within that function.
• Global variables: Declared outside functions, accessible throughout the program.

This program demonstrates how a local variable is limited to a function.

#include <stdio.h>

void testFunction() {
    int x = 10; // Local variable
    printf("Inside function: %d\n", x);
}

int main() {
    testFunction();
    // printf("Outside function: %d\n", x); // Error: x is not accessible here
    return 0;
}

This program demonstrates how a global variable is accessible across functions.

#include <stdio.h>

int globalVar = 100; // Global variable

void printGlobal() {
    printf("Global variable: %d\n", globalVar);
}

int main() {
    printGlobal();
    return 0;
}

Scope defines where a variable can be accessed.

• Local variables exist only inside their function.
• Global variables exist for the entire program.
• Block scope variables exist within loops or conditional blocks.

To write clean and efficient code:

• Use global variables sparingly to avoid unintended modifications.
• Prefer function parameters over global variables for better control.
• Always declare function prototypes before use.

In this lesson, you learned:

• How function prototypes help declare functions before use.
• The difference between local and global variables.
• The scope of variables in C.

Next, we will explore recursion in C!