Functions in C

Learn how to organize and reuse code using functions in the C programming language.

As your programs grow, it becomes helpful to organize your code into functions.
Functions let you name a block of code and call it whenever you need it — this keeps your code clean, reusable, and easier to understand.

What is a Function?

A function is like a mini-program you can define and reuse.
Every C program starts with a special function called main() — that’s where the program begins.

Here’s a simple function:

void greet() {
    printf("Hello from a function!\n");
}

This function uses the printf() function to print a message. Functions can contain other functions, variables, and logic just like the main program.

The \n at the end of the string is a newline character, which moves the cursor to the next line after printing.

To run the function, just call it by name:

int main() {
    greet();  // this runs the greet() function
    return 0;
}

Function Syntax

Syntax means the rules for how to write code correctly.

Here’s the basic structure of a function in C:

return_type function_name(parameter_list) {
    // code to execute
}

The parts are:

• return_type: What type of value the function returns (like int, void, etc.)
• function_name: The name you give to the function (like greet)
• parameter_list: Inputs the function can take (like int a, int b)
• code to execute: The block of code that runs when the function is called

Example with parameters

void say_hello(char name[]) {
    printf("Hello, %s!\n", name);
}

the char name[] part is a parameter that lets you pass in a name when you call the function. The square brackets [] indicate that name is an list of characters (a string).

Call it like this:

say_hello("Kevin");

Functions That Return Values

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

int main() {
    int result = add(2, 3);
    printf("Result: %d\n", result);
    return 0;
}

Why Use Functions?

• Reusability – write once, use many times
• Readability – break big problems into smaller steps
• Maintainability – change code in one place instead of many

Quick Tips

• Declare functions before calling them (or use a function prototype)
• Always match the number and type of arguments
• void means “no return value”

Summary

You now know how to:

• Write and call your own functions
• Pass data into functions with parameters
• Return results from functions
• Use functions to clean up and simplify your code

Next up: Setting Up the Toolchain, where we’ll install the software you need to compile and upload C programs to the Raspberry Pi Pico.

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