An Introduction to C Functions

Every C program is a collection of functions. Find out all about these useful reusable chunks of code, from how to define them to what void means.

You may have covered basic C syntax, with simple examples, and are now wondering how to go about creating larger, real-world programs. For a start, you'll want to organize your code into efficient parts, with a minimum of repetition. In C, as in most languages, the answer to your problems is the function.

Creating and using functions in C is straightforward and brings many benefits. Functions allow you to break down complex programs into smaller chunks. They provide reusability, both within your original program, and in related libraries or other separate programs.

To start, you'll need to learn about function declarations, prototypes, parameters, and return statements.

In C programming, a function is a named section of code that does something when you ask it to. It helps keep your code organized and lets you use the same action multiple times without repeating yourself.

The simplest example is one you’ve already been using: the main() function. The main function is particularly special because it acts as the main entry point for any C program. You can also use library functions, which are those that someone else has already written, or you can write your own user-defined functions.

These three aspects are fundamental to the use of functions.

This provides information about a function's name, return type, and parameters, enabling its use before its full definition. It is also called a function prototype. It follows this simple syntax:

Where:

For example, here's a simple function declaration:

When you call a function, it runs the code specified in its definition. It includes the function's name, return type, parameter list, and the statements that define its behavior. Here is the syntax:

Let's break down parts of the syntax:

Function body: This is the block of code enclosed in curly braces {}. It contains the instructions that define what the function does when it's called.

Return statement: If the function has a return type other than void, the return statement sends a value back to the caller. This value should match the specified return type.

Here's a simple example of a function definition:

Just like cooking, you may have a recipe (function definition) and some ingredients (arguments), but you still need to follow the instructions to get a result. A function call will run the function with given arguments; here's the syntax:

arguments: These are the values or expressions that you pass to the function as inputs. Separate each argument with a comma. The number, order, and data types of the arguments should match the function's parameter list.

result: If the function has a return type other than void, you can capture the returned value using a variable of the appropriate data type.

Here's an example of a function call:

By following these three steps—declaring the function, providing its implementation, and calling it with appropriate arguments—you can effectively use functions to perform various tasks in your program.

Parameters are variables declared in the function's declaration or definition that act as placeholders for the values passed to the function when called. They allow you to pass data into the function, enabling it to work with and manipulate that data. Return values are the values a function produces and sends back to the caller.

There are two methods of passing parameters.

With this approach, a function call copies the value of the actual argument into the function's parameter. Changes made to the parameter within the function do not affect the original argument.

For example:

Using this approach, you pass the memory address (pointer) of a value to a function. Changes made to the parameter inside the function affect the original value outside it.

Pointers, including their use as parameters, are one of the aspects of C that make it different from a language like Python.

In summary, the key difference between the two methods is how changes to the parameter inside the function affect the original value outside it. Pass by value creates a copy, while pass by reference (pointer) allows direct manipulation of the original value.

Other languages often have an equivalent of C's pointers, but they generally work at a higher level. One example is C#'s support for out variables.

Void functions in C programming are functions that do not return a value. Use them to perform actions or tasks without producing a result. They may change their parameters using pass by reference, but they do not have to.

Here's an example of a void function:

Void functions are useful for performing actions, printing messages, modifying data, or carrying out tasks without requiring a return value.

Practicing through the creation of functions enhances your grasp of understanding and application in C programming. Furthermore, it elevates code readability and ease of maintenance. Dive into various applications of functions to unlock their advantages.

When you've mastered the fundamentals of C functions and want to challenge yourself, consider delving into recursion.

Melanie Ngome is a passionate writer and aspiring frontend web developer. Currently pursuing a course in web development, she combines her love for writing and coding to break down complex concepts and share her journey in becoming a frontend web developer.