C files are the building blocks of many programs still in use today. They contain source code written in the C programming language, one of the oldest and most reliable languages in computer science.

While many new languages have emerged, C continues to power critical systems like operating systems, embedded hardware, and development tools. If you’re learning programming or working with existing software, understanding C files is essential.

What Is a .c File?

A .c file is a plain text file that holds C source code. These files are used to define the instructions a program will follow when it runs. When a developer writes a C program, the logic, flow, and actions are written inside one or more .c files.

The .c extension helps compilers recognize the file as C source code. Each .c file may contain one or many functions that perform different tasks. These functions are combined and linked with other files during compilation to create a working program.

C files are often part of larger software projects. In those cases, the program might include many .c files working together with header files (.h). This allows developers to split logic into smaller, manageable parts and reuse code across multiple areas.

File Extension: .c vs .h vs .cpp

Different file extensions serve different purposes in C and C++ development.

• .c – Main source code for C programs

• .h – Header files that hold function declarations and macros

• .cpp – C++ source code files (used for object-oriented programming)

In a C project, .c files contain the function logic, while .h files define the structure of those functions. The compiler uses .h files to understand how different parts of a program fit together. C++ uses .cpp, but follows a similar pattern.

What’s Inside a C File?

A typical .c file begins with one or more #include statements and contains at least one function—usually main().

Here’s a basic example:

#include <stdio.h>

int main() {
printf(“Hello, world!\n”);
return 0;
}

This program tells the computer to include the standard input/output library, then prints “Hello, world!” to the screen. The main() function is the starting point of the program.

How to Open and Edit a C File

You can open .c files in most text editors. Some popular ones include Visual Studio Code, Sublime Text, and Notepad++. These editors highlight C syntax, which makes the code easier to read.

If you’re writing more complex programs, using an IDE like Code::Blocks or Dev-C++ can help. IDEs offer features like code suggestions, auto-formatting, and file management tools, which make working with multiple .c files simpler.

How to Compile and Run a C File

To run a C file, you first need to compile it. This means converting the code into an executable file the computer can understand.

gcc hello.c -o hello
./hello

This example uses GCC, a common C compiler. The first command compiles the hello.c file and creates an output file named hello. The second command runs the program. Other compilers include Clang, MSVC (for Windows), and TCC (a small C compiler).

C File vs Header File: What’s the Difference?

C files contain the actual code that runs the program. These are where developers write logic, loops, conditionals, and function details. Every C program must have at least one .c file that defines its behavior.

Header files (.h) contain declarations. These act like labels or summaries that tell the compiler what functions exist and what they expect as input. This separation allows you to use the same function declarations in multiple .c files without rewriting the code, making programs modular and reusable.

Where Are C Files Used Today?

Even in 2025, C files remain important in many parts of tech. Operating systems like Linux and parts of Windows include code written in C. These systems need reliable, low-level access to memory and hardware—something C provides well.

Embedded systems also depend on C files. Devices like routers, sensors, and medical machines run code compiled from .c files because the language offers speed and control.

C is also used in tools like Git, compilers, and system-level libraries. Developers continue to maintain and extend these systems, and understanding .c files is key to working with them. Whether you’re exploring open-source tools or learning programming fundamentals, C files are likely to show up.