Your first C program

Before you can write C programs, you need a C compiler. The most widely used one is gcc — the GNU Compiler Collection. It is free, works on Linux, macOS, and Windows (via WSL or MinGW), and is what most tutorials, textbooks, and production systems use.

Installing a compiler

Linux: gcc is often pre-installed. If not: sudo apt install gcc (Debian/Ubuntu) or sudo dnf install gcc (Fedora).

macOS: Install the Xcode Command Line Tools: xcode-select --install. This gives you clang aliased as gcc — it is compatible and will work for everything in this track.

Windows: Install WSL 2 (Windows Subsystem for Linux) and run sudo apt install gcc inside it. This gives you a real Linux environment and is the recommended path.

Verify the installation:

gcc --version

You should see something like gcc (Ubuntu 13.2.0-23ubuntu4) 13.2.0. The exact version does not matter much for what this track covers.

The minimum C program

Create a file called hello.c. The .c extension is the convention for C source files.

#include <stdio.h>

int main(void) {
    printf("Hello, World!\n");
    return 0;
}

This is the smallest complete C program that does something visible. Five things to understand:

#include <stdio.h> is a preprocessor directive. Before compilation starts, the preprocessor reads this line and pastes the contents of the system header file stdio.h into your source. That header declares the printf function — without the include, the compiler would not know printf exists.

int main(void) is the entry point. Every C program starts execution here. The int return type tells the OS whether the program succeeded (0) or failed (non-zero). The void in the parameter list means "this function accepts no arguments." You will also see int main(int argc, char *argv[]) when you need to read command-line arguments.

printf("Hello, World!\n") prints a string to standard output. The \n is the escape sequence for a newline character. Without it, the terminal prompt appears on the same line as your output.

return 0 exits the program and tells the OS the program succeeded. A non-zero return value signals failure — by convention, different values mean different kinds of failure. This feeds into shell scripting, where you can check $? after running a command.

Semicolons end statements. Unlike Python, C does not use indentation or newlines to separate statements — it uses semicolons. Forgetting one is one of the most common beginner mistakes and the compiler will tell you precisely where.

Compiling and running

gcc hello.c -o hello
./hello

gcc hello.c compiles the source file. -o hello names the output file hello (without -o the output is named a.out by default). ./hello runs it.

Output:

Hello, World!

Reading a compiler error

Type a deliberate mistake — remove the semicolon after the printf line:

#include <stdio.h>

int main(void) {
    printf("Hello, World!\n")
    return 0;
}

Compile:

hello.c:5:5: error: expected ';' before 'return'
    5 |     return 0;
      |     ^~~~~~

The format is file:line:column: severity: message. Here, hello.c line 5 column 5 is where the compiler noticed the problem. The actual missing semicolon is at the end of line 4 — the compiler reports where it first realized something was wrong, which is usually the token after the mistake.

The compilation pipeline

When you run gcc hello.c -o hello, four things happen in sequence:

1. Preprocessing: #include directives are expanded, #define macros substituted.
2. Compilation: C source is translated to assembly language.
3. Assembly: assembly is translated to machine code (object files).
4. Linking: object files and libraries are combined into the final executable.

You do not need to invoke each step separately — gcc handles all of them. But knowing the stages matters for understanding error messages: a "linker error" is different from a "compiler error," and fixing them requires different actions. The Advanced tier covers this in depth.

A slight improvement

Add a newline print to see how multiple calls work:

#include <stdio.h>

int main(void) {
    printf("Hello, World!\n");
    printf("This is C.\n");
    return 0;
}

Compile and run. Notice that printf is called once per line — it does not automatically add a newline. This is intentional: sometimes you want to build up a line of output piece by piece.

Where to go next

You have a working compiler and understand the basic structure of a C program. Next: variables and primitive types — how C stores data, what int, char, float, and double mean, and how to declare variables.