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Hello, World & the Compilation Pipeline

How your .c file becomes an executable — the four steps every C program takes.

Examples: gcc, clang, .i / .s / .o files

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Hello, World

Every C program starts at main() and returns an int status code.

Example

hello.c

#include <stdio.h>

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

Output

Hello, World!

The 4-Step Pipeline

Compilation is not one step. Each phase produces a real file you can inspect.

Syntax

one command per stage

gcc -E hello.c -o hello.i   // preprocess
gcc -S hello.i -o hello.s   // compile to asm
gcc -c hello.s -o hello.o   // assemble
gcc hello.o   -o hello      // link

.c → .i → .s → .o → executable

#include & the Preprocessor

The preprocessor is a text substitution engine. It runs BEFORE the compiler sees your code.

Example

example

#define PI 3.14159
#define SQUARE(x) ((x) * (x))

#include <stdio.h>  // pastes the whole header here

int main(void) {
    printf("%f\n", SQUARE(PI));
    return 0;
}

WATCH OUT

Always wrap macro arguments in parentheses — SQUARE(a+1) without them becomes a+1*a+1.

Compile & Run

The compiler + linker turn your code into a binary the OS can execute.

Syntax

shell

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gcc -Wall -Wextra -O2 hello.c -o hello
./hello

Always compile with -Wall -Wextra. They catch real bugs.