C Compiler Process

 Hi Learners!!

                C Compilation Process:

The process of turning your C source code (.c file) into an executable program is called compilation. Unlike interpreted languages (Python, JavaScript), C is a compiled language — which means your code must be translated into machine code before the program can run.

Modern C compilation is usually done in four main phases:

1. Preprocessing Phase

(What happens first – handled by preprocessor)

Main tasks:

• Removes all comments
• Expands all macros (#define, #ifdef, etc.)
• Includes header files (#include <stdio.h>)
• Processes conditional compilation directives (#if, #ifdef, #ifndef, #else, #endif)
• Handles #pragma, #line, etc.

Important fact: After preprocessing → the code is still text, but much bigger and without any preprocessor directives.

File extension convention (not always created physically): .i or .ii (preprocessed source)

2. Compilation Phase (proper)

(Translates preprocessed C code → assembly code)

Main jobs of the compiler:

• Checks syntax (are all brackets, semicolons correct?)
• Checks semantic rules (do variables exist? correct types? etc.)
• Converts C language constructs into low-level assembly instructions
• Does many optimizations (especially in -O2, -O3 modes)
• Produces symbol table (where are variables and functions located?)

Output: Assembly code (human-readable but machine-oriented)

Common file extension: .s

3. Assembly Phase

(Turns assembly code into machine code – object code)

The assembler does:

• Translates mnemonics (mov, call, push...) into actual binary machine instructions
• Resolves local labels
• Creates relocatable object file

Output: Object file (.o or .obj)

This file contains:

• Machine code
• Symbol table (which functions/variables are defined here, which are needed from outside)
• Relocation information (addresses that will be fixed later)

4. Linking Phase

(The final and very important step)

The linker does:

• Takes one or more object files (.o)
• Combines them with libraries (like libc, libm, etc.)
• Resolves external references (where is printf actually located?)
• Performs relocation (fixes all addresses)
• Produces final executable file

Stage	Tool	Input	Output	Main job	Still text?
Preprocessing	Preprocessor	.c	.i	Macro expansion, #include	Yes
Compilation	Compiler	.i	.s	Syntax/semantic check, code generation	No (asm)
Assembly	Assembler	.s	.o / .obj	Machine code generation	No
Linking	Linker	.o + libs	executable	Combine files, resolve symbols	No