C compilation process - An introduction

Introduction

Knowing what goes on behind the scenes when we compile our code.

The compilation process

Overview of the compilation process

- ? The compilation process is about the conversion of the C source code into an executable that the operating system is capable of running.

- ? This process is composed of four main steps:

? ? 1. ?Preprocessor

? ? 2. ?Compilation

? ? 3. ?Assembly

? ? 4. ?Linker

- ? Let's take each step and dissect it to see its internals, but before diving in, there are two rules that we must be aware of:

? ? 1. ?Only source files are compiled.

? ? 2. ?Each source file is compiled separately.

- ? Our analysis will comprise three main points:

? ? 1. ?The input of the process.

? ? 2. ?The function of the process.

? ? 3. ?The output of the process.

Step 1 - Preprocessing

Input

A C source code file `(.c)` is fed into this process.

Function

- ? Inclusion of the header files in the source code.

? ? - ? If the program contains `#include` this line is to be replaced by the original content of the header file.

- ? Expansion of macros.

? ? - ? Every macro defined by the `#define` keyword is to be replaced with its value.

- ? Removal of the user comments.

Output

- ? The output of this process is an expanded C code file without any preprocessing statements.

- ? The output is of type `(.i)` or `(.pre)`.

- ? This file is known as a translation unit or compilation unit.

Viewing preprocessed code

To view the translation unit we use the `-E` flag of the GCC compiler.

gcc -E myProgram.c        

Step 2 - Compiler

Input

- ? The input of this step is the translation unit.

- ? The compiler operates on a single translation unit only, so when there are multiple source code files that need to be compiled, each one is taken on an individual step.

Function

- ? The function of the compiler is to produce an optimized assembly code file.

- ? One of the main functions of the compiler is performing lexical analysis and syntax analysis on the translation unit, thus any error during the compiler, is mainly a syntax error.

- ? The compiler resolves only the current code, what we mean by this it doesn't resolve any function that is not defined in the current translation unit such as `printf()` as this still needs to be linked by the linker.

Output of the process

- ? A list file: This file contains the corresponding assembly code for each of our instructions.

Viewing the assembly code

To view the assembly code uses the `-S` flag for the GCC compiler.

gcc -S myProgram.c        

Step 3 - Assembler

Input

- ? The input of this process is an assembly code that could be generated by the compiler or written by the user.

Function

- ? The conversion of the assembly code file into an object code file.

Output

- ? An object file.

Creating an object file from an assembly file

as myProgram.s -o myProgram.o        

Creating an object file from a C source code file

gcc -c myProgram.c        

Step 4 - Linker

Input

The linker takes three input files:

1. ?Linker file.

2. ?Object file.

3. ?Library files.

+ What is a linker file?

? ? It's a file written in a linker script that describes the segmentation

? ? of the memory of the target machine that the code is to be run on.

Function

- ? Linking of external libraries and object files together.

- ? Resolution of external referenced functions and objects, such as the `printf()` function.

- ? Performing memory allocation following the linker file.

Output

The binary file for the corresponding machine.