C Compiler, Linker & Execution Flow

What Happens When You Run a C Program?

A C program does not run directly. It passes through a sequence of steps that convert human-readable code into a machine-understandable executable.

1. Source Code (.c File)

You write your program in a plain text file with a .c extension.

hello.c
--------
#include <stdio.h>

int main(){
    printf("Hello World");
    return 0;
}

2. Preprocessing (Handled by Preprocessor)

The preprocessor handles lines beginning with #. It performs:

• Header file expansion → includes stdio.h, math.h etc.
• Macro substitution via #define
• Conditional compilation (#if, #ifdef)
• Removing comments

Output: .i file (preprocessed source)

gcc -E hello.c > hello.i

3. Compilation

The compiler converts the preprocessed code into assembly language.

Output: .s file (assembly code)

gcc -S hello.i

• Checks syntax
• Reports errors
• Optimizes the code
• Generates CPU-specific assembly

4. Assembling

Assembler converts assembly code into machine code.

Output: .o file or .obj (object file)

gcc -c hello.s

This object file is NOT ready to run—it still needs linking.

5. Linking

The linker connects your object file with required libraries.

Output: a.exe, a.out, or a named executable

gcc hello.o -o hello

6. Loading

Loader loads the executable file from storage into main memory (RAM).

• Allocates memory for code
• Allocates memory for global variables
• Sets up runtime stack & heap

7. Execution

After loading, the CPU begins executing instructions starting from main().

Full Compilation Process (Diagram)

Source Code (.c)
       ↓
 Preprocessor
       ↓
 Compiled Code (.s)
       ↓
 Assembler
       ↓
 Object File (.o)
       ↓
 Linker + Libraries
       ↓
 Executable File (.exe / .out)
       ↓
 Loader
       ↓
 Program Execution (CPU)

Why Understanding Execution Flow Matters?

• Makes debugging easier
• Helps understand linking errors
• Improves understanding of memory layout
• Helps when working with multiple source files
• Useful in embedded and system-level programming

Practice Questions

1. Explain all stages of the C compilation process.
2. What is the role of the preprocessor?
3. What is an object file?
4. What does the linker do?
5. Why is the loader required?

Practice Task