C - Compilation and Execution Process in C

A C program does not execute directly. It passes through four main stages before producing output:

1. Preprocessing

2. Compilation

3. Linking

4. Execution

1. Preprocessing

• Handled by the preprocessor

• Processes all lines starting with #

• No machine code is generated at this stage

Main Preprocessor Tasks:

• Macro expansion (#define)

• Header file inclusion (#include)

• Conditional compilation (#if, #ifdef)

• Removal of comments

Example:

#include <stdio.h>
#define PI 3.14

Output:
A modified source file with expanded macros and included headers.

2. Compilation

• Handled by the compiler

• Converts preprocessed code into assembly / object code

• Checks for syntax errors

Compiler Responsibilities:

• Syntax checking

• Type checking

• Conversion to object code (.obj or .o file)

Example error detected:

int a = "10";   // type mismatch error

If errors exist → compilation stops.

3. Linking

• Handled by the linker

• Combines:

  • Object code

  • Library functions

• Produces a final executable file

Linking Tasks:

• Resolves function calls (e.g., printf)

• Combines multiple object files

• Attaches standard libraries

Example:

printf("Hello");

printf() definition is linked from the standard library.

Output:

• Executable file (.exe in Windows, a.out in Linux)

4. Execution

• Handled by the operating system

• Executable file is loaded into memory

• Program starts execution from main() function

Example:

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

Output:

Hello World

Flow of Compilation and Execution

Source Code (.c)
        ↓
Preprocessor
        ↓
Compiler
        ↓
Object Code (.o)
        ↓
Linker
        ↓
Executable File
        ↓
Execution (Output)

Summary Table

Key Exam Points

• C is a compiled language

• Errors can occur during compilation or linking

• Execution starts from main()

• Libraries are linked before execution.