Difference between Preprocessor and Compiler in C

What is a Preprocessor in C?

The preprocessor is a tool that runs before the actual compilation begins. It handles all the preprocessing directives, which are lines in your code that start with a # symbol. These directives are instructions to the compiler to perform specific tasks before compiling the code.

Common Preprocessor Directives

• #include (file inclusion): Adds contents from another file.
• #define: Defines constants or macros.
• #ifdef, #ifndef, #endif: Conditional compilation.

Example:

#include <stdio.h>
#define QUIZ_SCORE 100
 
int main()
{
    printf("Score: %d\n", QUIZ_SCORE);
    return 0;
}

The preprocessor will replace QUIZ_SCORE with 100 before the compiler sees the code.

What is a Compiler in C?

The compiler is a program that converts your human-readable C code into machine code (binary code) that the computer can understand and execute.

It starts working after the preprocessing step is complete.

Compilation Stages:

• Lexical Analysis – Breaks code into tokens.
• Syntax Analysis – Checks for grammar and structure.
• Semantic Analysis – Ensures the code makes logical sense.
• Code Optimization – Improves performance.
• Code Generation – Converts to machine code.

Step-by-Step Compilation Flow

Here’s a simplified version of the C compilation process:

Source Code (.c file)
      ↓
Preprocessor
      ↓
Modified Code (no # directives)
      ↓
Compiler
      ↓
Assembly Code
      ↓
Assembler
      ↓
Object Code (.o file)
      ↓
Linker
      ↓
Executable File (.exe or a.out)

Difference between Preprocessor and Compiler in C

Sanfoundry Global Education & Learning Series – 1000 C Tutorials.