Preprocessor Directives

C Preprocessor

In a C program, all lines that start with # are processed by the preprocessor, a special program invoked by the compiler.

In simple terms:

The preprocessor takes a C program and produces another C program where all preprocessor directives (# lines) are handled and removed.

---

Types of Preprocessor Directives

There are four main types of preprocessor directives:

1. Macros

2. File Inclusion

3. Conditional Compilation

4. Other Directives

---

Important Preprocessor Directives

All preprocessor commands:

• Begin with #

• Must be the first non-blank character on a line

• Typically start in the first column for readability

List of Common Directives

Sr. No	Directive	Description
1	#define	Substitutes a preprocessor macro
2	#include	Inserts a header file
3	#undef	Undefines a macro
4	#ifdef	True if macro is defined
5	#ifndef	True if macro is not defined
6	#if	Tests a compile-time condition
7	#else	Alternative for #if
8	#elif	Combination of #else and #if
9	#endif	Ends conditional block
10	#error	Prints error message
11	#pragma	Issues compiler-specific instructions

---

Important Facts About the Preprocessor

---

1️⃣ File Inclusion (#include)

When using #include, the contents of the included file are copied into the current file after preprocessing.

Using Angle Brackets

#include <math.h>

• Searches in standard system directories

Using Double Quotes

#include "yourheaderfile.h"

• Searches in the current directory first

• Then in system directories

---

2️⃣ Macros (#define)

When using #define, the preprocessor replaces matching tokens with the defined value.

Example: Constant Macro

#include <stdio.h>
#define max 100

int main()
{
    printf("max is %d", max);
    return 0;
}

Output:

max is 100

⚠️ Note: The word max inside double quotes ("") is not replaced.

---

Example: Function-like Macro

#include <stdio.h>
#define MULTIPLY(a, b) a*b

int main()
{
    printf("%d", MULTIPLY(2, 3));
    return 0;
}

---

3️⃣ Conditional Compilation

Preprocessors support conditional compilation using #if, #ifdef, #ifndef, etc.

Example:

#ifndef MESSAGE
#define MESSAGE "You wish!"
#endif

---

Platform-Specific Compilation Example

#ifdef __unix__
    #include <unistd.h>
#elif defined _WIN32
    #include <windows.h>
#endif

• __unix__ → Defined on Unix systems

• _WIN32 → Defined on Windows systems

Most Windows compilers define _WIN32.
Some define WIN32.

You can define macros manually during compilation:

-D_WIN32

---

4️⃣ Header File Multiple Inclusion Problem

If a header file is included multiple times, it may cause:

• Redefinition errors

• Duplicate declarations

Solution: Use Include Guards

#ifndef HEADER_FILE
#define HEADER_FILE

// header content

#endif

---

5️⃣ Standard Predefined Macros

C provides some built-in macros:

Macro	Description
__FILE__	Current file name
__DATE__	Compilation date
__TIME__	Compilation time
__LINE__	Current line number

Example:

#include <stdio.h>

int main()
{
    printf("Current File : %s\n", __FILE__);
    printf("Current Date : %s\n", __DATE__);
    printf("Current Time : %s\n", __TIME__);
    printf("Line Number : %d\n", __LINE__);
    return 0;
}