C Preprocessor Directives

01What is the Preprocessor?

Definition

The C Preprocessor is a tool that processes your code before compilation. It handles directives that start with # - like including files, defining constants, and conditional compilation.

Compilation Process

Source Code

.c file

→

Preprocessor

#include, #define

→

Compiler

Translation

→

Executable

.exe / a.out

Key Preprocessor Directives

• #include - Include header files
• #define - Define macros and constants
• #ifdef / #ifndef / #endif - Conditional compilation
• #pragma - Compiler-specific instructions

What's Really Happening?

The preprocessor is like a smart text editor that runs BEFORE the compiler sees your code:

#include: Literally copy-pastes the entire header file into your code. If you include 10 files, your code grows by 10 files worth!

#define: Find-and-replace. Every time it sees PI, it replaces with 3.14159. NO memory is used!

#ifdef: Includes or excludes entire blocks of code based on conditions. Different code for Windows vs Linux!

See it yourself: Run gcc -E file.c to see the preprocessed output!

02#include Directive

The #include directive tells the preprocessor to copy the contents of another file into your code. There are two forms:

<angle brackets>

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

Used for standard library headers. Searches system include directories.

"double quotes"

main.c

#include "myheader.h"
#include "utils/helper.h"
#include "../config.h"

Used for your own headers. Searches current directory first.

Creating Your Own Header File

mymath.h

1// mymath.h - Header file (declarations)
2#ifndef MYMATH_H    // Header guard - prevents double inclusion
3#define MYMATH_H
4
5// Function declarations (prototypes)
6int add(int a, int b);
7int subtract(int a, int b);
8int multiply(int a, int b);
9
10#endif

mymath.c

1// mymath.c - Implementation file (definitions)
2#include "mymath.h"
3
4int add(int a, int b) {
5    return a + b;
6}
7
8int subtract(int a, int b) {
9    return a - b;
10}
11
12int multiply(int a, int b) {
13    return a * b;
14}

main.c

1// main.c - Using the header
2#include <stdio.h>
3#include "mymath.h"
4
5int main() {
6    printf("5 + 3 = %d\n", add(5, 3));
7    printf("5 - 3 = %d\n", subtract(5, 3));
8    printf("5 * 3 = %d\n", multiply(5, 3));
9    return 0;
10}

03#define Macros

The #define directive creates macros - text replacements done by the preprocessor before compilation.

Simple Constants

constants.c

1#include <stdio.h>
2
3// Define constants
4#define PI 3.14159
5#define MAX_SIZE 100
6#define GREETING "Hello, World!"
7
8int main() {
9    double area = PI * 5 * 5;     // PI replaced with 3.14159
10    int arr[MAX_SIZE];             // MAX_SIZE replaced with 100
11    printf("%s\n", GREETING);     // GREETING replaced with string
12    return 0;
13}

Macros with Parameters

macro_functions.c

1#include <stdio.h>
2
3// Macro functions (no function call overhead)
4#define SQUARE(x) ((x) * (x))
5#define MAX(a, b) ((a) > (b) ? (a) : (b))
6#define MIN(a, b) ((a) < (b) ? (a) : (b))
7#define ABS(x) ((x) < 0 ? -(x) : (x))
8
9int main() {
10    int n = 5;
11    printf("Square of %d: %d\n", n, SQUARE(n));   // Output: 25
12    printf("Max(3, 7): %d\n", MAX(3, 7));         // Output: 7
13    printf("Min(3, 7): %d\n", MIN(3, 7));         // Output: 3
14    printf("Abs(-10): %d\n", ABS(-10));           // Output: 10
15    return 0;
16}

Macro Pitfall: Always Use Parentheses!

main.c

// BAD - No parentheses
#define SQUARE_BAD(x) x * x
// Problem: SQUARE_BAD(3+2) expands to 3+2 * 3+2 = 3+6+2 = 11 (WRONG!)
// GOOD - With parentheses
#define SQUARE_GOOD(x) ((x) * (x))
// SQUARE_GOOD(3+2) expands to ((3+2) * (3+2)) = 5 * 5 = 25 (CORRECT!)

#undef - Undefine a Macro

main.c

#define DEBUG 1
// ... code that uses DEBUG ...
#undef DEBUG    // Now DEBUG is undefined
// DEBUG no longer exists here

04Conditional Compilation

Conditionally include or exclude code based on defined macros. Useful for debugging, platform-specific code, and feature flags.

#ifdef / #ifndef / #endif

debug_example.c

1#include <stdio.h>
2
3#define DEBUG    // Comment this line to disable debug mode
4
5int main() {
6    int x = 10;
7    
8    #ifdef DEBUG
9        printf("[DEBUG] x = %d\n", x);    // Only compiled if DEBUG is defined
10    #endif
11    
12    printf("Result: %d\n", x * 2);
13    
14    #ifndef RELEASE
15        printf("[DEV] Development build\n");  // Only if RELEASE is NOT defined
16    #endif
17    
18    return 0;
19}

#if / #elif / #else / #endif

version_check.c

1#include <stdio.h>
2
3#define VERSION 2
4
5int main() {
6    #if VERSION == 1
7        printf("Running Version 1\n");
8    #elif VERSION == 2
9        printf("Running Version 2\n");
10    #else
11        printf("Unknown Version\n");
12    #endif
13    
14    #if defined(DEBUG) && VERSION >= 2
15        printf("Debug mode for V2+\n");
16    #endif
17    
18    return 0;
19}

Platform-Specific Code

platform.c

1#include <stdio.h>
2
3void clearScreen() {
4    #ifdef _WIN32
5        system("cls");       // Windows
6    #else
7        system("clear");     // Linux/Mac
8    #endif
9}
10
11int main() {
12    #if defined(_WIN32) || defined(_WIN64)
13        printf("Running on Windows\n");
14    #elif defined(__linux__)
15        printf("Running on Linux\n");
16    #elif defined(__APPLE__)
17        printf("Running on macOS\n");
18    #else
19        printf("Unknown platform\n");
20    #endif
21    
22    return 0;
23}

05Header Guards

Header guards prevent a header file from being included multiple times, which would cause "redefinition" errors.

Without Header Guard

main.c

// myheader.h
struct Point {
    int x, y;
};
// If included twice:
// ERROR: redefinition of 'struct Point'

✓With Header Guard

main.c

// myheader.h
#ifndef MYHEADER_H
#define MYHEADER_H
struct Point {
    int x, y;
};
#endif  // MYHEADER_H

Modern Alternative: #pragma once

main.c

// myheader.h
#pragma once    // Non-standard but widely supported
struct Point {
    int x, y;
};

#pragma once is simpler but not part of the C standard. Traditional header guards are more portable.

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Mistake 1: Semicolon after #define

main.c

// WRONG
#define MAX 100;    // Semicolon included in replacement!
int arr[MAX];       // Becomes: int arr[100;]; - ERROR!
// CORRECT
#define MAX 100
int arr[MAX];       // Becomes: int arr[100];

Mistake 2: Space in macro name

main.c

// WRONG
#define MAX VALUE 100    // 'MAX' is defined as 'VALUE 100'
// CORRECT
#define MAX_VALUE 100

Mistake 3: Missing parentheses in macro

main.c

// WRONG
#define DOUBLE(x) x + x
int result = DOUBLE(5) * 2;  // 5 + 5 * 2 = 5 + 10 = 15 (WRONG!)
// CORRECT
#define DOUBLE(x) ((x) + (x))
int result = DOUBLE(5) * 2;  // ((5) + (5)) * 2 = 10 * 2 = 20

Mistake 4: Side effects in macro arguments

main.c

#define SQUARE(x) ((x) * (x))
int a = 5;
int result = SQUARE(a++);  
// Expands to: ((a++) * (a++))
// a is incremented TWICE! Result is undefined behavior.
// Solution: Use inline functions for complex operations

08Frequently Asked Questions

Q:What are predefined macros I can use?

A: The compiler provides useful macros:__FILE__ (current filename),__LINE__ (line number),__DATE__ (compile date),__TIME__ (compile time),__func__ (current function name). These are invaluable for debugging and logging.

Q:When should I use #define vs const?

A: Use const for type-safe constants that respect scope. Use #define for conditional compilation, macro functions, and string constants. Modern C prefersconst and inline functions over macros because they're type-checked and debuggable.

Q:Why do I need header guards?

A: Without guards, if file A includes header.h and file B includes both A and header.h, the header contents are included twice — causing "redefinition" errors. #ifndef/#define/#endif ensures the header is only processed once per compilation unit.

Q:Why wrap macro arguments in parentheses?

A: Macros do text substitution, not evaluation.#define SQ(x) x*x with SQ(1+2) becomes1+2*1+2 = 5, not 9! With parentheses:#define SQ(x) ((x)*(x)) gives ((1+2)*(1+2)) = 9.

Q:What's the difference between <header.h> and "header.h"?

A: Angle brackets <> search system include directories (standard library headers). Quotes "" search the current directory first, then system directories. Use <> for standard headers, "" for your own headers.

Q:Can I see what the preprocessor outputs?

A: Yes! Use gcc -E file.c to output the preprocessed code without compiling. This shows all macro expansions and included files — invaluable for debugging macro issues.

Q:What is the difference between #include <file> and #include "file"?

A: Angle brackets (<file.h>) search system include paths first, used for standard library headers. Double quotes ("file.h") search the current directory first, then system paths — use for your own headers.

Q:How do I define macros from the command line?

A: Use the -D flag with gcc:gcc -DDEBUG -DVERSION=2 file.c. This is equivalent to having#define DEBUG and #define VERSION 2at the top of your file. Very useful for build configurations.

Q:What are LINE and FILE used for?

A: These predefined macros expand to the current line number and filename. They're invaluable for debugging and logging:printf("Error at %s:%d", __FILE__, __LINE__);tells you exactly where an error occurred.

08Summary

What You Learned:

✓#include: Include header files (use <> for standard, "" for your own)
✓#define: Create constants and macro functions
✓Conditional: #ifdef, #ifndef, #if, #elif, #else, #endif
✓Header Guards: Prevent multiple inclusion with #ifndef/#define/#endif
✓Best Practices: Always use parentheses in macros, no semicolons in #define

09Advanced Preprocessor Techniques

Stringification (#)

The # operator converts a macro argument to a string literal.#define PRINT_VAR(x) printf(#x " = %d\n", x)turns PRINT_VAR(count) intoprintf("count" " = %d\n", count). Useful for debugging macros.

Token Pasting (##)

The ## operator concatenates tokens.#define CONCAT(a, b) a##b makesCONCAT(my, Var) become myVar. This is powerful for generating function names or variable prefixes programmatically.

Variadic Macros

C99 introduced variadic macros with ... and __VA_ARGS__. Example: #define DEBUG(...) printf(__VA_ARGS__) accepts any number of arguments. This is commonly used for logging macros that wrap printf.

Detecting Compiler Features

Use predefined macros to detect the environment: __GNUC__ for GCC,_MSC_VER for MSVC, __STDC_VERSION__ for C standard version. This enables writing portable code that adapts to different compilers and standards.

←Previous: UNIX/Linux System Calls Next: Recursion→

10When to Use the Preprocessor

Good Uses vs Alternatives

Use the preprocessor for header guards, conditional compilation, and platform detection. For constants, prefer const or enum over #define when possible—they provide type checking. For inline code, use inline functions over macros for type safety.

01What is the Preprocessor?

Definition

The C Preprocessor is a tool that processes your code before compilation. It handles directives that start with # - like including files, defining constants, and conditional compilation.

Compilation Process

Source Code

.c file

→

Preprocessor

#include, #define

→

Compiler

Translation

→

Executable

.exe / a.out

Key Preprocessor Directives

• #include - Include header files
• #define - Define macros and constants
• #ifdef / #ifndef / #endif - Conditional compilation
• #pragma - Compiler-specific instructions

What's Really Happening?

The preprocessor is like a smart text editor that runs BEFORE the compiler sees your code:

#include: Literally copy-pastes the entire header file into your code. If you include 10 files, your code grows by 10 files worth!

#define: Find-and-replace. Every time it sees PI, it replaces with 3.14159. NO memory is used!

#ifdef: Includes or excludes entire blocks of code based on conditions. Different code for Windows vs Linux!

See it yourself: Run gcc -E file.c to see the preprocessed output!

02#include Directive

The #include directive tells the preprocessor to copy the contents of another file into your code. There are two forms:

<angle brackets>

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

Used for standard library headers. Searches system include directories.

"double quotes"

main.c

#include "myheader.h"
#include "utils/helper.h"
#include "../config.h"

Used for your own headers. Searches current directory first.

Creating Your Own Header File

mymath.h

1// mymath.h - Header file (declarations)
2#ifndef MYMATH_H    // Header guard - prevents double inclusion
3#define MYMATH_H
4
5// Function declarations (prototypes)
6int add(int a, int b);
7int subtract(int a, int b);
8int multiply(int a, int b);
9
10#endif

mymath.c

1// mymath.c - Implementation file (definitions)
2#include "mymath.h"
3
4int add(int a, int b) {
5    return a + b;
6}
7
8int subtract(int a, int b) {
9    return a - b;
10}
11
12int multiply(int a, int b) {
13    return a * b;
14}

main.c

1// main.c - Using the header
2#include <stdio.h>
3#include "mymath.h"
4
5int main() {
6    printf("5 + 3 = %d\n", add(5, 3));
7    printf("5 - 3 = %d\n", subtract(5, 3));
8    printf("5 * 3 = %d\n", multiply(5, 3));
9    return 0;
10}

03#define Macros

The #define directive creates macros - text replacements done by the preprocessor before compilation.

Simple Constants

constants.c

1#include <stdio.h>
2
3// Define constants
4#define PI 3.14159
5#define MAX_SIZE 100
6#define GREETING "Hello, World!"
7
8int main() {
9    double area = PI * 5 * 5;     // PI replaced with 3.14159
10    int arr[MAX_SIZE];             // MAX_SIZE replaced with 100
11    printf("%s\n", GREETING);     // GREETING replaced with string
12    return 0;
13}

Macros with Parameters

macro_functions.c

1#include <stdio.h>
2
3// Macro functions (no function call overhead)
4#define SQUARE(x) ((x) * (x))
5#define MAX(a, b) ((a) > (b) ? (a) : (b))
6#define MIN(a, b) ((a) < (b) ? (a) : (b))
7#define ABS(x) ((x) < 0 ? -(x) : (x))
8
9int main() {
10    int n = 5;
11    printf("Square of %d: %d\n", n, SQUARE(n));   // Output: 25
12    printf("Max(3, 7): %d\n", MAX(3, 7));         // Output: 7
13    printf("Min(3, 7): %d\n", MIN(3, 7));         // Output: 3
14    printf("Abs(-10): %d\n", ABS(-10));           // Output: 10
15    return 0;
16}

Macro Pitfall: Always Use Parentheses!

main.c

// BAD - No parentheses
#define SQUARE_BAD(x) x * x
// Problem: SQUARE_BAD(3+2) expands to 3+2 * 3+2 = 3+6+2 = 11 (WRONG!)
// GOOD - With parentheses
#define SQUARE_GOOD(x) ((x) * (x))
// SQUARE_GOOD(3+2) expands to ((3+2) * (3+2)) = 5 * 5 = 25 (CORRECT!)

#undef - Undefine a Macro

main.c

#define DEBUG 1
// ... code that uses DEBUG ...
#undef DEBUG    // Now DEBUG is undefined
// DEBUG no longer exists here

04Conditional Compilation

Conditionally include or exclude code based on defined macros. Useful for debugging, platform-specific code, and feature flags.

#ifdef / #ifndef / #endif

debug_example.c

1#include <stdio.h>
2
3#define DEBUG    // Comment this line to disable debug mode
4
5int main() {
6    int x = 10;
7    
8    #ifdef DEBUG
9        printf("[DEBUG] x = %d\n", x);    // Only compiled if DEBUG is defined
10    #endif
11    
12    printf("Result: %d\n", x * 2);
13    
14    #ifndef RELEASE
15        printf("[DEV] Development build\n");  // Only if RELEASE is NOT defined
16    #endif
17    
18    return 0;
19}

#if / #elif / #else / #endif

version_check.c

1#include <stdio.h>
2
3#define VERSION 2
4
5int main() {
6    #if VERSION == 1
7        printf("Running Version 1\n");
8    #elif VERSION == 2
9        printf("Running Version 2\n");
10    #else
11        printf("Unknown Version\n");
12    #endif
13    
14    #if defined(DEBUG) && VERSION >= 2
15        printf("Debug mode for V2+\n");
16    #endif
17    
18    return 0;
19}

Platform-Specific Code

platform.c

1#include <stdio.h>
2
3void clearScreen() {
4    #ifdef _WIN32
5        system("cls");       // Windows
6    #else
7        system("clear");     // Linux/Mac
8    #endif
9}
10
11int main() {
12    #if defined(_WIN32) || defined(_WIN64)
13        printf("Running on Windows\n");
14    #elif defined(__linux__)
15        printf("Running on Linux\n");
16    #elif defined(__APPLE__)
17        printf("Running on macOS\n");
18    #else
19        printf("Unknown platform\n");
20    #endif
21    
22    return 0;
23}

05Header Guards

Header guards prevent a header file from being included multiple times, which would cause "redefinition" errors.

Without Header Guard

main.c

// myheader.h
struct Point {
    int x, y;
};
// If included twice:
// ERROR: redefinition of 'struct Point'

✓With Header Guard

main.c

// myheader.h
#ifndef MYHEADER_H
#define MYHEADER_H
struct Point {
    int x, y;
};
#endif  // MYHEADER_H

Modern Alternative: #pragma once

main.c

// myheader.h
#pragma once    // Non-standard but widely supported
struct Point {
    int x, y;
};

#pragma once is simpler but not part of the C standard. Traditional header guards are more portable.

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Mistake 1: Semicolon after #define

main.c

// WRONG
#define MAX 100;    // Semicolon included in replacement!
int arr[MAX];       // Becomes: int arr[100;]; - ERROR!
// CORRECT
#define MAX 100
int arr[MAX];       // Becomes: int arr[100];

Mistake 2: Space in macro name

main.c

// WRONG
#define MAX VALUE 100    // 'MAX' is defined as 'VALUE 100'
// CORRECT
#define MAX_VALUE 100

Mistake 3: Missing parentheses in macro

main.c

// WRONG
#define DOUBLE(x) x + x
int result = DOUBLE(5) * 2;  // 5 + 5 * 2 = 5 + 10 = 15 (WRONG!)
// CORRECT
#define DOUBLE(x) ((x) + (x))
int result = DOUBLE(5) * 2;  // ((5) + (5)) * 2 = 10 * 2 = 20

Mistake 4: Side effects in macro arguments

main.c

#define SQUARE(x) ((x) * (x))
int a = 5;
int result = SQUARE(a++);  
// Expands to: ((a++) * (a++))
// a is incremented TWICE! Result is undefined behavior.
// Solution: Use inline functions for complex operations

08Frequently Asked Questions

Q:What are predefined macros I can use?

Q:When should I use #define vs const?

Q:Why do I need header guards?

Q:Why wrap macro arguments in parentheses?

A: Macros do text substitution, not evaluation.#define SQ(x) x*x with SQ(1+2) becomes1+2*1+2 = 5, not 9! With parentheses:#define SQ(x) ((x)*(x)) gives ((1+2)*(1+2)) = 9.

Q:What's the difference between <header.h> and "header.h"?

Q:Can I see what the preprocessor outputs?

A: Yes! Use gcc -E file.c to output the preprocessed code without compiling. This shows all macro expansions and included files — invaluable for debugging macro issues.

Q:What is the difference between #include <file> and #include "file"?

Q:How do I define macros from the command line?

A: Use the -D flag with gcc:gcc -DDEBUG -DVERSION=2 file.c. This is equivalent to having#define DEBUG and #define VERSION 2at the top of your file. Very useful for build configurations.

Q:What are LINE and FILE used for?

08Summary

What You Learned:

✓#include: Include header files (use <> for standard, "" for your own)
✓#define: Create constants and macro functions
✓Conditional: #ifdef, #ifndef, #if, #elif, #else, #endif
✓Header Guards: Prevent multiple inclusion with #ifndef/#define/#endif
✓Best Practices: Always use parentheses in macros, no semicolons in #define

09Advanced Preprocessor Techniques

Stringification (#)

Token Pasting (##)

The ## operator concatenates tokens.#define CONCAT(a, b) a##b makesCONCAT(my, Var) become myVar. This is powerful for generating function names or variable prefixes programmatically.

What You Will Learn

01What is the Preprocessor?

Definition

Compilation Process

Key Preprocessor Directives

What's Really Happening?

02#include Directive

<angle brackets>

"double quotes"

Creating Your Own Header File

03#define Macros

Simple Constants

Macros with Parameters

Macro Pitfall: Always Use Parentheses!

#undef - Undefine a Macro

04Conditional Compilation

#ifdef / #ifndef / #endif

#if / #elif / #else / #endif

Platform-Specific Code

05Header Guards

Without Header Guard

✓With Header Guard

Modern Alternative: #pragma once

!Code Pitfalls: Common Mistakes & What to Watch For

Mistake 1: Semicolon after #define

Mistake 2: Space in macro name

Mistake 3: Missing parentheses in macro

Mistake 4: Side effects in macro arguments

08Frequently Asked Questions

Q:What are predefined macros I can use?

Q:When should I use #define vs const?

Q:Why do I need header guards?

Q:Why wrap macro arguments in parentheses?

Q:What's the difference between <header.h> and "header.h"?

Q:Can I see what the preprocessor outputs?

Q:What is the difference between #include <file> and #include "file"?

Q:How do I define macros from the command line?

Q:What are __LINE__ and __FILE__ used for?

08Summary

What You Learned:

09Advanced Preprocessor Techniques

Stringification (#)

Token Pasting (##)

Variadic Macros

Detecting Compiler Features

10When to Use the Preprocessor

Good Uses vs Alternatives

Test Your Knowledge

Related Tutorials

Header Files in C

Multi-File Programs in C

C11 Standard Features

Have Feedback?

What You Will Learn

01What is the Preprocessor?

Definition

Compilation Process

Key Preprocessor Directives

What's Really Happening?

02#include Directive

<angle brackets>

"double quotes"

Creating Your Own Header File

03#define Macros

Simple Constants

Macros with Parameters

Macro Pitfall: Always Use Parentheses!

#undef - Undefine a Macro

04Conditional Compilation

#ifdef / #ifndef / #endif

#if / #elif / #else / #endif

Platform-Specific Code

05Header Guards

Without Header Guard

✓With Header Guard

Modern Alternative: #pragma once

!Code Pitfalls: Common Mistakes & What to Watch For

Mistake 1: Semicolon after #define

Mistake 2: Space in macro name

Mistake 3: Missing parentheses in macro

Q:What are LINE and FILE used for?

Q:What are LINE and FILE used for?