Preprocessor Directives — #define, #include, Macros

🎯 Learning Objectives

• Use #define for symbolic constants and macros
• Create function-like macros with parameters
• Use conditional compilation (#ifdef, #ifndef)
• Understand the preprocessor execution model

1. Preprocessor Overview

1.1 Key Directives

2. Macros

2.1 Object-like Macros

c
#define PI 3.14159265
#define MAX_BUFFER_SIZE 4096
#define ERROR_MSG "An error occurred"

2.2 Function-like Macros

c
#define SQUARE(x) ((x) * (x))
#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define MIN(a, b) ((a) < (b) ? (a) : (b))

c
#define BAD_SQUARE(x) x * x
int result = BAD_SQUARE(2 + 3);  // 2 + 3 * 2 + 3 = 11 (WRONG!)
// FIX: #define SQUARE(x) ((x) * (x))
// SQUARE(2+3) = ((2+3) * (2+3)) = 25

3. Conditional Compilation

c
#define DEBUG 1
#ifdef DEBUG
    printf("Debug: x = %d, y = %d\n", x, y);
#endif
// Platform-specific code
#ifdef _WIN32
    #include <windows.h>
#elif defined(__linux__)
    #include <unistd.h>
#else
    #error "Unsupported platform"
#endif
// Header guard (in header files)
#ifndef MY_HEADER_H
#define MY_HEADER_H
// Header content...
#endif

4. Common Predefined Macros

5. 📝 Practice Questions

Q1: Write a macro to compute the absolute value.

c
#define ABS(x) ((x) < 0 ? -(x) : (x))

Q2: What is the value of BAD_PRODUCT(3+2, 4+1) if #define BAD_PRODUCT(a,b) a*b?

Answer: 3+2*4+1 = 3+8+1 = 12. The correct version #define PRODUCT(a,b) ((a)*(b)) gives ((3+2)*(4+1)) = 25. Q3: Why are header guards necessary?

Answer: Header guards prevent a header file from being included multiple times in the same translation unit. Without them, multiple definitions would cause compilation errors (e.g., duplicate struct definitions, function declarations). Q4: When should you use a function-like macro instead of a function?

Answer: Macros are useful when: (1) you need to operate on different types (type-generic), (2) you need access to the caller's context (LINE, FILE), (3) the overhead of function call matters (inline expansion). Modern C prefers inline functions over macros. Q5: Explain the output of: #define STR(x) #x printf("%s", STR(hello));

Answer: #x converts the parameter to a string literal. STR(hello) expands to "hello". Output: hello.

6. 🔗 Cross-References

• Week 7 - File I/O: Preprocessor for debugging output
• BSCS4032 (Compiler Design): Preprocessor as first compilation phase Join Discord PreviousFile I/ONextHome