Preprocessor Directives

🔵 C Programming Lesson 15 Advanced

The preprocessor is the first step in the compilation pipeline. It acts as a text substitution tool, parsing all directives starting with the pound (#) symbol.

1 Macros & Header Guards

Common preprocessor directives include:

#define: Creates text substitution macros.
#include: Injects the content of specified header files directly into your source code.
Conditional Compilation (#ifndef, #define, #endif): Prevents compiler errors caused by double function declarations. Often used as **Header Guards** in header files:
```
#ifndef MY_HEADER_H
#define MY_HEADER_H
// declarations go here
#endif
```

2 Declaring Macros and Conditional Compilation

Let's run a program defining macro constants, function macros, and checking conditions:

C — Preprocessor directives ▶ Run Code

#include <stdio.h>

// Macro constant
#define PI 3.14159

// Inline functional macro (parentheses prevent order of operation bugs)
#define SQUARE(x) ((x) * (x))

int main() {
    printf("Value of PI: %f\n", PI);
    printf("Square of (5 + 1): %d\n", SQUARE(5 + 1)); // ((5+1)*(5+1)) = 36

    // Conditional compilation checks
    #ifdef PI
        printf("PI macro is declared!\n");
    #else
        printf("PI macro is not declared!\n");
    #endif

    return 0;
}

3 Code Challenge

Challenge: Define a function macro called `MAX(x, y)` using the ternary conditional operator. Write code inside `main()` to test it by comparing two integer values, and print out the maximum value.