• C Preprocessor runs before compilation and processes directives that start with #.
• Preprocessor is used for macros, header files, conditional compilation, and code reuse.
• Modular programming means dividing a program into smaller files (modules) using header (.h) and source (.c) files.
• Inline functions reduce function call overhead for small functions.

1) C Preprocessor Directives

• #include : includes header files.
• #define : defines macros/constants.
• #undef : removes a macro definition.
• #if, #elif, #else, #endif : conditional compilation.
• #ifdef, #ifndef : compile code based on macro defined or not.
• #pragma : compiler-specific instructions.

#include <stdio.h>
#define PI 3.14159
#define MSG "Welcome to C"

int main() {
    printf("%s\n", MSG);
    printf("PI = %.5f\n", PI);
    return 0;
}

2) Macros

• Object-like macro: constant replacement.
• Function-like macro: behaves like a function (use parentheses to avoid errors).

#include <stdio.h>

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

int main() {
    int x = 10, y = 25;

    printf("MAX = %d\n", MAX(x, y));
    printf("SQUARE(5) = %d\n", SQUARE(5));

    return 0;
}

3) Conditional Compilation

• Useful to enable/disable debug code and support different platforms.

#include <stdio.h>
#define DEBUG 1

int main() {

#if DEBUG
    printf("Debug mode ON\n");
#else
    printf("Debug mode OFF\n");
#endif

    return 0;
}

4) Header Files

• Header file (.h) contains function prototypes, macros, and declarations.
• Types:
  • System headers: <stdio.h>, <stdlib.h> (use < >)
  • User headers: "myfile.h" (use quotes)
• Use include guards to avoid multiple inclusion.

// mymath.h
#ifndef MYMATH_H
#define MYMATH_H

int add(int a, int b);

#endif

// mymath.c
#include "mymath.h"

int add(int a, int b) {
    return a + b;
}

// main.c
#include <stdio.h>
#include "mymath.h"

int main() {
    printf("Sum = %d\n", add(10, 20));
    return 0;
}

5) Modular Programming Concepts

• Split program into multiple files:
  • .h for declarations (what is available)
  • .c for definitions (how it works)
• Benefits:
  • Easy maintenance and debugging
  • Code reusability
  • Team development becomes easier

6) Inline Functions

• inline suggests compiler to replace function call with function body (for small functions).
• It can improve speed but may increase program size.
• Mostly used in headers for small utility functions.

#include <stdio.h>

static inline int cube(int x) {
    return x * x * x;
}

int main() {
    printf("Cube of 3 = %d\n", cube(3));
    return 0;
}

7) Compilation of Multiple Files

• Compile and link:
  • gcc main.c mymath.c -o app
  • ./app

8) Quick Notes

• Preprocessor directives start with #.
• Macros are replaced before compilation (no type checking).
• Header files store prototypes and declarations.
• Modular programming makes large programs manageable.
• Inline functions can make small repeated operations faster.