C Preprocessor

What Does the Preprocessor Do?

Before your C program is compiled, the preprocessor:

• Includes header files (#include)
• Defines constants & macros (#define)
• Conditionally compiles code (#ifdef, #ifndef)
• Removes comments
• Expands macros

Preprocessor Directives

Major preprocessor directives include:

• #include – include header files
• #define – define macros/constant values
• #undef – undefine macros
• #ifdef, #ifndef – conditional compilation
• #if, #else, #elif – compile based on conditions
• #pragma – compiler-specific instructions

1. #include Directive

Used to include header files into your program.

• #include <stdio.h> → Standard header
• #include "myfile.h" → User-defined header

Example: #include

#include <stdio.h>

int main() {
    printf("Hello Preprocessor!");
    return 0;
}

2. #define Directive

• Used to create symbolic constants
• No memory is allocated
• Compiler replaces macro everywhere

#define PI 3.14
#define MAX 100

3. Macro with Arguments

#define SQUARE(x) (x * x)

#include <stdio.h>

int main() {
    printf("%d", SQUARE(5));
    return 0;
}

4. Conditional Compilation

Used to compile parts of code depending on conditions.

#include <stdio.h>

#define INDIA

int main() {

#ifdef INDIA
    printf("Indian Version");
#else
    printf("Global Version");
#endif

    return 0;
}

5. #undef Directive

Used to delete a defined macro.

#define RATE 10
#undef RATE

6. #pragma Directive

Used for compiler-specific features.

#pragma startup myStart
#pragma exit myEnd

Practice Questions

1. What is a preprocessor?
2. Explain macro substitution with an example.
3. Differentiate between #include <file.h> and "file.h".
4. What is conditional compilation?
5. What does #pragma do?

Practice Task