`#define` Statement in C

The #define directive is a preprocessor command that tells the C preprocessor to replace a name with a value (or code) everywhere it appears in the program — before compilation begins. It is the most basic form of a macro in C.

Syntax

#define MACRO_NAME replacement_text

MACRO_NAME — by convention written in UPPERCASE
replacement_text — the value or code that replaces the name
No semicolon (;) at the end

Symbolic Constants

The most common use: replacing magic numbers with readable names.

#include <stdio.h>
#define PI       3.14159
#define MAX_SIZE 100
#define GREETING "Hello, World!"

int main() {
    float r = 5.0;
    float area = PI * r * r;

    printf("Area = %.2f\n", area);
    printf("%s\n", GREETING);

    return 0;
}

Output:

Area = 78.54
Hello, World!

The preprocessor replaces every occurrence of PI with 3.14159 before the compiler sees the code.

Object-like Macros

#define TRUE  1
#define FALSE 0
#define NULL  0
#define NEWLINE '\n'

Undefining a Macro

You can remove a macro definition mid-file using #undef:

#define LIMIT 50
// ... use LIMIT ...
#undef LIMIT
// LIMIT no longer exists below this line

`#define` vs `const`

	`#define`	`const`
Handled by	Preprocessor	Compiler
Type checking	None	Yes
Memory	None allocated	Allocated
Scope	File-wide only	Follows block rules
Debugging	Harder	Easier

Prefer const for typed constants. Use #define for compile-time substitutions like include guards.

Fundamentals

Functions

Array

String

Structure

Union

Pointer

File Handling

Preprocessor

#define Statement

`#define` Statement in C

Syntax

Symbolic Constants

Object-like Macros

Undefining a Macro

`#define` vs `const`

​#define Statement in C

​Syntax

​Symbolic Constants

​Object-like Macros

​Undefining a Macro

​#define vs const

`#define` Statement in C

Syntax

Symbolic Constants

Object-like Macros

Undefining a Macro

`#define` vs `const`