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#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.

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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

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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.

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Object-like Macros

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

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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

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#define vs const

#defineconst
Handled byPreprocessorCompiler
Type checkingNoneYes
MemoryNone allocatedAllocated
ScopeFile-wide onlyFollows block rules
DebuggingHarderEasier
Prefer const for typed constants. Use #define for compile-time substitutions like include guards.