C Preprocessor and Macros

C Preprocessor and Macros

Conditional compilation, include guards, predefined macros, macro forms, and the main preprocessor directives in C and C++.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

C Preprocessor and Macros

Core directives

#include
#define
#undef
#if
#ifdef
#ifndef
#elif
#else
#endif
#error
#line
#pragma

Common macro shapes

Object-like macros

#define BUFFER_SIZE 1024

Function-like macros

#define MAX(a, b) ((a) > (b) ? (a) : (b))

Stringification and token pasting

#define STR(x) #x
#define JOIN(a, b) a##b

Predefined macros you will see often

__FILE__
__LINE__
__DATE__
__TIME__
__STDC__
__STDC_VERSION__
__cplusplus in C++ translation units

Include guards and conditional compilation

#ifndef WIDGET_H
#define WIDGET_H

/* declarations */

#endif

Modern C++ code may also use #pragma once, but include guards remain the portable baseline pattern.

Macro risks

arguments may be evaluated more than once
operator precedence surprises are common without parentheses
names are globally substituted before compilation
macros ignore scope and namespaces

When macros are still appropriate

include guards or #pragma once
compile-time feature switches
platform configuration boundaries
very low-level interoperability with C APIs or compiler extensions

Prefer language features when possible

constexpr instead of constant-like macros
inline functions or templates instead of function-like macros
enum or enum class instead of groups of integer macros

Example in practice

int main() {
    // Pick one facility from this reference page.
    // Write the smallest program that exercises its main precondition,
    // complexity rule, or lifetime constraint before scaling up.
    return 0;
}