Preprocessor

 

C

Compiler support

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Strings library

Algorithms

Numerics

Date and time utilities

Input/output support

Localization support

Concurrency support (C11)

Technical Specifications

Symbol index

 

C language

Basic concepts

Keywords

Preprocessor

Statements

Expressions

Initialization

Declarations

Functions

Miscellaneous

History of C

Technical Specifications

 

Preprocessor

#if#ifdef#ifndef#else#elif#elifdef#elifndef#endif (C23)(C23)

#define#undef#,## operators

#include__has_include (C23)

#error#warning (C23)

#pragma_Pragma (C99)

#line

#embed__has_embed (C23)(C23)

 

The preprocessor is executed at translation phase 4, before the compilation. The result of preprocessing is a single file which is then passed to the actual compiler.

Directives

The preprocessing directives control the behavior of the preprocessor. Each directive occupies one line and has the following format:

• # character
• preprocessing instruction (one of define, undef, include, if, ifdef, ifndef, else, elif, elifdef, elifndef(since C23), endif, line, embed(since C23), error, warning(since C23), pragma)
• arguments (depends on the instruction)
• line break

The null directive (# followed by a line break) is allowed and has no effect.

Capabilities

The preprocessor has the source file translation capabilities:

• conditionally compile of parts of source file (controlled by directive #if, #ifdef, #ifndef, #else, #elif, #elifdef, #elifndef(since C23) and #endif).
• replace text macros while possibly concatenating or quoting identifiers (controlled by directives #define and #undef, and operators # and ##)
• include other files (controlled by directive #include and checked with __has_include(since C23))
• cause an error or warning(since C23) (controlled by directive #error or #warning respectively(since C23))

The following aspects of the preprocessor can be controlled:

• implementation defined behavior (controlled by directive #pragma and operator _Pragma(since C99))
• file name and line information available to the preprocessor (controlled by directives #line)

References

• C17 standard (ISO/IEC 9899:2018):

• 6.10 Preprocessing directives (p: 117-129)

• C11 standard (ISO/IEC 9899:2011):

• 6.10 Preprocessing directives (p: 160-178)

• C99 standard (ISO/IEC 9899:1999):

• 6.10 Preprocessing directives (p: 145-162)

• C89/C90 standard (ISO/IEC 9899:1990):

• 3.8 PREPROCESSING DIRECTIVES

See also

C documentation for Predefined Macro Symbols

C documentation for Macro Symbol Index

C++ documentation for Preprocessor