Conditional inclusion

 

C

Language

headers

Type support

Dynamic memory management

Error handling

Program utilities

Variadic function support

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

Technical Specifications

 

C language

ASCII chart

Basic concepts

translation phases

comment

identifier

scope

lifetime

name space

type

arithmetic types

object

main function

memory model

Keywords

Preprocessor

Statements

Expressions

Initialization

Declarations

Functions

Miscellaneous

history of C

Technical Specifications

 

Preprocessor

#if#ifdef#ifndef#else#elif#endif

#define#undef#,## operators

#include

#error

#pragma

#line

 

The preprocessor supports conditional compilation of parts of a source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef and #endif directives.

Syntax

#if expression
#ifdef expression
#ifndef expression
#elif expression
#else
#endif

Explanation

The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif directives, then optionally includes at most one #else directive and is terminated with the #endif directive. Any inner conditional preprocessing blocks are processed separately.

Each of #if, #elif, #else, #ifdef and #ifndef directives control a code block until the first #elif, #else, #endif directive not belonging to any inner conditional preprocessing blocks.

#if, #ifdef and #ifndef directives test the specified condition (see below), and if it evaluates to true, compiles the controlled code block. In that case subsequent #else and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else or #elif directive (if any) is processed. In the former case, the code block controlled by the #else directive is unconditionally compiled. In the latter case, the #elif directive acts as if it were a #if directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent #elif and #else directives. The conditional preprocessing block is terminated by the #endif directive.

Conditional evaluation

#if, #elif

The expression is a constant expression, using only literals and identifiers, defined using #define directive. Any identifier, which is not literal, non defined using #define directive, evaluates to 0.

The expression may contain unary operators in form defined identifier or defined (identifier) which return 1 if the identifier was defined using #define directive and 0 otherwise. If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with ​0​.

#ifdef, #ifndef

Checks if the identifier was defined using #define directive.

#ifdef identifier is essentially equivalent to #if defined( identifier).

#ifndef identifier is essentially equivalent to #if !defined( identifier).

Example

Run this code

#define ABCD 2
#include <stdio.h>
 
int main(void)
{
 
#ifdef ABCD
    printf("1: yes\n");
#else
    printf("1: no\n");
#endif
 
#ifndef ABCD
    printf("2: no1\n");
#elif ABCD == 2
    printf("2: yes\n");
#else
    printf("2: no2\n");
#endif
 
#if !defined(DCBA) && (ABCD < 2*4-3)
    printf("3: yes\n");
#endif
}

Output:

1: yes
2: yes
3: yes

References

• C11 standard (ISO/IEC 9899:2011):

• 6.10.1 Conditional inclusion (p: 162-164)

• C99 standard (ISO/IEC 9899:1999):

• 6.10.1 Conditional inclusion (p: 147-149)

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

• 3.8.1 Conditional inclusion

See also

C++ documentation for Conditional inclusion