20.2 Preprocessing Directives

The compiler accepts several specialized preprocessor directives in addition to the standard directives. All of these are listed in the following table.

Table 20-1. Preprocessor Directives
Directive	Meaning	Example
`#define`	Define preprocessor macro	`#define SIZE 5` `#define FLAG` `#define add(a,b) ((a)+(b))`
`#elif`	Short for `#else #if`	`see #ifdef`
`#else`	Conditionally include source lines	`see #if`
`#endif`	Terminate conditional source inclusion	`see #if`
`#error`	Generate an error message	`#error Size too big`
`#if`	Include source lines if constant expression true	`#if SIZE < 10` `c = process(10)` `#else` `skip();` `#endif`
`#ifdef`	Include source lines if preprocessor symbol defined	`#ifdef FLAG` `do_loop();` `#elif SIZE == 5` `skip_loop();` `#endif`
`#ifndef`	Include source lines if preprocessor symbol not defined	`#ifndef FLAG` `jump();` `#endif`
`#include`	Include text file into source	`#include <stdio.h>` `#include "project.h"`
`#line`	Specify line number and file name for listing	`#line 3 final`
`#pragma`	Compiler-specific options	`#pragma config WDT=OFF`
`#undef`	Undefines preprocessor symbol	`#undef FLAG`
`#warning`	Generate a warning message	`#warning Length not set`

Macro expansion using arguments can use the # character to convert an argument to a string, and the ## sequence to concatenate arguments. If two expressions are being concatenated, consider using two macros in case either expression requires substitution itself, so for example

#define paste1(a,b) a##b

#define paste(a,b) paste1(a,b)

lets you use the paste macro to concatenate two expressions that themselves may require further expansion. Remember that once a macro identifier has been expanded, it will not be expanded again if it appears after concatenation.

For implementation-defined behavior of preprocessing directives, see the 22.13 Preprocessing Directives section.