Preprocessor Directives

Preprocessor directives must be the first non-whitespace text specified on a line. All directives are prefixed with the pound or number-sign character ('#'). For example:

#pragma PRINT
#include <stdio.h>
#define DEBUG 1

Whitespace is allowed before and after the number-sign ('#'). A number-sign ('#') that appears alone on a line is interpreted as a null preprocessor directive. For example:

# define myfavnum 45
#
   #    include <stdio.h>

The entire proprocessor directive must be contained in a single source line. Line continuations, backslash ('\') followed by a new-line character, may be used in preprocessor directives since these are removed by the preprocessor. For example:

#define mycode                      \ 
        {                           \ 
        volatile unsigned char i;   \ 
        for (i=0; i<100; i++);      \ 
        }

The number-sign ('#') and the preprocessor directive must be explicitly specified and may not come from a macro expansion. For example:

#define mydef #define otherdef 16

mydef

In this case, mydef is expanded to define otherdef 16 since # is interpreted as a stringize operator. This expansion is processed by the compiler and a syntax error is generated.

The following table lists the preprocessor directives and gives a brief description of each.

Directive	Description
#define	Defines a preprocessor macro or constant.
#elif	Initiates an alternative branch of the if condition, when the previous #if, #ifdef, #ifndef, or #elif branch was not taken.
#else	Initiates an alternative branch when the previous #if, #ifdef, or #ifndef branch was not taken.
#endif	Ends a #if, #ifdef, #ifndef, #elif, or #else block.
#error	Outputs an error message defined by the user.
#if	Evaluates an expression for conditional compilation.
#ifdef	Evaluates an expression for conditional compilation. The argument to be evaluated is the name of a definition.
#ifndef	Same as #ifdef but the evaluation succeeds if the definition is not defined.
#include	Reads source text from an external file. The notation sequence determines the search sequence of the included files. The compiler searches for include files specified with less-than/greater-than symbols ('<', '>') in the include file directory. Include files specified with double-quotes (" ") are searched for in the current directory.
#line	Specifies a line number and an optional filename. This specification is used in error messages to identify the error position. For line synchronization with debug information or the listing file, this preprocessor directive must be combined with the NOPREPROCESS Compiler directive.
#message	Outputs a information message defined by the user.
#pragma	Allows you to specify directives that may be included on the compiler command line. Pragmas may contain the same directives that are specified on the command line.
#undef	Deletes a preprocessor macro or constant definition.
#warning	Outputs a warning message defined by the user.