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
#include <stdio.h>
#define DEBUG 1

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

Directive	Description
define	Defines a preprocessor macro or constant.
undef	Deletes a preprocessor macro or constant definition.
include	Reads source text from an external file. The notation sequence determines the search sequence of the included files. C51 searches for include files specified with less-than/greater-than symbols (< >) in the include file directory. C51 searches for include files specified with double-quotes (" ") in the current directory.
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.
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.
line	Specifies a line number together with an optional filename. These specifications are used in error messages to identify the error position.
endif	Ends an if, ifdef, ifndef, elif, or else block.
error	Outputs an error message defined by the user. This directive instructs the compiler to emit the specified error message.
pragma	Allows you to specify control directives that may also be setup in the project compiler options window.