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Cx51 User's Guide
Cx51 Introduction Compiling Programs Language Extensions 
Preprocessor Header Files 
How #include Works Including Files Once Only Macros Simple Macros Complex Macros Macro Operators Stringize (#) Token Pasting (##) Defined Predefined Macros Undefining and Redefining Macro Caveats Preprocessor Directives Conditional Compilation #define #elif #else #endif #error #if #ifdef #ifndef #include #line #message #pragma #undef #warning Advanced Programming 8051 Derivatives Error Messages Library Reference AppendixDefined
The preprocessor defined operator is used in constant expressions to determine if an identifier is defined (by the #define preprocessor directive). If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows:
defined (identifier)
or
defined identifier
The defined operator may be used in #if or #elif directives only.
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