 Cx51 User's Guide | |
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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 Appendix#ifndef
The #ifndef directive is a special case of the #if directive that tests a name to determine if it is not defined.
#ifndef name
This directive is equivalent to:
#if !defined(name)
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