C #ifndef Directive Explained

#ifndef is a conditional compilation directive in C language, used to check if a certain identifier has been defined during compilation, and if not, execute the code following the directive. Its syntax form is:

#ifndef identifier
code to be executed
#endif

If the identifier has already been defined, the condition is false, and the code after the conditional compilation directive will be skipped. If the identifier is not defined, the condition is true, and the code after the conditional compilation directive will be executed.

#ifndef is used to prevent repeated definitions, commonly used for header file protection, which can prevent redefinition errors caused by repeated inclusion of header files. When a header file is included multiple times, only the code following the conditional compilation directive will be executed during the first inclusion, and subsequent inclusions will skip this part of the code. This ensures that macro definitions, structure declarations, etc. in the header file will only be compiled once, avoiding redefinition errors.