C #ifndef

The #ifndef directive is used for conditional compilation. It checks whether a specific macro is not defined, and if the macro is not defined, it includes a block of code for compilation. If the macro is defined, the code within the #ifndef block is excluded from compilation.

Here’s the basic syntax of the #ifndef directive:

#ifndef macro_name
   // Code to compile if macro_name is not defined
#endif

• macro_name is the name of the macro that you want to check for absence (i.e., not being defined).

Here’s an example of how the #ifndef directive is used:

#include <stdio.h>

// Uncomment the next line to disable debug mode
//#undef DEBUG

int main() {
#ifndef DEBUG
    printf("Debug mode is disabled.\n");
#else
    printf("Debug mode is enabled.\n");
#endif

    return 0;
}

In this example:

1. The DEBUG macro is not defined initially. However, you can see that there’s a line //#undef DEBUG, which is commented out. If you uncomment this line, it will undefine the DEBUG macro.
2. Inside the program, the #ifndef directive checks whether the DEBUG macro is not defined. Since it is not defined initially, the code within the #ifndef block is included in compilation.
3. As a result, “Debug mode is disabled.” is printed when the program is executed.

The #ifndef directive is often used for conditional compilation to include or exclude code based on the absence of specific macros. This can be useful for enabling or disabling features, debugging code, or defining different code paths based on configuration options.