C #ifdef

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

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

#ifdef macro_name
   // Code to compile if macro_name is defined
#endif

• macro_name is the name of the macro that you want to check for definition.

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

#include <stdio.h>

#define DEBUG // Define DEBUG macro

int main() {

    #ifdef DEBUG
        printf("Debug mode is enabled.\n");
    #else
        printf("Debug mode is disabled.\n");
    #endif

    return 0;
}

In this example:

1. The DEBUG macro is defined using #define. This macro serves as a switch to enable or disable debugging code.
2. Inside the program, the #ifdef directive checks whether the DEBUG macro is defined. Since it is defined, the code within the #ifdef block is included in compilation.
3. As a result, “Debug mode is enabled.” is printed when the program is executed.

You can use the #ifdef directive to conditionally include or exclude code based on the presence or absence of specific macros. This is commonly used for enabling or disabling debugging code, selecting different code paths for different platforms, or configuring feature flags in your C programs.