#if defined

  1. Preprocessor Directive: #if defined
  2. The preprocessor directive #if defined is used to check if a particular macro or identifier has been defined in the program.

  3. It allows conditional compilation based on whether or not a particular macro or identifier has been defined.

  4. Macro or Identifier:

  5. The macro or identifier can be any valid C identifier that we want to check for its definition.

  6. It is usually defined using the #define directive before the #if defined check.

  7. Conditional Compilation:

  8. The #if defined directive evaluates the condition and determines whether the code block following it should be compiled or not.

  9. If the macro or identifier is defined, the code block will be compiled, otherwise it will be skipped.

  10. Code Block:

  11. The code block following the #if defined directive contains the code that will be compiled if the condition is true.

  12. It can include any valid C statements or expressions.

  13. End of Conditional Compilation:

  14. The #endif directive marks the end of the conditional compilation block.
  15. It is used to close the if block and resume normal compilation.

Example:

#define DEBUG_ENABLED

#if defined(DEBUG_ENABLED)
    printf("Debugging is enabled.\n");
    // Additional debug code here
#endif

// Other code here

In the above example, the macro DEBUG_ENABLED is defined using the #define directive. The #if defined(DEBUG_ENABLED) checks if the macro is defined. Since it is defined, the code block inside the #if defined block will be compiled, and the message "Debugging is enabled." will be printed.