#ifdef in C

 The #ifdef directive in C is used for conditional compilation.

 It checks whether a particular macro is defined.

 If the macro is defined, the code between #ifdef and #endif is included in the compilation; otherwise, it is skipped.

Syntax of #ifdef in C 

#ifdef macro_name
// code to be compiled if macro_name is defined
#endif

Example:

#include<stdio.h>

#define DEBUG 1

int main() {
    
#ifdef DEBUG
    printf("Debugging is enabled.\n");
#endif

    printf("This line is always printed.\n");

    return 0;
}
		

Output:

Debugging is enabled.
This line is always printed.

In this example:

 The DEBUG macro is defined with a value of 1.

 The #ifdef DEBUG directive checks if DEBUG is defined.

 If DEBUG is defined, the message "Debugging is enabled. " is printed.

 The message "This line is always printed." is printed regardless of whether DEBUG is defined or not.

Uses of ifdef in C 

 Debugging: It's commonly used to enable or disable debugging statements in the code.

#ifdef DEBUG
    printf("Debugging message: variable value = %d\n", variable);
#endif

 Platform-specific code: To write platform-specific code that is only compiled for certain platforms.

#ifdef __linux__
    // Linux-specific code
#endif

 Feature toggles: To enable or disable certain features of the program based on configuration.

#if,def FEATURE_A
    // Code related to Feature A
#endif

 By using #ifdef, you can write more flexible and maintainable code that can be customized for different scenarios or platforms.