The **`#ifndef` directive** is one of the widely used used directives in C. It allows conditional compilations. During the compilation process, the preprocessor is supposed to determine if any provided #key# macros: A macro in computer science is a rule or pattern that specifies how a certain input should be mapped to a replacement output. #key# exist before any subsequent code is included. 



# Syntax

```
#ifndef `macro_definition`

// macro_definition must not be defined if the preprocessor wants to include the source code during compilation. 
```

>The `#ifndef` directive must always be closed by an `#endif` directive, otherwise it will cause an error.

# Code
Example of the use of `#ifndef` in C:



#include <stdio.h>

// we define a variable 
#define xyz 32


int main()
{
   // now we use #ifndef to see if xyz is defined or not
   // in this example, it is defined, we ll move to the 
   // #else part of the code. 
   #ifndef xyz
   printf("Error printing lottery number");
   #else
   printf("Your lottery number is %d", xyz);
   #endif

   return 0;
}

The output: 
```
Your lottery number is 32
```
Now let's remove the `#define xyz 32` statement in the code: 

#include <stdio.h>

int main()
{
   // now we use #ifndef to see if xyz is defined or not
   // in this example, it is not defined, we ll move to the 
   // #ifndef execution of the code. 
   #ifndef xyz
   printf("Error printing lottery number");
   #else
   printf("Your lottery number is %d", xyz);
   #endif

   return 0;
}

What is the #ifndef directive in C?

Used in C programming, the #ifndef directive conditions code inclusion based on macro existence, essential for preventing redundant code.