Conditional Compilation In C: #ifndef #else #endif

In this video tutorial lets learn about preprocessor command or directives like #ifndef, #else and #endif. These directives are used for conditional compilation. #ifndef works completely opposite to #ifdef

Related Read:
Conditional Compilation In C: #ifdef #else #endif

How Does #ifndef Work?

The block of code between #ifndef and #endif works only if the macro name is NOT defined. If the macro name is defined, the compiler will skip the entire block of code inside #ifndef from compiling.

When To Use #ifndef

We’ve explained this in detail for video tutorial: Conditional Compilation In C: #ifdef #else #endif It holds good for this video tutorial too.

Video Tutorial: Conditional Compilation In C: #ifndef #else #endif

Source Code: Conditional Compilation In C: #ifndef #else #endif

#include<stdio.h>

#define iOS

int main()
{
    #ifndef iOS
        printf("I Love Apple Devices\n");
    #else
        printf("Code for Non Apple Devices\n");
    #endif // iOS

    return 0;
}

Output:
Code for Non Apple Devices

#include<stdio.h>


int main()
{
    #ifndef iOS
        printf("I Love Apple Devices\n");
    #else
        printf("Code for Non Apple Devices\n");
    #endif // iOS

    return 0;
}

Output:
I Love Apple Devices

In above programs, if iOS macro name is defined, then the set of code present inside #else block gets compiled and executed. If macro name iOS is not defined, then the code inside #ifndef block gets compiled and executed.

Note:

1. Using #else is optional. You can just use #ifndef and #endif.

2. Working of #ifndef is opposite to that of #ifdef.

3. This is similar to if else condition, but here the block of code which do not match the criteria doesn’t even get compiled. It’s treated like regular comments.

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