C programming - Use of extern variable

Question

Answer 1

Difference between extern int and int =>

• Extern int A only declares that there is some identifier A which will be used in the program
• int A declares that there is some identifier A which will be used in the program as well as allocates memory for it to store some value.

Here, you are only declaring the identifier A and memory allocation is not there .

When memory is not allocated, then it is a compiler error to initialize the variable .

Comments

@kapil @manojk

I understood your point but please suggest what should be the correction in above program to get output as 20.

Thanks

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Confirm me once that in A allocate the memory as well. Is it like the primitive datatypes are allocated memory while declaring?

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int main()
{
    extern int a;
    printf("%d",a);
    
}
int a = 20;
//

or another way this code produced 20 is 

extern int a= 20;
int main()
{

printf("%d",a);
return 0;
}

or 

extern int a =0 ;
int main()
{
a = 20;
printf("%d",a);
return 0;
}

@Mayankprakash Now it is produced output 20 by defined the variable ‘a’ globally

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@`JEET as i know while declaring variables memory is not allocated but defining variables memory is allocated. for example 

int a; //declaration and no memory is allocated

int a = 10; //definition as well as declaration with init to 10 and memory is allocated

so i have same doubt as yours

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thanks

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It's a Linker Error: Because no memory is allocated for a anywhere in the program (it is never defined as int a; outside the function or in another linked file), the linker will fail to find a memory address for a

Answer 2

It will not compile.

Hence, the answer should be c.

Reason: When a variable is declared extern, then it will be only treated as declared which means no definition is given for that variable which again means that no memory allocation is done. So, this is what extern is used for.

Let me know if you want to understand it in more depth as there is one exception as well.

Comments

Please check out this link. This is the best article written on extern.

I think it will surely suffice your needs.

https://www.geeksforgeeks.org/understanding-extern-keyword-in-c/

And please do let us know if it's still not clear.

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i think by default all the functions is extern but not the variables and variables by default is auto.

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@`JEET

int main()
{
    extern int a = 20;
    printf("%d",a);
    
}
//why this gives error but followig code is not

extern int a =20;
int main()
{
    printf("%d",a);
    
}

//why tis happens