Generally, if we see in programs, variables are defined many times in many programs which requires more data to store these. But using this keyword, U can define only one time and can use it wherever and as many times we want .

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Extern :
All variables we have seen so far have limited scope (the block
in which they are declared) within the program. However, in some
applications it may be useful to have data which is accessible from
within any other file or which remains in existence for the entire
execution of the program.
Suppose if a variable is declared in file1 and we want to access
the variable in another file2 and file3, then how to access these
variables in other files??? Simple, the answer to this is “Extern”.
Extern keyword is used for the global variables that are defined
in one file and can be used those variables in other files by using
extern keyword.
extern int a;
Let us see an example:
file1.c file2.c
#include<stdio.h> #include<stdio.h>
int main() int a=1;
{ int main()
extern int a; {return 0;}
printf("%d",a);
return 0;
}
In this example, the variable ‘a‘ is created and initialized in
file2.c and is used in file1.c as external variable. For information on
how to compile a program whose source code is split among multiple
files, you can simply type the command gcc -o a file1.c file2.c, and
run the program with ./a.
 When you use extern keyword before the global variable
declaration, the compiler understands you want to access a

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variable being defined in another program or file, and hence
not to allocate any memory for this one. Instead, it simply
points to the global variable defined in the other file.
a)
#include <stdio.h>
extern int i; //extern variable
int main()
{
printf("%d",i);
return 0;
}
Output: Compilation error, undefined symbol i.
b)
#include<stdio.h>
extern int i;
int main()
{
i = 1;
return 0;
}
This program throws error in compilation. Because ‘i’ is
declared but not defined anywhere. Essentially, the ‘I’ isn’t
allocated any memory. In (b), the program is trying to change
the value to 1 of a variable that doesn’t exist at all.
 If the global variables are not initialized then, they are
initialized to their default values.
Int – 0 , Float – 0.0, Char - 0 , String – null
#include <stdio.h>
int i; char c; float f; char *str;
int main()
{
printf("%d %d %f %s",c,i,f,str);

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return 0;
}
Output: 0 0 0.000000 (null)
 If we declare a variable as extern, it means that the definition
for that variable is defined somewhere else in the program or
in other file. It doesn’t allocate the memory for these variables.
It’s just the declaration. So we can declare it as many times as
we can.
#include <stdio.h>
extern int i; //Declaring the variable i.
int i=1; //Initializing the variable.
extern int i; //Again declaring the variable i.
int main()
{
extern int i; //Again declaring the variable i.
printf("%d",i);
return 0;
}
Output: 1
 A particular extern variable can be declared many times but we
can initialize at only one time. (even outside or inside the
function of same program)
#include <stdio.h>
extern int i; //Declaring the variable
int i=25; //Initializing the variable
int main()
{
printf("%d",i);
return 0;
}
int i=20; //Initializing the variable
Output: Compilation error: Multiple initialization
variable i.

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 To define a variable i.e. allocate the memory for extern
variables it is necessary to initialize the variables.
#include <stdio.h>
extern int i=1; //extern variable
int main()
{
printf("%d",i);
return 0;
}
Output: 1
 We cannot initialize extern variable locally i.e. within any block
either at the time of declaration or separately. We can only
initialize extern variable globally.
a)
#include <stdio.h>
int main()
{
extern int i=1; // Try to initialize extern variable
// locally.
printf("%d",i);
return 0;
}
Output: Compilation error: Cannot initialize extern
variable.
b)
#include <stdio.h>
int main()
{
extern int i; // Declaration of extern variable i.
int i=1; // Try to locally initialization of
// extern variable i.
printf("%d",i);
return 0;
}
Output: Compilation error: Multiple declaration of
variable i.

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