External storage (also called global storage or variables:

in turboc++
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//program to understand global variable .
#include <stdio.h>
#include <stdlib.h>
extern int a=5;// or simply int a //global variable; can be used with keyword extern
float b=4.5;//global variable;can be used with extern keyword
void function();
int main()
{
    printf("value of a and b from main function are=%d and %f\n",a,b);
    function();//function calling
    return 0;
}
void function()//function body line
{

        printf("from user defined function a=%d,b=%f\n",a,b);//printing value
}

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in codeblocks:
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//program to understand global variable .
#include <stdio.h>
#include <stdlib.h>
#include<conio.h>
extern int a=5;// or simply int a //global variable; can be used with keyword extern
float b=4.5;//global variable;can be used with extern keyword
void function();
int main()
{
    printf("value of a and b from main function are=%d and %f\n",a,b);
    function();//function calling
    getch();
    return 0;
}
void function()//function body line
{

        printf("from user defined function a=%d,b=%f\n",a,b);//printing value
}