Global Variable in C Programming | C Programming

Global Variable in C Programming  

A local variable is one that is declared inside a function and can only be used by that function. If you declare a variable outside all functions then it is a global variable and can be used by all functions in a program.       

    

// Global variables    

int a;   

 int b;     

int Add()    

{      

 return a + b;    

}    

 int main()    

{    

// Local variable        

 a = 5;         

b = 7;        

answer = Add();         

printf("%d\n",answer);       

 return 0;    

}       

Call by Value and Call by reference      

The arguments passed to function can be of two types namely    

1. Values passed / Call by Value    

2. Address passed / Call by reference    

The first type refers to call by value and the second type refers to call by reference.      For instance, consider program 1:             

main()           

{                   

int x=50, y=70;                     

interchange(x,y);                    

 printf(“x=%d y=%d”,x,y);          

 }             

interchange(x1,y1)           

int x1,y1;           

{                   

int z1;                   

z1=x1;                   

x1=y1;                   

y1=z1;   

                  

printf(“x1=%d y1=%d”,x1,y1);           

}       

Here the value to function interchange is passed by value.  

Consider program 2              

main()           

{                   

int x=50, y=70;                     

interchange(&x,&y);                     

printf(“x=%d y=%d”,x,y);           

}             

interchange(x1,y1)           

int *x1,*y1;           

{                   

int z1;                   

z1=*x1;                  

 *x1=*y1;                   

*y1=z1;                     

printf(“*x=%d *y=%d”,x1,y1);          

 }       

Here the function is called by reference. In other words, the address is passed by using symbol &, and the value is accessed by using symbol *.    

The main difference between them can be seen by analyzing the output of program1 and program2.      

The output of program1 that is call by value is   

 x1=70 y1=50     

x=50 y=70       

But the output of program2 that is call by reference is      

*x=70 *y=50     

x=70 y=50       

This is because in the case of call by value the value is passed to a function named interchange and there    the value got interchanged and got printed as    
     

x1=70 y1=50  
    

and again since no values are returned back and therefore original values of x and y as in the main function namely 

x=50 y=70 got printed.