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, 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 case of call by value the value is passed to function named as 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 main function namely 
x=50 y=70 got printed.

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