GATE CSE 1999 | Question: 15

Question

What will be the output of the following program assuming that parameter passing is

1. call by value

2. call by reference

3. call by copy restore

procedure P{x, y, z}; 
begin 
    y:y+1; 
    z: x+x; 
end; 
begin 
    a:= b:= 3; 
    P(a+b, a, a); 
    Print(a); 
end  

Answer 1

• Call by Value : 3
• Call by Reference : 12
• Call by Copy-Restore : 12

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The $3$ parameter passing mechanisms are simulated in the following 'C' codes. 

PS: C language only supports Call by Value and even in the case of pointers, the value of the pointer is getting passed explicitly in a pointer variable. This is different from call by reference (say in C++) where this happens implicitly. The following code for Call-by-Reference and Call-by-copy-restore is just a simulation of the parameter passing behaviour and their implementation in any language need not be the same.

1. Call by Value

   #include <stdio.h> 
   int foo(int x,int y,int z)
   { 
       y = y+1; 
       z = x+x; 
   } 
   int main(void)
   { 
       int a = 3; 
       int b = 3; 
       foo(a+b,a,a); 
       printf("%d",a); 
       return 0; 
   }  
   

2. Call by Reference (Call by reference is simulated by passing address in C) :

   #include <stdio.h>
   int foo(int *x,int *y,int *z)
   { 
       *y = *y+1; 
       *z = *x+*x;
   } 
   int main(void) 
   { 
       int a = 3; 
       int b = 3; 
       int c = a+b; 
       foo(&c,&a,&a); 
       printf("%d",a); 
       return 0; 
   }   
   

3. Call by Copy-Restore:

   #include <stdio.h> 
   void foo(int *x,int *y,int *z)
   { 
       *y = *y+1; 
       *z = *x+*x;
   } 
   int main(void)
   { 
       int a=3; 
       int b=3; 
       int c=a+b; 
       int d,e;
       d = c;//copy 
       e = a;//copy 
       foo(&d,&e,&e); 
       a = e;//restore 
       c = d;//restore 
       printf("%d",a); 
       return 0;
   }

Comments

call by reference function is like macro function in C. answer should be 14. please check and update

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@Aditya There seems to be two issues with your approach:-

1. In copy-restore, you seem to be restoring the value of the expression \(a+b\). To the best of my knowledge, copy-restore only restores values for variables/arguments/expressions that have an associated l-value. \(a+b\) doesn’t seem to have an inherent l-value, right?
2. How do you know the order of restoring the values for variables/arguments/expressions is left-to-right? \(12\) seems to be the answer only when the last variable to be restored is \(z\), preceded by \(y\). If we were to restore \(z\), followed by \(y\), then \(a\) seems to possess a value of \(4\) instead.

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yes,  by reference  a = 14