Programming in C

Question

#include <stdio.h>
void f(int,int);
int main(void){
    int a = 5;
    f(a,a);
    printf("%d",a);
}
void f(int x, int y){
    x--;
    y=y+15;
}

If copy-restore is used, then what will be the value of print(a)?

Answer 1

on calling f() new activation record pushed and its local variables x and y get value 5 each. now x-- will make x 4 and y=y+15 make y 20 but there will be no effect on a ie actual variable so answer would be 5

Comments

We are not calling the function by call by value here (even though that's how C calls functions) but via copy-restore mechanism. Assuming that, what is the answer?

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ok in copy restore mechanism it just work like call by refrence where the actual parameter change in real time as when we change formal parameter effect occur at actual but the only difference in copy restore is the effect does not occur in real time instead when the called function completes

so answer would be 20 as 20 be the last to be copied to variable a

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That's what I thought as well. But the answer from where I got this question is saying either 4 or 20 which I don't think is correct.

Answer 2

The answer would be either 4 or 20.
Let me tell you why basically Call by Copy/Restore is a special case of call-by-reference where the provided reference is unique to the caller. The final result of the referenced values will not be saved until the end of the function

The actual implementation of above program would be :

#include <stdio.h>
void f(int x, int y){
    x--;
    y=y+15;
}

int main(void)
{
    int a = 5;
    {
    int cr_0000 = a;    // Copy for first argument
    int cr_0001 = a;    // Copy for second argument
    incr(&cr_0000, &cr_0001);
    a = cr_0001;        // Restore for second argument
    a = cr_0000;        // Restore for first argument
    }
    printf("%d\n", a);
    return 0;
}

Now as you can see it depends on programmer which value he/she pick and thus answer would change accordingly.

Reference :https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_copy-restore

Answer 3

answer is 4 or 20

Comments

Well if y is calculated at the last step before stepping out of function, why are we also using x value?

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becoz they r calling  f(x,y) ,,,