call by value, reference, name...

**magic101** · 05-05-2008

hey everyone.. i am new to C and need your help with telling me the differences between call by value, reference name and copy-restore...

right now i am trying to figure out what the outputs for this code are gonna be based on those cases

Code:

#include <stdio.h>

      int a;

      void f(int x, int y) {
        x += a;
        a += 2*y;
      }

      int main() {
        a = 3;
        f(a, 4);
        printf("%d\n", a);
        return 0;
      }

i figure out that by reference the output of this code will be 14... but what about by value? name? and copy-restore??

any help is appreciated .. thanks

**magic101** · 05-05-2008

is call by value answer 11? reference 14?

i am not sure about name and the other one still... please i am sure people reading this thread can help...

**robwhit** · 05-05-2008

Your example uses call by value. There is no call by reference in that code.

**cpjust** · 05-05-2008

I think you mean "Pass by reference". Which means passing pointers to variables instead of the values.

Code:

#include <stdio.h>

      int a;

      void f(int* x, int y) {
        *x += a;
        a += 2*y;
      }

      int main() {
        a = 3;
        f( &a, 4);
        printf("&#37;d\n", a);
        return 0;
      }

I've never heard of "copy-restore", and I'm not sure what you mean by "reference name"?

**magic101** · 05-05-2008

Originally Posted by robwhit

Your example uses call by value. There is no call by reference in that code.

ok... i know that the symentics of the code are by value.... soo the answer is 11...
if this code would be rewritten to be by reference then the output would be 14....

how should this code be rewritten to be by name and by copy-restore??

i already understand how it works by value and reference.... i am more concerned about the other two cases....

thanks

**magic101** · 05-05-2008

here i broke it down:

for by value:

Code:

f(int x, int &y){
// x will be 3 as passed argument
x += a;
// now a is added to x so x will be 6
// but now nothing is done with x anymore
a += 2*y;
// a is still 3 so the result is 11
}

for by reference:

Code:

 f(int x, int &y){
// x will be 3 as passed argument
x += a;
// now a is added to x so x will be 6
// but because & is used x is the same as a
// meaning if you change x it will change a
a += 2*y;
// a is now 6 so the result is 14
}

i need help with by name and copy - restore

thank you

**Adak** · 05-05-2008

All parameters in C are passed by copying. The "pass by reference" only exists because a copy of a pointer is still a pointer, which has a valid address.

Call by name, call by copy restore?

Are you talking about how the stack calls can handle parameter name changing, when you call or return from a called function?

**magic101** · 05-05-2008

Originally Posted by Adak

All parameters in C are passed by copying. The "pass by reference" only exists because a copy of a pointer is still a pointer, which has a valid address.

Call by name, call by copy restore?

Are you talking about how the stack calls can handle parameter name changing, when you call or return from a called function?

hi

i am using these considerations:

Copy-restore: A hybrid between call-by-value and call-by-reference.
Call-by-name: The effect is as though the procedure was a macro.
Used in Algol. Can be used for inline expansion of functions.

for example:

Code:

int a;
void unsafe(int x) {
x= 2; a= 0;
}
int main() {
a= 1; unsafe(a); printf("&#37;d\n", a);
}

Under call-by-reference, it prints 0. Under copy-restore, it prints 2.

i still don't understand why 2 there and what the output of function i posted before should be =(

**cpjust** · 05-05-2008

Originally Posted by magic101

Copy-restore: A hybrid between call-by-value and call-by-reference.
Call-by-name: The effect is as though the procedure was a macro.
Used in Algol. Can be used for inline expansion of functions.

I don't think anyone here has any clue what you're talking about.
In C there are only 2 possible ways of passing data to a function: By Value, or By Reference (i.e. pointer).
I never heard of Algol until today. Does anyone still use it?

**robwhit** · 05-05-2008

Originally Posted by cpjust

I think you mean "Pass by reference". Which means passing pointers to variables instead of the values.

Right, I did.

I've never heard of those other things either.

**Elysia** · 05-06-2008

Code:

f(int x, int &y){

This is not valid C.
The & on variables are used for references in C++, but can't be used in C.
This is also why I use the term pass by pointer instead of pass by reference for C.

Code:

void foo(int* x);

Here we can pass by pointer.

And FYI, there are only 3 ways to pass arguments in C and C++:

- By value
- By pointer
- By reference (C++ only)

So I don't know where you're getting the rest or how you're mixing or confusing code.

**laserlight** · 05-06-2008

An example of call by value would be:

Code:

void foo(int v)
{
    /* v's value is a copy of x's value */
    /* ... */
}

/* ... */
int x = 0;
foo(x);

An example of simulated call by reference in C would be:

Code:

void bar(int *p)
{
    /* p refers to x, but pointer notation is used */
    /* ... */
}

/* ... */
int x = 0;
bar(&x);

An example of simulated call by copy-restore in C would be:

Code:

void baz(int *p)
{
    /* copy */
    int *local = malloc(sizeof(int));
    if (local == NULL)
    {
        return;
    }
    *local = *p;

    /* ... */

    /* restore */
    *p = *local;
    free(local);
}

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