Call by reference question

I have a question regarding call by reference - essentially using a function i made and calling it in the main function. I'm passing 2 variables (pointers) into it and the value is copied in the corresponding function but the variable itself is not changed in the calling environment (main).

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Code:

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void somefunction(arg1, arg2, arg3, arg4, *ans1, *ans2)
// function does some arithmetic and spits out 2 output values/variables *ans1 and *ans2

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NOT SHOWED is the header file that has the function prototype etc

int main(void)
{
int = arg1, arg2, arg3, *ans1, **ans11;
double = arg4, *ans2; **ans22

ans11 = &ans1
ans22 = &ans2

timediff(arg1, arg2, arg3, arg4, *ans1, *ans2);

printf("answers are %d and %d", ans11, ans22);

return 0;
}

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I didn't copy my program in but made a close generic approximation version of it... hopefully that can help a little...

Thought the best way to do it is try and use a pointer to a pointer...

Any thoughts?

What is your question? You might want to show us the smallest and simplest (compilable) program that demonstrates the problem instead of showing remarkably C-like pseudocode.

You can NEVER change anything inside a function that you haven't got the address of. In other words, if what you pass to a function is not a pointer to what you are trying to modify, you will only modify the variable that is local to the function.

For example:
pass a variable to modify an integer:

Code:

---

int func(int *p)
{
  *p = 7;
}

int main()
{
...
  int x = 3;
  func(&x);
...
}

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Note that the same applies to pointers:

Code:

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void func2(const char **ptr)
{
  if (x % 2)  // odd
  {
      *ptr = "Odd";
  }
  else
  {
      *ptr = "Even";
  }
}

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--
Mats

Quote:

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Originally Posted by matsp

You can NEVER change anything inside a function that you haven't got the address of. In other words, if what you pass to a function is not a pointer to what you are trying to modify, you will only modify the variable that is local to the function.

For example:
pass a variable to modify an integer:

Code:

---

int func(int *p)
{
  *p = 7;
}

int main()
{
...
  int x = 3;
  func(&x);
...
}

---

Note that the same applies to pointers:

Code:

---

void func2(const char **ptr)
{
  if (x % 2)  // odd
  {
      *ptr = "Odd";
  }
  else
  {
      *ptr = "Even";
  }
}

---

--
Mats

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Thank you.

Are you saying that I am on the right track with a pointer to a pointer?

Quote:

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Originally Posted by badtwistofate

Are you saying that I am on the right track with a pointer to a pointer?

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Yes, if your intention is really to change the pointer, not just what it points to.