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Passing Parameters via the Stack

Passing Parameters via the Stack

• Parameters are passed in higher order languages (HOL) by:

  • Value

    • A copy of the actual argument's value is made in memory.

    • i.e., The caller and callee have two independent variables with the same value. If the callee modifies the parameter value, the effect is NOT visible to the caller.

  • Reference

    • The address of the passed argument is copied into the formal parameter.

    • Inside the function, the address is used to access the actual argument used in the call.

    • If the callee modifies the parameter value, the effect is visible to the caller.

• Pointers in C

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  int x = 12;       // sets aside one .word for variable x
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  int *y = &x;      // sets aside one .word as a pointer to integer y
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                    // but no allocation for y
  

Functions and Parameters

• Pass-by-value example

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  void swap(int a, int b)
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  {
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      int temp = a;
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      a = b;
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      b = temp;
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  }
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  void main(void)
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  {
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      int x = 2, y = 3;
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      swap(x, y);
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  }
  

  Before making the call to swap the values for x and y are pushed onto the stack and when it returns back to the main it pulls the values back off the stack.

  swap will work only within the stack frame which will eventually go away when swap returns.

• Pass-by-reference example

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  1
  void swap(int *a, int *b)
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  {
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      int temp = *a;
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      *a = *b;
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      *b = temp;
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  }
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  void main(void)
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  {
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      int x = 2, y = 3;
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      swap(&x, &y);
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  }
  

  Before making the call to swap the values for x and y are pushed onto the stack and when it returns back to the main it pulls the values back off the stack.

  The values x and y are swapped because the swapping was done by using their addresses.

Using Recursion

• When you have a programming language that supports recursion, there are a lot of things that have to be accounted for. Some of this is not easy and will cause some really bad problems if not done right.

  • e.g., Dividing a number by 10 using recursion in the function utoa.