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Address of the function

Discussion in 'C' started by back from retirement, Dec 3, 2008.

  1. back from retirement

    back from retirement New Member

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    I was reading about recursive functions....I found something not well answered in my text books....
    In the recursive function of fibonacci numbers....I used the following code....

    Code:
    int fib(int n)
    {
        if(n==1||n==0)
             return(1);
        else
             return(fib(n-1)+fib(n-2));
    }
    
    So when I was trying to find the value of fib(4), I noticed that fib(4) is calling fib(3) and fib(2). Again fib(3) is calling fib(2) and fib(1). My query is, in case of the two calls, are both fib(2)'s being called from the same function address???
     
    Last edited: Dec 3, 2008
  2. shabbir

    shabbir Administrator Staff Member

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    Whenever any function is called ( inline exceptional ) the {program execution} address and memory location changes with stack pointer change.

    Try looking at the assembly code for this and would see what is happening.
     
  3. xpi0t0s

    xpi0t0s Mentor

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    That's not quite true Shabbir. The function address, i.e. the location of the code, remains constant. Each call to the function gets its own stack frame, whether the function is called from itself or from somewhere else. The two fib(2) calls are made from the same function address, but from different stack frames.

    e.g.
    fib(4) -> frame 1, calls:
    - fib(3) -> frame 2, calls:
    - - fib(2) -> frame 3, calls:
    - - - fib(1) -> frame 4, doesn't call anyone, just returns 1
    - - - fib(0) -> frame 4, doesn't call anyone, just returns 1
    - - fib(1) -> frame 3 (reuse), doesn't call anyone, just returns 1
    - fib(2) -> frame 2, calls:
    - - fib(1) -> frame 3 (reuse), doesn't call anyone, just returns 1
    - - fib(0) -> frame 3 (reuse), doesn't call anyone, just returns 1

    This can be demonstrated by printing the address of n; this won't give the exact stack frame pointer, but it will show what's going on:

    Code:
    int fib(int n)
    {
    	printf("entering fib(n=%d); &n=%p\n",n,&n);
        if(n==1||n==0)
             return(1);
        else
             return(fib(n-1)+fib(n-2));
    }
    
    void go4e_15378()
    {
    	printf("fib4=%d\n",fib(4));
    }
    
    Output (added comments with --):

    entering fib(n=4); &n=0012FDC4
    entering fib(n=3); &n=0012FCEC
    entering fib(n=2); &n=0012FC14 --*
    entering fib(n=1); &n=0012FB3C -- 2 frame 4's
    entering fib(n=0); &n=0012FB3C
    entering fib(n=1); &n=0012FC14 --*
    entering fib(n=2); &n=0012FCEC
    entering fib(n=1); &n=0012FC14 --*
    entering fib(n=0); &n=0012FC14 -- this and *** are frame 3
    fib4=5

    So as you can see from the following two lines extracted from the above output:
    entering fib(n=2); &n=0012FC14
    entering fib(n=2); &n=0012FCEC

    the two fib(2) calls have n at a different place in memory.
     
  4. shabbir

    shabbir Administrator Staff Member

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    By Function address I meant the execution point and not the function address in the memory where it is loaded for execution.

    I have corrected the by adding function execution address as I thought that was misleading.
     
  5. back from retirement

    back from retirement New Member

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    What I wanted was the way xpi0t0s answered it....maybe there is something wrong in the language of my questuion....sorry for that anyway.....
    In my compiler I ran the program designed by xpi0t0s, and it gave me the following output....

    Code:
    entering fib(n=4); &n=10EF:2200
    entering fib(n=3); &n=10EF:21F6
    entering fib(n=2); &n=10EF:21EC
    entering fib(n=1); &n=10EF:21E2
    entering fib(n=0); &n=10EF:21E0
    [b]entering fib(n=1); &n=10EF:21EA[/b]
    entering fib(n=2); &n=10EF:21F4
    [b]entering fib(n=1); &n=10EF:21EA[/b]
    entering fib(n=0); &n=10EF:21E8
    fib4=5
    
    So it definitely comes from the same function address....with different frames....

    Thanks immensely for answering the question.....it was urgently needed..... :D
     
    Last edited: Dec 3, 2008
  6. shabbir

    shabbir Administrator Staff Member

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    yeah some confusion from you and added something from me as well

    But all is well that ends well
     

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