Pointer to struct

This is a discussion on Pointer to struct within the C Programming forums, part of the General Programming Boards category; Hi Revisiting c programming after oh ages (15 years :-() and writing a pointer to a struct. The program at ...

  1. #1
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    Pointer to struct

    Hi

    Revisiting c programming after oh ages (15 years :-() and writing a pointer to a struct.
    The program at the bottom gives me

    Struct Alpha using values is 9, 7, 2.434000:
    Address of pointer ptr1 is 134510852
    Size of struct is 12
    Values are 9 , 7 and 0.000000
    Using -> notation x is 9
    Using -> notation y is 7
    Using -> notation depth is 2.434000



    However I cannot see for the life of me why
    printf("Values are %d , %d and %f\n",*ptr1, *ptr1, *ptr1);

    gives
    Values are 9 , 7 and 0.000000

    Why does the same reference (right term?) give different results and more specifically the first two values in the instance of my struct ?

    TIA Paul

    Code:
    /* 
     * File:   newmain.c
     * Author: paulj
     *
     * Created on 10 June 2009, 20:11
     */
    
    #include <stdio.h>
    #include <stdlib.h>
    
    int main(int argc, char** argv) {
    
        struct Position {
            int x;
            int y;
            float depth;
        };
    
        struct Position Alpha;
            Alpha.x = 9;
            Alpha.y = 7;
            Alpha.depth = 2.434;
        ;
        struct Position * ptr1;
        ptr1 = &Alpha;
    
        printf("Struct Alpha using values is %d, %d, %f:\n", Alpha.x, Alpha.y, Alpha.depth);
        printf("Address of pointer ptr1 is %d\n",ptr1);
        printf("Size of struct is %d\n",sizeof(Alpha));
        printf("Values are %d , %d and %f\n",*ptr1, *ptr1, *ptr1);
        
      
    
        printf("Using -> notation x is %d\n",ptr1->x);
        printf("Using -> notation y is %d\n",ptr1->y);
        printf("Using -> notation depth is %f\n",ptr1->depth);
    
        return (EXIT_SUCCESS);
    }

  2. #2
    spurious conceit MK27's Avatar
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    Hi Paul. Looks like you have some "brushing up" to do!

    Quote Originally Posted by Paul Johnston View Post
    However I cannot see for the life of me why
    printf("Values are %d , %d and %f\n",*ptr1, *ptr1, *ptr1);

    gives
    Values are 9 , 7 and 0.000000
    Actually, it says "Values are 9 , 9 and 0.000000" (I did test it). That is because *ptr1, *ptr1, and *ptr1 are all the same thing, where as ptr1->x, ptr1->y, and ptr1->depth are three different things.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  3. #3
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    That's because ptr1 is pointing to a compound variable type ie a struct instead of to each of its members.

  4. #4
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    Test

    I retried it on a second machine (Open Solaris 0609 using NetBeans 6.5.1 ) and it does return
    Values are 9 , 7 and 0.0000

    Cutting and pasting into NetBeans on windows does the same as does manually compiling with gcc 3.4.3 on a Unix box.
    Guess I need to look into compound variable types !

    Regards Paul

  5. #5
    Deathray Engineer MacGyver's Avatar
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    Compiling as C90:

    Code:
    : In function `main':
    :24: warning: ISO C90 forbids mixed declarations and code
    :28: warning: int format, pointer arg (arg 2)
    :30: warning: int format, Position arg (arg 2)
    :30: warning: int format, Position arg (arg 3)
    :30: warning: double format, Position arg (arg 4)
    : At top level:
    :11: warning: unused parameter 'argc'
    :11: warning: unused parameter 'argv'
    Compiling as C99:

    Code:
    : In function `main':
    :28: warning: int format, pointer arg (arg 2)
    :30: warning: int format, Position arg (arg 2)
    :30: warning: int format, Position arg (arg 3)
    :30: warning: double format, Position arg (arg 4)
    : At top level:
    :11: warning: unused parameter 'argc'
    :11: warning: unused parameter 'argv'
    In other words, your program is technically broken with your pointer usage. Arguing about what gets printed is kind of pointless. Why are you deferencing a pointer to a struct and giving that to printf as two ints and a float? Are you trying some form of digitial suicide?

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