Pointer to struct

Paul Johnston · 06-10-2009, 03:24 PM

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);
}

MK27 · 06-10-2009, 03:37 PM

Hi Paul. Looks like you have some "brushing up" to do!

Quote:

Originally Posted by Paul Johnston

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.

itCbitC · 06-10-2009, 03:37 PM

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

Paul Johnston · 06-11-2009, 02:46 AM

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

MacGyver · 06-11-2009, 03:01 AM

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?

06-10-2009, 03:24 PM	#1
Paul Johnston Registered User Join Date: Jun 2009 Posts: 2	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 = Α 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); }
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06-10-2009, 03:37 PM	#3
itCbitC Registered User Join Date: Oct 2008 Location: TX Posts: 1,262	That's because ptr1 is pointing to a compound variable type ie a struct instead of to each of its members.
itCbitC is offline

06-11-2009, 02:46 AM	#4
Paul Johnston Registered User Join Date: Jun 2009 Posts: 2	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
Paul Johnston is offline

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