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cannot initialize structure variables

This is a discussion on cannot initialize structure variables within the C Programming forums, part of the General Programming Boards category; Hi, i have a problem that is rather basic but since i am not a frequent developper this presents a ...

  1. #1
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    cannot initialize structure variables

    Hi,

    i have a problem that is rather basic but since i am not a frequent developper this presents a difficulty to me that i do not understand. So what i have is :


    Code:
    #include <stdio.h>
    #include "w.h"
    
    
    void main(){
    
    WheelCnt *wheel;
    
    wheel->x =0;
    wheel->y = 0;
    
    printf("%d\n",wheel->y);
    
    }
    and w.h that looks like this:


    Code:
    #define MAXWSIZE 200
    
    
    
    typedef struct WheelCnt{   
    	int q[MAXWSIZE];
    	int x;
    	int y ;
    }WheelCnt;
    the error i get when i run the program is :

    Segmentation fault (core dumped)

    and this is because i am trying to do this:

    Code:
    wheel->x =0;
    wheel->y = 0;
    Can someone help


    thank you !!!

  2. #2
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    wheel is just a pointer to a WheelCnt structure, there is no actual structure. The pointer has a garbage value, and you are accessing memory you don't own. That results in undefined behavior, which often manifests as a seg fault.

    You need to declare it as a struct and use the . (dot) operator:
    Code:
    WheelCnt wheel;
    wheel.x = 0;
    or malloc space for it and use the -> operator:
    Code:
    WheelCnt *wheel;
    wheel = malloc(sizeof(*wheel));
    wheel->x = 0;

  3. #3
    Algorithm Dissector iMalc's Avatar
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    You don't have an instance of a structure in that pogram. All you have is a pointer and that pointer points to nowhere. Yes writing through that will cause a crash.
    What are you trying to do? Do you just want a local variable that is a structure, or do you need to dynamically allocate it for some reason?
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  4. #4
    SAMARAS std10093's Avatar
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    Warning!
    Code:
    void main(){  }
    That hurts.If i was the compiler i guess i would cry.
    The right thing to write is this
    Code:
    int main(void)
    {
        ..
        return 0;
    }
    Also about the malloc do not forget to include stdlib.h (malloc is written there).And i would suggest you to write (if trying to using the malloc function)
    Code:
    WheelCnt *wheel;
    wheel = malloc(sizeof(WheelCnt ));
    wheel->x = 0;
    because i think that it is more clear,as when you define a struct ,it's like you have your own type of data (like the language has it's primitive types by default int,char,etc).When you malloc memory you use the sizeof(int) -in a more complex statement usually-,so this is why i suggest it.

  5. #5
    qny
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    The advantage (in my opinion) to using sizeof object rather than sizeof (type) is that you have one less place to change if you decide to use another type later on: the compiler knows the type of the object.

    Code:
    int *x;
    /* ... */
    x = malloc(100 * sizeof *x); // if the type changes, this line can be left alone
    Code:
    double *x;
    /* ... */
    x = malloc(100 * sizeof *x); // if the type changes, this line can be left alone
    Compare with
    Code:
    int *y;
    /* ... */
    y = malloc(100 * sizeof (int)); // if the type changes, this line must be changed too
    Code:
    double *y;
    /* ... */
    y = malloc(100 * sizeof (double)); // if the type changes, this line must be changed too
    Salem and AndiPersti like this.

  6. #6
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    Quote Originally Posted by std10093 View Post
    Also about the malloc do not forget to include stdlib.h (malloc is written there).
    Good call.
    And i would suggest you to write (if trying to using the malloc function)
    Code:
    WheelCnt *wheel;
    wheel = malloc(sizeof(WheelCnt ));
    wheel->x = 0;
    because i think that it is more clear,as when you define a struct ,it's like you have your own type of data (like the language has it's primitive types by default int,char,etc).When you malloc memory you use the sizeof(int) -in a more complex statement usually-,so this is why i suggest it.
    I disagree there, or rather, some people may find it more clear, but it's also more error prone. I prefer the use of *variable_name because it ensures you always malloc the right amount of memory regardless of whether you change the type of variable_name, etc.

    EDIT: Also, I would argue that it is not a terribly complex bit of syntax, really quite simple. variable_name is a pointer, *variable_name is the thing it points to. You want to malloc the size of the the thing it points to. Use of *variable_name is done all the time when returning data to the caller via output parameters to functions.
    Last edited by anduril462; 09-12-2012 at 04:21 PM.

  7. #7
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    Quote Originally Posted by std10093 View Post
    Warning!
    Code:
    void main(){  }
    That hurts.If i was the compiler i guess i would cry.
    It is possible that the OP is writing code for an embedded device, in which case "void main()" is correct. I suspect this might be the case due to the variable names - "WheelCnt" might stand for "Wheel Control," and the int members 'x' and 'y' sound like coordinates - but I might very well be wrong.

  8. #8
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    > That hurts.If i was the compiler i guess i would cry.
    hahahaha xD

    But you should seriously check out this link.

  9. #9
    SAMARAS std10093's Avatar
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    Wonderful things in this post.Bravo to all!

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