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peoblem with queues

This is a discussion on peoblem with queues within the C Programming forums, part of the General Programming Boards category; i have a structure with Code: typedef struct queue_s { struct queue_s *q_next; struct queue_s *q_prev; } queue_t; queue_t devices={&devices,&devices}; ...

  1. #1
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    peoblem with queues

    i have a structure with


    Code:
    typedef struct queue_s 
    {
       struct queue_s *q_next;
       struct queue_s *q_prev;
    } queue_t;
    
    
    queue_t devices={&devices,&devices};
    now i am confused , what value will be assigned to the two members of the object devices.

    please help ....

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    Aren't you missing a data element in your structure?

    q_next will be the link to the following element in the queue, q_prev to the preceding element:
    Code:
              ----------       ----------       ----------
              | data   |       | data   |       | data   |
    NULL<-----| q_prev |<------| q_prev |<------| q_prev |
              | q_next |------>| q_next |------>| q_next |------>NULL
              ----------       ----------       ----------
    Bye, Andreas

  3. #3
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    Quote Originally Posted by AndiPersti View Post
    Aren't you missing a data element in your structure?

    q_next will be the link to the following element in the queue, q_prev to the preceding element:
    Code:
              ----------       ----------       ----------
              | data   |       | data   |       | data   |
    NULL<-----| q_prev |<------| q_prev |<------| q_prev |
              | q_next |------>| q_next |------>| q_next |------>NULL
              ----------       ----------       ----------
    Bye, Andreas


    no i don't need a data element in the structure, i know that prev will point to previous element and next to the next element...just wanted to know what will be values of q_next and q_prev in the condition mentioned in the problem...]

    thanks in advance!!

  4. #4
    SAMARAS std10093's Avatar
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    They will point to the structure just constructed i would say

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    i mean...what values will they contain...NULL values??

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    Quote Originally Posted by rinky05 View Post
    no i don't need a data element in the structure, i know that prev will point to previous element and next to the next element...just wanted to know what will be values of q_next and q_prev in the condition mentioned in the problem...]

    thanks in advance!!
    'devices' is a struct of type queue_t, you assign it's own address to the two members *q_next and *q_prev. I guess this is the inital state of your queue, where it's self referential.

    Try adding this after the line that confuses you:

    Code:
    printf("%p %p %p\n", &devices, devices.q_next, devices.q_prev);
    This doesn't mean that you won't need a data element in the struct however, although it's possible to create without one it's pointless since you can not add anything to your queue.
    Last edited by Subsonics; 08-21-2012 at 08:01 AM.
    rinky05 likes this.

  7. #7
    SAMARAS std10093's Avatar
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    Quote Originally Posted by rinky05 View Post
    i mean...what values will they contain...NULL values??
    No.Here is the story.The struct is created,memory is allocated,so the struct exsists.Then the pointers that you have as members of the struct are set to point to this same struct.The struct has been created,so it not NULL.I would suggest you to add that printf subsonics said

  8. #8
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by rinky05 View Post
    now i am confused , what value will be assigned to the two members of the object devices.
    "devices" will be a struct containing two pointers that both point to the "devices" struct, i.e. itself.
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