Thread: Accessing arrays using pointers

  1. #1
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    Accessing arrays using pointers

    I came across the below code snippet in a project and was not sure how value of variable "response" is computed. Here as we can see, pic_data holds two one dimensional arrays but "response" access both the single dimensional array as two dimensional array. Can anyone please explain how this works?

    Code:
    #define MAX 100
    #define MAXBUF 100
    
    u32 response;
    u32 index;
    
    typedef struct {  
          u16         flag;  
          u16         status;  
    } __attribute__ ((packed)) register;
    
    typedef struct{  
            register      *rq[MAX];  
           u64            buf[MAXBUF];
    }Data;
    
    Data *pic_data;
    
    void getres(Data *pic_data) {   
     response = *((u32*)&(pic_data->rq[index][pic_data->buf[index]]));
    }
    Last edited by pkumarn; 08-30-2013 at 11:28 AM.

  2. #2
    Hurry Slowly vart's Avatar
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    rq is array of pointers

    You do not provide the initialization of pointer but suppose it is pointing to the array of structs reqister then
    Code:
    &(pic_data->rq[index][pic_data->buf[index]])
    will be pointer to some struct in this external array pointed by rq[index]

    This struct contains 2 16 bit unsigned integers, so casting pointer to u32 and then dereferencing pointer will return u32 combined from 2 u16 integers.

    Of course this solution is not portable. So this code is written for one specific architecture in mind. And one specific compiler (based on usage of __attribute__)
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  3. #3
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    Thanks for the quick reply.

    To dig more into it. I understand the part of pic_data->rq[index] typecasts to u32 gives me value of both 'flag' and 'status' but what would this fetch pic_data->buf[index]?

  4. #4
    Hurry Slowly vart's Avatar
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    As I said pq[index] is pointing not to the single struct but to arrays of structs
    So it looks like pic_data->buf[index] is used as index in this external arrays
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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