Accessing arrays using pointers

[pkumarn]

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?

#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]]));
}

[vart]

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

&(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__)

[pkumarn]

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]?

[vart]

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