使用指针访问数组

Question

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?

Note: below code is not full fledged code snippet of a larger code block.

#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]]));
}

Answer 1

That line isn't accessing a 2D array, it's accessing a 1D array of pointers, and then dereferencing the pointer it gets out.

Let's break it down into steps. Starting with:

response = *((u32*)&(pic_data->rq[index][pic_data->buf[index]]));

We can rewrite as:

register *r = pic_data->rq[index]; // figure out which element of 'rq' to use
u64 offset = pic_data->buf[index]; // figure out what offset to use from 'buf'
response = *(u32 *)&r[offset];     // get the right register and extract value
                                   // into a 32-bit word

Editorial note: register is a reserved word, don't use it as a type name. Your function paramater pic_data also shadows the global variable of the same name. Be careful out there!