使用指针算术引用结构成员

Question

I've got a struct definition,

struct liste{
    unsigned int size;
    unsigned int capacity;
    char* data;
};

and an instance,

struct liste lst = {3, 4, "hi"};

and what I'm trying to do is get the data member without directly calling lst.data . So far I've been able get a pointer, dataC ;

char* dataC = (char *) ((char *)&lst + 2 * sizeof(unsigned int));

whereby printing dataC and &lst.data as pointers gives the same output. I thought dereferencing dataC and casting the result to a char * would yield a pointer identical lst.data but I get a segfault.

Is there something I'm missing??

Answer 1

According to your code, dataC does not store the address of the data "hi", but the address of the pointer of lst.data. You can see the code below. dataC is the address of lst.data. *dataC is the address of the string "hi", the same as lst.data.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include<stdint.h>
struct liste{
    unsigned int size;
    unsigned int capacity;
    char *data;
};

int main()
{
    
    struct liste lst = {3, 4,"hi"};
    char **dataC = (char **) ((char *)&lst + 2 * sizeof(unsigned int));
    printf("datac = %s\n",(*dataC));

}

Answer 2

Your code is neither portable nor rugged, because there might be struct padding inside the struct. If you need to do something like you attempt here, you should be using the offsetof macro, which is used to get the byte offset of a member inside a struct. Example:

#include <stdio.h>
#include <stddef.h> // offsetof

struct liste{
    unsigned int size;
    unsigned int capacity;
    char* data;
};

int main (void)
{
  struct liste lst = {3, 4, "hi"};
  char** ptrptr = (char**) ((char*)&lst + offsetof(struct liste, data)) ;
  puts(*ptrptr);
}

Notably the (char*)&list part has nothing to do with the data we are looking for being of char type. This is simply a way of iterating through a larger data type byte by byte, which C allows if we use character pointers. We end up with a character pointer pointing at the location of (the pointer) data inside the struct. By casting the result to char** , we make it clear that whatever we are pointing at is a char* .

Similarly, we could get the capacity member like this:

unsigned int** pp_cap = (unsigned int**) ((char*)&lst + offsetof(struct liste, capacity)) ;
printf("%d\n", *pp_cap);