If I have a code:

typedef struct s_ {
   int a;
   char* b;
} s;

int main()
{
  s* st = malloc(sizeof(s));
  st->b = malloc(20*sizeof(char));
  st->a = 1;
  st->b = "foo";
}

Is it possible here to access data in char array using offset? For example offset here is 4 bytes, I know it and can calculate using for example offsetof() macro, but I can't access data using pointer arithmetics like:

printf("%s", (char*)(st 4));

I would be very happy if someone could help here :)

CodePudding user response：

The answer may be surprising: st 4 actually increments the pointer by 32 bytes!

This is because the type of st is struct s_ * and when you add 4 to that, it is incremented by 4 times the size of the struct.

In order to move by 4 bytes, you need to cast the pointer to char* first and then increment it.

Try this: printf("%s", *(char**)((char*)st 4));

Edit: Added *(char**).

It is needed because by incrementing the pointer, we don't get the beginning of the string, we get the address of the pointer to the beginning of the string. So we need to cast it to the proper type and dereference it.

CodePudding user response：

You can calculate the byte address of the char * element b (which is a char ** value) using (char *)st offsetof(s, b); therefore you can access the string using code like this:

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct s_
{
    int   a;
    char *b;
} s;

int main(void)
{
    s *st = malloc(sizeof(s));
    st->b = malloc(20 * sizeof(char));
    st->a = 1;
    strcpy(st->b, "foo");

    char *str = *(char **)((char *)st   offsetof(s, b));
    printf("[%s]\n", str);
    return 0;
}

The output is a line containing [foo].

Now you know why you don't want to have to do this — let the compiler solve it for you:

printf("[%s]\n", st->b);

This question is getting close to Is it possible to dynamically define a struct in C?

CodePudding user response：

If you use printf("%s", (char*)(st 4));,result have been offset 4*struct s

You want to printf the fourth character，could write like this

char *ptr = null;
ptr = st;
printf("[%s]",ptr);