Thread: Casting from struct to char array and vice versa

Originally Posted by Edelweiss

Hi I want to ask about casting in C.

If I have a struct c in which contains 2 integers a and b,

Code:

typedef struct{
char a;
int b;
}c;

No you don't. You have an int and a char.

Originally Posted by Edelweiss

and I want cast it to a char array, so I did this

Why?

Originally Posted by Edelweiss

Code:

main(){
char* ch;
c c1;
c* pter = (c*)malloc(sizeof(c));
c* revert = (c*)malloc(sizeof(c));

pter = &c1;
pter->a = 5;
pter->b=6;
ch = (char)pter;

revert = (c*)t;

What is that supposed to be?

Originally Posted by Edelweiss

When I tried to revert back to c struct and access the values of a and b I encounter some access reading violation reading location error.
Any ideas what is wrong here? is this the way to do it?

Yeah, you are casting a pointer to a structure to be a character and assigning that to a character pointer. You also have a memory leak. Your compiler should be having a fit.


Quzah.

I don't understand why you make it so complicated, with the use of malloc and all. I might demonstrate with something like:

Code:

#include <stdio.h>

typedef struct {
    char a;
    int b;
} c;

int main(void) {
    c c1 = {'a', 123};
    char *ch;
    c *revert;
    ch = (char*)&c1;
    revert = (c*)ch;

    printf("priting revert c.a is %c\n", revert->a);
    printf("priting revert c.b is %d\n", revert->b);
    return 0;
}

Hmm... I bet you could hide data in a structure's padding by casting back and forth (you could just use a union instead of casting, but I suppose if it's just for the sake of the experience...). I guess it should be legal to hide stuff in the padding. You would be assuming things about how your structure worked, but since unions would legally let you do this anyway, I suppose you should be able to.

Code:

struct foo
{
    char c;
    long long l;
};

Let's say that after c we have padding to align l on the next word boundary. Casting to a character pointer (or sticking this structure in a union containing an array of characters) would let us plug whatever we want into the padding.

At least it seems like you should be able to.


Quzah.

Originally Posted by Edelweiss

this char array will then be sent to another function which will unpack it back to the struct, in which I just cast the struct back on the char array.

Why do you have to go through this intermediate char array?

Originally Posted by quzah

I bet you could hide data in a structure's padding by casting back and forth (you could just use a union instead of casting, but I suppose if it's just for the sake of the experience...). I guess it should be legal to hide stuff in the padding. You would be assuming things about how your structure worked, but since unions would legally let you do this anyway, I suppose you should be able to.

Yes, I agree. Once you are dealing with the object as if it were an array of char, you can modify its bytes as you wish.