Assigning Arrays inside a Struct to a new Struct with Arrays

Question

So I have a struct with arrays inside it like so:

struct struct1 {
unsigned char   data1[32];
unsigned char   data2[32];
char   *id;
};

and a second struct defined as

typedef struct
{
   uint8_t id;  
   uint8_t data1[32]; 
   uint8_t data2[32]; 
} struct2;

Struct1 with data already inside it is passed to me via a function like so:

bool func1(struct struct1 * const struct1)

and I need to create a NEW struct2 and pass all the data from struct1 into it.

I thought I could just assign the pointers like so

struct2 *new_struct;

new_struct->id = struct1->id;
new_struct->data1 = struct1->data1;
new_struct->data2 = struct1->data2;

but I guess array pointers in C cannot be changed (or at least that's what I got from reading up on it).

So how do I create a new struct2 and pass the data I need into it from struct1?

Answer 1

array pointers in C cannot be changed

There is no such thing as an "array pointer". Either you have an array, or you have a pointer.

In your case data1 and data2 are arrays, so there is no pointer you could have reassigned. Your only choice is to copy the data stored in the arrays from one struct to the other.

You can use simple assignment (=) between struct variables of the same type, but in your case you have different types, so you need to copy each member separately. The easiest way to do that is to use memcpy (from <string.h>).

#include <string.h>

// ...
struct2 new_struct;

new_struct.id = *struct1->id;
memcpy(new_struct.data1, struct1->data1, sizeof new_struct.data1);
memcpy(new_struct.data2, struct1->data2, sizeof new_struct.data2);

Notes:

• new_struct is not a pointer here. In your example you dereference an uninitialized pointer, which has undefined behavior.
• I dereferenced struct1->id because struct2.id is a single char, not a pointer. I assume this is what you want to happen.