copy Array to Member of Struct in C

Question

I'm trying to transmit data wireless. My payload-struct looks like this:

typedef struct
{
   uint8_t data[3];
} data_t

I also have an array in my code:

uint8_t data_to_be_copied[3];

if I try to simply assign each other I get the following error:

msg->data = data_to_be_copied;

incompatible types when assigning to type 'uint8_t[3]' from type 'uint8_t *'

using a for-loop my microcontroller crashes:

for (int i = 0; i < 3; i++) {
  msg->data[i] = data_to_be_copied[i];
}

Same happens, when I try to use memcpy:

memcpy(msg->data, data_to_be_copied, 3);

What am I missing here?

Thanks for your help.

Answer 1

msg->data is an array.

As such, it does not have an l-value, and you cannot change it.

That's why you're getting a compilation error for the first option mentioned above.

For the second and third options mentioned above, you're getting a memory access violation during runtime, probably because your msg variable is not initialized to point to a valid memory address.

Assuming that you have data_t* msg declared in the right place, you still need to initialize it:

data_t* msg = (data_t*)malloc(sizeof(data_t));

You should also make sure that you free(msg) at a later point in the execution of your program.

BTW, you have not specified the entire structure of your program, but a simple data_t msg instance might be sufficient for your needs (in which case, you don't need to use the malloc and the free).

And as mentioned in a comment by @Shahbaz, if you do end up using malloc and free, then you need to make sure that your platform (OS or BSP) supports dynamic memory allocation (AKA heap).

Answer 2

You have some possibilities:

memcpy (msg->data, data_to_be_copied, 3);

or change data_to_be_copied to

data_t data_to_be_copied;

and assign the whole thing: *msg = data_to_be_copied

or if msg has actually more members, declare a struct that has an array of three chars as its only member.

If it crashes, then most likely msg is NULL or some uninitialised pointer.