how to do a copy of data from one structure pointer to another structure member

Question

I am passing the address of a structure to the function.

void validate(void *ptr)
{
    // A variable of type Msgmt Structure
    Msgmt msg;
    memset(&msg, 0, MSG_SIZE);

    // Type casting the void pointer to this structure dev_t
    dev_t *elem = (dev_t*)ptr;
    msg.msg_id = 10;  
    if (elem->devCategory == VAL){  
        // This statement crashes the code
        // memcpy(msg.messageData.value.data, elem->data, LEN);

        // This statement goes through 
        memcpy(&msg.messageData.value.data,elem->data,LEN) ; 
    }

    // ... do something
}

messageData is a union within struct msg and value is a member of union and data is an array of unsigned char.

how should I do a memcpy of elem->data to the msg.messageData.value.data ? ( elem->data is an array of unsigned char )

thanks !!!

Answer 1

The statement crashes, because the pointer is NULL.

Here you clear the members.

    memset(&msg, 0, MSG_SIZE);

    // Type casting the void pointer to this structure dev_t
    dev_t *elem = (dev_t*)ptr;

Here is the only thing that you initiliaze something.

    msg.msg_id = 10;  
    if (elem->devCategory == VAL){  

And here you access the NULL pointer, which luckily for you crashes.

        // This statement crashes the code
        // memcpy(msg.messageData.value.data, elem->data, LEN);

And here you will corrupt other memory most likely, without crashing.

        // This statement goes through 
        memcpy(&msg.messageData.value.data,elem->data,LEN) ; 
    }

    // ... do something
}

You must assign msg.messageData.value.data some defined memory, before you can use it as a target for memcp .