trying to copy struct members to byte array in c

Question

I am attempting to copy the members of a struct containing a mixture of ints, char's and arrays of chars into a byte array to send to a serial line. So far I have

struct msg_on_send
{
    char descriptor_msg[5];
    int  address;
    char space;
    char cmdmsg[5];
    char CR;
    char LF;
};

void switch_output_on()
{
    int member;
    struct msg_on_send SendMsg_on[sizeof member] =
     {

    };
    unsigned char buffer [ sizeof SendMsg_on[0] ];
    showbytes(buffer, serialize(buffer, SendMsg_on));
}

/*************************************************************************** 
*   Function:   ArrayBuild                                                 *
*   Purpose:    Uses memcopy to transfer the struct members sequentially   *
*               into an array of char                                      *
*   Arguments:                                                             *
*   Returns:    size_t i = a count of the number of bytes in the array     *
***************************************************************************/    

size_t ArrayBuild(unsigned char *dst, const struct msg_on_send *object)
{
    size_t i = 0;

    memcpy(&dst[i], &object->descriptor_msg, sizeof object->descriptor_msg);
    i += sizeof object->descriptor_msg;

    memcpy(&dst[i], &object->address, sizeof object->address);
    i += sizeof object->address;

    memcpy(&dst[i], &object->space, sizeof object->space);
    i += sizeof object->space;

    memcpy(&dst[i], &object->cmdmsg, sizeof object->cmdmsg);
    i += sizeof object->cmdmsg;

    memcpy(&dst[i], &object->CR, sizeof object->CR);
    i += sizeof object->CR;

    memcpy(&dst[i], &object->LF, sizeof object->LF);
    i += sizeof object->LF;

    return i;
}

/*********************************************************************** 
*   Function:   USARTWrite                                             *
*   Purpose:    Writes the array data to the USART data register       *
*   Arguments:  void *object = struct member                           *
*               size_t size =  size of array remaining                 *
*   Returns:    None                                                   *
***********************************************************************/

void USARTWrite(const void *object, size_t size)        
{
    const unsigned char *byte;
    for ( byte = object; size--; ++byte )
    {
        printf("%02X", *byte);
    }
    putchar('\n');
}

As I obtained this code, I don't fully understand how it works. I can see that the memcpy takes each element of the struct and makes it into a serial stream indexed by the 'i' variable, but I don't know how the USARTWrite function packetises this into a string, or how to load the array with my struct initialisation.

Sorry this post is a bit long, but I'm just starting this programming lark, and trying to get my head around this concept.

Thanks Dave

EDIT:

wow, many good answers quickly - thanks guys.

slaz: That seems logical to me, I hadn't really thought about that approach as I haven't really got my head around pointers yet, but I am beginning to see that they are an essential part of C, so I duly will have a look.

• This line of code sends the data to my UART, what would I replace the array containing the message contents with? It seems like I am missing a logical step here where I have a variable telling me where my structure starts and how big it is, but no array to send

  USART_SendData(USART1, message_on_contents[array_count]);
  

Harper Shelby: Thank you for that description, it makes it much clearer in my mind.

rgds

Dave

Answer 1

You don't have to actually copy the struct into an array of bytes. You could optionally do this:

struct msg_on_send myMessage;

// code to set myMessage to whatever values...

// get a byte pointer that points to the beginning of the struct    
uint8_t *bytePtr = (uint8_t*)&myMessage;

// pass that into the write function, and it will write the amount of bytes passed in
USARTWrite(bytePtr, sizeof(myMessage));

The power of pointers! :)