Thread: maping an unsigned char ofer a structure

My problem is the following. I have an unsigned char array packet[] and the following structures:

Code:

typedef struct set_serv_prop_s
{

	byte serviceType;
	union
	{
		heartbeatCommand_t	*heartbeatCommand;
		announcement_t		*announcement;
	} set_serv_propType;

} set_serv_prop_t;

typedef struct announcement_s
{

	union
	{
		announAllocCommand_t	*announAllocCommand;
		announAllocResponse_t	*announAllocResponse;
		announDownloadCommand_t	*announDownloadCommand;
		announRemCommand_t		*announRemCommand;
		announRemResponse_t		*announRemResponse;
		announMarkRemCommand_t	*announMarkRemCommand;
		announDownAbortCommand_t *announDownAbortCommand;
	} announcementType;

} announcement_t;

typedef struct announAllocCommand_s
{

	byte	encodingLaw;
	uint16	reserved;
	uint32	announcementSize;

} announAllocCommand_t;

the service type field tells what king of package it is.

i try to map it:

Code:

void read_set_serv_prop_from_pkt(set_serv_prop_t **head,
								unsigned char* packet)
{
	int i;

	*head = (set_serv_prop_t*) packet.payload;
	
	for(i=0;i<packet.size;i++)
		printf("&#37;x ",packet.payload[i]);
	printf("\n");
	
	if((*head)->serviceType==0x05)
	{
		announAllocCommand_t *allocCommand = (*head)->set_serv_propType.announcement->announcementType.announAllocCommand;
			
			fprintf(OUTPUT, "\t%d\n",allocCommand->encodingLaw);
			fprintf(OUTPUT, "\t%d\n",allocCommand->reserved);
			fprintf(OUTPUT, "\t%d\n\n", allocCommand->announcementSize);
	}
}

and i get the output:

Code:

5 0 0 0 0 0 fa 0
        49
        12846
        -2147446592

5 - Service Type
0 - Encoding law
0 0 - Reserved
0 0 fa 0 -Announcement size

the output is totally wrong. What is the problem here? and how can I solve it ?

Please help... I'm very pressed by time

Code:

typedef struct announAllocCommand_s
{

	byte	encodingLaw;
	uint16	reserved;
	uint32	announcementSize;

} announAllocCommand_t;

This struct is probaboy ending up 8 bytes long. Is that what you expect?
The reason is:

Code:

byte encodingLaw;  // :@0, 1 byte long. 
uint16 reserved; //: @2, 2 bytes long - 1 byte alignment inserted before this. 
uint32 announcementSize;  //: @4 - 4 bytes long.

--
Mats

You could probably do something like this:

Code:

typedef struct announAllocCommand_s
{
        byte    serviceType;
	byte	encodingLaw;
	uint16	reserved;
	uint32	announcementSize;

} announAllocCommand_t;

typedef struct set_serv_prop_s
{

	union
	{
	        byte serviceType;
		heartbeatCommand_t	heartbeatCommand;
		announcement_t		announcement;
	} set_serv_propType;

} set_serv_prop_t;

Note that I've removed the star from the "announceMent" and "heartbeadCommand", as I believe this is what you want - I expect that you are NOT sending pointers across the comms channel.

--
Mats

Originally Posted by spank

this last post makes sense to me. thank you very much. now i understand padding (or at least I think so )

Most compilers have a way that you can turn padding off, but unfortunately, every compiler manufacturer have their own idea of HOW you do that.

Padding is there to improve performance [and in some processors, bigger data items than bytes HAS to be aligned to certain boundaries, or the processor will throw an exception and abort the execution]. So if you have a single byte followed by a bigger type, e.g. a 32-bit integer, the 32-bit integer should be aligned to an even 4 byte (32-bit) address to make it efficent [and/or correct where relevant].

--
Mats