Thread: Confused by pointer offset calculation

I am trying to work out the difference (in bytes) between two members of a packed structure.

The structure is defined like this:

Code:

struct __attribute__((packed)) jcamsys_msg_image
{
        unsigned char           magic[2];
        uint32_t                msglen;
        uint16_t                msgtype;                                // JC_MSG_... or JC_MSG_...
        uint16_t                reqid;
        uint16_t                cam;
        uint16_t                image_type;                             // at the moment only JC_IMAGE_TYPE_JPEG
        char                    crypted;                                // TRUE if data that follows is encrypted
        uint32_t                datalen;
        uint16_t                width;
        uint16_t                height;
        unsigned char           data[JC_MAX_MSGLEN-JC_MAX_HEADERLEN];   // image data
};

Code:

int jc_msg_image(struct jcamsys_key* key, int *fd, int reqid, int crypted, int cam, unsigned char*data, int datasize, int datalen, int image_type, int width, int height)
{
        struct jcamsys_msg_image*       msgi =(struct jcamsys_msg_image*)sendbuf;
        int w=0;

        msgi->magic[0]  = MAGIC0;
        msgi->magic[1]  = MAGIC0;
!!!     msgi->msglen    = ( ((uintptr_t)msgi->data - (uintptr_t)msgi->msgtype) ) + datalen;                         // len = remaining header size + data size        
        msgi->msgtype   = JC_MSG_IMAGE;
        msgi->reqid     = jc_nextreqid(reqid);
        msgi->cam       = cam;
        msgi->image_type= JC_IMAGE_TYPE_JPEG;
        msgi->crypted   = crypted;
        msgi->datalen   = datalen;
        msgi->width     = width;
        msgi->height    = height;

        if (crypted==TRUE)
                jcam_crypt_buf_copy(key, msgi->data, data, datasize, datalen);                          // copy as we crypt
        else    memcpy(msgi->data, data, datalen);                                                      // else just copy

printf("sending image message msgi->msglen=%d\n",msgi->msglen); fflush(stdout);
        if (jc_sockwrite(fd, (unsigned char*)&sendbuf, msgi->msglen)<0)
                return(JC_ERR);
        else    return(msgi->reqid);
}

The issue is that:
((uintptr_t)msgi->data - (uintptr_t)msgi->msgtype)

gives some huge number, it should be 15 ?

What I am trying to say is "as an integer" what the difference between the address of structure member data and address of structure member msgtype.

Anyone know what I am doing wrong?

Many Thanks,
Jon

That works a treat, thanks :-)

I am getting careless in my old age...

Returns 17 not my expected 15 as GCC quite reasonably aligns data[] to a 32 bit boundary.