Question

What I have is this

struct Record
{
unsigned char cat;
unsigned char len[2]={0x00, 0x1b};  // can't put short here because that 
                                   // whould change the size of the struct
unsigned char dat[253];
};
Record record;
unsigned short recordlen = *((unsigned short*)record.len);

This result in recordlen=0x1b00 instead of 0x001b

Same with *reinterpret_cast<unsigned short*>(record.len)

Can you explain why ? How should I be doing this ?

Answer 1

Can you explain why?

Because you cannot assume a specific endianness of your computer architecture.

The natural follow-up question is what do you do about it. Fortunately, you can force a specific byte order by calling one of these functions htonl, htons, ntohl, or ntohs. They work regardless of the computer architecture on which you run them:

On the sending end, you convert from host order to network order; on the receiving end, you convert from network order to host order.

// Sending end
unsigned short recordlen = calculate_len();
*reinterpret_cast<unsigned short*>(record.len) = htons(recordlen);

// Receiving end
unsigned short recordlen = ntohs(*reinterpret_cast<unsigned short*>(record.len));