How to manage endianess of double from network

Question

I have a BIG problem with the answer to this question Swap bits in c++ for a double

Yet, this question is more or less what I search for: I receive a double from th

Answer 1

I could not make John Källén code work on my machine. Moreover, it might be more useful to convert the double into bytes (8 bit, 1 char):

template<typename T>
string to_byte_string(const T& v)
{
    char* begin_ = reinterpret_cast<char*>(v);
    return string(begin_, begin_ + sizeof(T));
}

template<typename T>
T from_byte_string(std::string& s)
{
    assert(s.size() == sizeof(T) && "Wrong Type Cast");
    return *(reinterpret_cast<T*>(&s[0]));
}

This code will also works for structs which are using POD types.

If you really want the double as two ints

double d;
int* data = reinterpret_cast<int*>(&d);

int first = data[0];
int second = data[1];

Finally, long int will not always be a 64bit integer (I had to use long long int to make a 64bit int on my machine).

Answer 2

If your doubles are in IEEE 754 format that you should be relatively OK. Now you have to divide their 64 bits into two 32-bit halves and then transmit them in big-endian order (which is network order);

How about:

void send_double(double d) {
    long int i64 = *((reinterpret_cast<int *>)(&d)); /* Ugly, but works */
    int hiword = htonl(static_cast<int>(i64 >> 32));
    send(hiword);
    int loword = htonl(static_cast<int>(i64));
    send(loword);
}

double recv_double() {
    int hiword = ntohl(recv_int());
    int loword = ntohl(recv_int());
    long int i64 = (((static_cast<long int>) hiword) << 32) | loword;
    return *((reinterpret_cast<double *>(&i64));
}

Answer 3

Assuming you have a compile-time option to determine endianness:

#if BIG_ENDIAN
template <typename T>
void swap_endian(T& pX)
{
   // Don't need to do anything here... 
}
#else
template <typename T>
void swap_endian(T& pX)
{
    char& raw = reinterpret_cast<char&>(pX);
    std::reverse(&raw, &raw + sizeof(T));
}
#endif

Of course, the other option is to not send double across the network at all - considering that it's not guaranteed to be IEEE-754 compatible - there are machines out there using other floating point formats... Using for example a string would work much better...