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 the network and I want to encoded it properly in my machine.

In the case I receive an int I perform this code using ntohl :

int * piData = reinterpret_cast<int*>((void*)pData);

//manage endianness of incomming network data 
unsigned long ulValue = ntohl(*piData);
int iValue = static_cast<int>(ulValue);

But in the case I receive an double, I don't know what to do.

The answer to the question suggest to do:

template <typename T>
void swap_endian(T& pX)
{
    char& raw = reinterpret_cast<char&>(pX);
    std::reverse(&raw, &raw + sizeof(T));
}

However , if I quote this site:

The ntohl() function converts the unsigned integer netlong from network byte order to host byte order. When the two byte orders are different, this means the endian-ness of the data will be changed. When the two byte orders are the same, the data will not be changed.

On the contrary @GManNickG's answer to the question always does the inversion with std::reverse .

Am I wrong considering that this answer is false ? ( in the extent of network management of endianess which the use of ntohl suggest though it was not precisely said in the title of the OP question).

In the end: Should I split my double into two parts of 4 bytes and apply the ntohl function on the two parts ? Are there more cannonical solutions ?

There's also this interesting question in C, host to network double?, but it limits to 32 bits values. And the answer says doubles should be converted to strings because of architecture differences... I'm also gonna work with audio samples, should I really consider converting all the samples to strings in my database ? ( the doubles come from a database that I query over the network)

Answer 1

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 2

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...

Answer 3

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 4

If you want to know system endianless

ONLY #if __cplusplus > 201703L

#include <bit>
#include <iostream>

using namespace std;

int main()
{
    if constexpr (endian::native == endian::big)
        cout << "big-endian";
    else if constexpr (endian::native == endian::little)
        cout << "little-endian";
    else
        cout << "mixed-endian";
}

For more info: https://en.cppreference.com/w/cpp/types/endian