portable ntohl and friends

Question

I'm writing a small program that will save and load data, it'll be command line (and not interactive) so there's no point in including libraries I need not include.

When using sockets directly, I get the ntohl functions just by including sockets, however here I don't need sockets. I'm not using wxWidgets, so I don't get to use its byte ordering functions.

In C++ there are lot of new standardised things, for example look at timers and regex (although that's not yet fully supported) but certainly timers!

Is there a standardised way to convert things to network-byte ordered?

Naturally I've tried searching "c++ network byte order cppreference" and similar things, nothing comes up.

BTW in this little project, the program will manipulate files that may be shared across computers, it'd be wrong to assume "always x86_64"

@trojanfoe it shouldn't matter :P I am using GCC 4.9.something though if that's what you mean. — Alec Teal, Mar 10 '15 at 15:08
When you say " I get the ntohl functions just by including sockets", it does matter. You mean including `sockets.h`, but what about linking? — trojanfoe, Mar 10 '15 at 15:09
@trojanfoe: On which platform does `ntohl` come from "sockets.h"? — Lightness Races in Orbit, Mar 10 '15 at 15:11
@LightnessRacesinOrbit I don't know - I am asking the OP to clarify. — trojanfoe, Mar 10 '15 at 15:11
@LightnessRacesinOrbit (I don't care. I want the OP to be more specific). — trojanfoe, Mar 10 '15 at 15:12
@trojanfoe either way if they were in "file.h" including that and HOPING they were macros, or inline functions is hardly "portable" - yes it'd be dumb to link to such short functions but none the less. — Alec Teal, Mar 10 '15 at 15:13
on posix platforms : http://pubs.opengroup.org/onlinepubs/9699919799/functions/htonl.html - that's as portable (and standardized) as it'll get — Sander De Dycker, Mar 10 '15 at 15:21

score 0 · Answer 1 · edited May 23 '17 at 11:44

0

Is there a standardised way to convert things to network-byte ordered?

No. There isn't.

Boost ASIO has equivalents, but that somewhat violates your requirements.

edited May 23 '17 at 11:44

Community

1
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answered Mar 10 '15 at 15:12

Lightness Races in Orbit

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Other than a runtime check, is there a standard way to detect byte ordering? (perhaps a macro?) – Alec Teal Mar 10 '15 at 15:15
@AlecTeal: No. C++ is an abstraction. It doesn't care what byte ordering your machine uses. That's deliberate, and good. – Lightness Races in Orbit Mar 10 '15 at 15:26

Maxim Egorushkin · Answer 2 · 2015-03-10T16:31:57.430

There are no C++ standard functions for that, but you can compose the required functionality from the C++ standard functions.

Big-endian-to-host byte-order conversion can be implemented as follows:

#include <boost/detail/endian.hpp>
#include <boost/utility/enable_if.hpp>
#include <boost/type_traits/is_arithmetic.hpp>
#include <algorithm>

#ifdef BOOST_LITTLE_ENDIAN
#   define BE_TO_HOST_COPY std::reverse_copy
#elif defined(BOOST_BIG_ENDIAN)
#   define BE_TO_HOST_COPY std::copy
#endif

inline void be_to_host(void* dst, void const* src, size_t n) {
    char const* csrc = static_cast<char const*>(src);
    BE_TO_HOST_COPY(csrc, csrc + n, static_cast<char*>(dst));
}

template<class T>
typename boost::enable_if<boost::is_integral<T>, T>::type
be_to_host(T const& big_endian) {
    T host;
    be_to_host(&host, &big_endian, sizeof(T));
    return host;
}

Host-to-big-endian byte-order conversion can be implemented in the same manner.

Usage:

uint64_t big_endian_piece_of_data;
uint64_t host_piece_of_data = be_to_host(big_endian_piece_of_data);

doron · Answer 3 · 2015-03-10T17:11:17.037

0

The following should work correctly on any endian platform

int32_t getPlatformInt(uint8_t* bytes, size_t num)
{
    int32_t ret;
    assert(num == 4);
    ret = bytes[0] << 24;
    ret |= bytes[1] << 16;
    ret |= bytes[2] << 8;
    ret |= bytes[3];
    return ret;

}

You network integer can easily be cast to an array of chars using:

uint8_t* p = reiterpret_cast<uint8_t*>(&network_byte_order_int)

edited Mar 10 '15 at 17:11

answered Mar 10 '15 at 15:20

doron

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If `sizeof(int) < 4` because `<< 24` shift would be undefined behaviour with such an `int`. – Maxim Egorushkin Mar 10 '15 at 16:41
Fixed the types for the pedants out there. – doron Mar 10 '15 at 17:12

score 0 · Answer 4 · answered Mar 10 '15 at 15:22

GCC has __BYTE_ORDER__ which is as good as it will get! It's easy to detect if the compiler is GCC and test this macro, or detect if it is Clang and test that, then stick the byte ordering in a config file and use the pre-processor to conditionally compile bits of code.

score 0 · Answer 5 · edited May 23 '17 at 12:32

The code from Doron that should work on any platform did not work for me on a big-endian system (Power7 CPU architecture).

Using a compiler built_in is much cleaner and worked great for me using gcc on both Windows and *nix (AIX):

uint32_t getPlatformInt(const uint32_t* bytes) 
  {
    uint32_t ret;
    ret = __builtin_bswap32 (*bytes));
    return ret;
  }

See also How can I reorder the bytes of an integer in c?

portable ntohl and friends

5 Answers5