I'm working on an implementation of the memcache protocol which, at some points, uses 64 bits integer values. These values must be stored in "network byte order".
I wish there was some uint64_t htonll(uint64_t value)
function to do the change, but unfortunately, if it exist, I couldn't find it.
So I have 1 or 2 questions:
I have in mind a basic implementation but I don't know how to check the endianness at compile-time to make the code portable. So your help is more than welcome here ;)
Thank you.
Here is the final solution I wrote, thanks to Brian's solution.
uint64_t htonll(uint64_t value)
{
// The answer is 42
static const int num = 42;
// Check the endianness
if (*reinterpret_cast<const char*>(&num) == num)
{
const uint32_t high_part = htonl(static_cast<uint32_t>(value >> 32));
const uint32_t low_part = htonl(static_cast<uint32_t>(value & 0xFFFFFFFFLL));
return (static_cast<uint64_t>(low_part) << 32) | high_part;
} else
{
return value;
}
}
#define htonll(x) ((1==htonl(1)) ? (x) : ((uint64_t)htonl((x) & 0xFFFFFFFF) << 32) | htonl((x) >> 32))
#define ntohll(x) ((1==ntohl(1)) ? (x) : ((uint64_t)ntohl((x) & 0xFFFFFFFF) << 32) | ntohl((x) >> 32))
The test (1==htonl(1)) simply determines (at runtime sadly) if the hardware architecture requires byte swapping. There aren't any portable ways to determine at compile-time what the architecture is, so we resort to using "htonl", which is as portable as it gets in this situation. If byte-swapping is required, then we swap 32 bits at a time using htonl (remembering to swap the two 32 bit words as well).
Here's another way to perform the swap that is portable across most compilers and operating systems, including AIX, BSDs, Linux, and Solaris.
#if __BIG_ENDIAN__
# define htonll(x) (x)
# define ntohll(x) (x)
#else
# define htonll(x) ((uint64_t)htonl((x) & 0xFFFFFFFF) << 32) | htonl((x) >> 32))
# define ntohll(x) ((uint64_t)ntohl((x) & 0xFFFFFFFF) << 32) | ntohl((x) >> 32))
#endif
The important part is to use __BIG_ENDIAN__
or __LITTLE_ENDIAN__
; and not __BYTE_ORDER__
, __ORDER_BIG_ENDIAN__
or __ORDER_LITTLE_ENDIAN__
. Some compilers and operating systems lack __BYTE_ORDER__
and friends.