On Thu, Jul 19, 2012, at 12:02 PM, Greg Banks wrote:
> Hmm.
>
> static void test_byteorder(void)
> {
> const char b64[8] = { 0, 0, 0, 0, 0, 0, 0, 1 };
> const char b32[4] = { 0, 0, 0, 1 };
>
> char i64[8];
> char i32[4];
>
> Here's your first problem, the alignment of all these variables is
> uncontrolled. The code is only working by accident because the C
> runtime environment gives you an aligned stack, and also because x86
> doesn't care so much about alignment.
Yeah, OK - I'll fix that first. that's pretty easy.
> In the header file lib/byteorder64.h
>
> /* http://stackoverflow.com/a/4410728/94253 */
>
> #if defined(__linux__)
> # include <endian.h>
> #elif defined(__FreeBSD__) || defined(__NetBSD__)
> # include <sys/endian.h>
> #elif defined(__OpenBSD__)
> # include <sys/types.h>
> # define be16toh(x) betoh16(x)
> # define be32toh(x) betoh32(x)
> # define be64toh(x) betoh64(x)
> #elif defined(WORDS_BIGENDIAN)
> #define CYRUS_BYTESWAP
> #endif
>
> That could do with some improvement.
Looks like stackoverflow might indeed be full of it. I guess we should
really test for each of those in configure.in and then #define the right
inline instruction.
> Solaris also has an optimised ntohll() in libc which uses the 64b bswap
> instruction, although it's not inlined.
>
> http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/lib/libc/amd64/gen/byteorder.s
>
> and gcc has a builtin which uses the instruction on platforms which have
> it
>
> http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-g_t_005f_005fbuiltin_005fbswap64-3333
>
> which presumably will work on some platforms where Linux's <endian.h>
> (which re-implements the same thing differently) is not available.
>
> Disassembling cunit/byteorder64.o, I don't see any bswap instructions in
> there, but nm shows undefined symbols for libc's htonl and ntohl.
> Writing a function that calls ntohll() and nothing else, it seems that
> ntohll() expands to a no-op. Running gcc -E confirms this.
>
> Aha..
>
> #ifdef be64toh
> #define htonll(x) htobe64(x)
> #define ntohll(x) be64toh(x)
> #elif defined (CYRUS_BYTESWAP)
> /* little-endian 64-bit host/network byte-order swap functions */
> extern unsigned long long _htonll(unsigned long long);
> extern unsigned long long _ntohll(unsigned long long);
> #define htonll(x) _htonll(x)
> #define ntohll(x) _ntohll(x)
> #else
> #define htonll(x) (x)
> #define ntohll(x) (x)
> #endif
>
> The <endian.h> on my desktop defines a be64toh(). The same header on
> ci.cyrusimap.org doesn't.
Yeah, damn. Is it really ancient, or 32 bit, or something?
Bron.
