Re: Is detecting endianness at compile-time unworkable?

From: Michael Felt <aixtools@felt.demon.nl>
Date: Jul 31, 2018, 14:01 UTC
Message-ID: <287cdba8-19c1-2fe8-4aff-d0385b38e92c@felt.demon.nl>
In-Reply-To: <87wotdt649.fsf@evledraar.gmail.com>

I hope a I have a "leap forward"

On 7/30/2018 11:39 AM, Ævar Arnfjörð Bjarmason wrote:

Show 5 quoted lines

> Perhaps it's worth taking a step back here and thinking about whether
> this whole thing is unworkable. It was hard enough to get this to work
> on the combination of Linux, *BSD and Solaris, but I suspect we'll run
> into increasingly obscure platforms where this is hard or impossible
> (AIX, HP/UX etc.)

While I still cannot say for HP/UX it does seem there is a potential solution based on the status for _LITTLE_ENDIAN and _BIG_ENDIAN. At least, gcc on POWER and xlc on POWER provides one or the other - and my hope is that gcc on other platforms also provides them.

For "other" compilers that do not provide them - a modification to CFLAGS to define one or the other should make "make" work.

Details (note - I am not a programmer, so by definition at least one of my "macros" will be wrong :)

AIX and xlc root@x066:[/]xlc -qshowmacros -E /dev/null | grep -i endi 1506-297 (S) Unable to open input file null. A file or directory in the path name does not exist.. #define __HHW_BIG_ENDIAN__ 1 #define __BIG_ENDIAN__ 1 #define __THW_BIG_ENDIAN__ 1 #define _BIG_ENDIAN 1

On SLES12 (le) and xlc suse12test:~/images/littleEndian/sles # xlc -qshowmacros -dM -E x.c | grep -i endi #define _LITTLE_ENDIAN 1 #define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__ #define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__ #define __LITTLE_ENDIAN__ 1 #define __ORDER_BIG_ENDIAN__ 4321 #define __ORDER_LITTLE_ENDIAN__ 1234 #define __ORDER_PDP_ENDIAN__ 3412 #define __VEC_ELEMENT_REG_ORDER__ __ORDER_LITTLE_ENDIAN__

Based on what I can see on gcc on POWER and xlc on POWER I think an approach (simplified) can be:

#if undefined(_BIG_ENDIAN) && undef(_LITTLE_ENDIAN) #error "one of _BIG_ENDIAN or _LITTLE_ENDIAN must be defined. Try adding the correct value to CFLAGS" #else defined(_BIG_ENDIAN) && defined(_LITTLE_ENDIAN) #error "Only one of _BIG_ENDIAN and _LITTLE_ENDIAN may be defined, not both" #endif

And then logic based on the value set. This should also make cross-compile possible by unsetting an incorrect default and setting the correct value.

p.s. Is there a setting I need to set somewhere so I receive a copy of the email sent after it is received by the list. I could send myself a copy, but I much prefer it comes from the maillist - as verification it was received.