Re: Is detecting endianness at compile-time unworkable?

[Michael Felt <aixtools@felt.demon.nl>]

I hope a I have a "leap forward"


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