From: Junio C Hamano <gitster@pobox.com>
To: Charles Bailey <cbailey32@bloomberg.net>
Cc: git@vger.kernel.org
Subject: Re: [PATCH] Add extra logic required to detect endianness on Solaris
Date: Thu, 01 May 2014 12:18:22 -0700 [thread overview]
Message-ID: <xmqq61lpcnpd.fsf@gitster.dls.corp.google.com> (raw)
In-Reply-To: <xmqqa9b1coml.fsf@gitster.dls.corp.google.com> (Junio C. Hamano's message of "Thu, 01 May 2014 11:58:26 -0700")
Junio C Hamano <gitster@pobox.com> writes:
> Charles Bailey <cbailey32@bloomberg.net> writes:
>
>> #if !defined(__BYTE_ORDER)
>> +/* Known to be needed on Solaris but designed to potentially more portable */
>> +
>> +#if !defined(__BIG_ENDIAN)
>> +#define __BIG_ENDIAN 4321
>> +#endif
>> +
>> +#if !defined(__LITTLE_ENDIAN)
>> +#define __LITTLE_ENDIAN 1234
>> +#endif
>> +
>> +#if defined(_BIG_ENDIAN)
>> +#define __BYTE_ORDER __BIG_ENDIAN
>> +#endif
>> +#if defined(_LITTLE_ENDIAN)
>> +#define __BYTE_ORDER __LITTLE_ENDIAN
>> +#endif
>
> The existing support is only for platforms where all three macros
> (BYTE_ORDER, LITTLE_ENDIAN and BIG_ENDIAN) are defined, and the
> convention used on such platforms where BYTE_ORDER is set to either
> one of the *_ENDIAN macros to tell the code which byte order we
> have. This mimics the convention where __BYTE_ORDER and other two
> macros are already defined with two leading underscores, and in such
> a case we do not have to do anything. We make the final decision to
> use or bypass bswap64() in our ntohll() implementation based on the
> variables with double leading underscores.
>
> This patch seems to address two unrelated issues in that.
>
> (1) The existing support does not help a platform where the
> convention is to define either _BIG_ENDIAN (with one leading
> underscore) or _LITTLE_ENDIAN and not both, which is Solaris
> but there may be others.
>
> (2) There may be __LITTLE_ENDIAN and __BIG_ENDIAN macros already
> defined on the platform. Or these may not have been defined at
> all. You avoid unconditionally redefing these.
>
> I find the latter iffy.
>
> What is the reason for avoiding redefinition? Is it because you
> know the original values they have are precious? And if so in what
> way they are precious? If the reason of avoiding redefinition is
> because you do not even know what their values are (so that you are
> trying to be safe by preserving), what other things can you say
> about their values you are preserving?
>
> Specifically, do you know that these two are defined differently, so
> that defining __BYTE_ORDER to one of them and comparing it to
> __BIG_ENDIAN is a good way to tell if the platform is big endian?
>
> I would understand it if (2) were "we undefine if these are defined
> and then define them as 4321 and 1234 respectively, in order to
> avoid a compiler warning against redefinition of a macro", but that
> is not what I am seeing, so I am not sure what you meant to achieve
> by that "if !defined()" constructs.
>
> Thanks.
Just a thought.
I am wondering if you may want to go the other way around. That is,
instead of using "we have byte-order, big and little and the way to
determine endianness is to see byte-order matches which of the
latter two", use "there may be either big or little but not both
defined, and that is how you learn the byte-order".
And make these two macros match what Solaris happens to use.
I am not sure which variant I like better myself, though.
compat/bswap.h | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)
diff --git a/compat/bswap.h b/compat/bswap.h
index 120c6c1..e87998e 100644
--- a/compat/bswap.h
+++ b/compat/bswap.h
@@ -101,19 +101,24 @@ static inline uint64_t git_bswap64(uint64_t x)
#undef ntohll
#undef htonll
-#if !defined(__BYTE_ORDER)
-# if defined(BYTE_ORDER) && defined(LITTLE_ENDIAN) && defined(BIG_ENDIAN)
-# define __BYTE_ORDER BYTE_ORDER
-# define __LITTLE_ENDIAN LITTLE_ENDIAN
-# define __BIG_ENDIAN BIG_ENDIAN
-# endif
+#if !defined(_BIG_ENDIAN) && !defined(_LITTLE_ENDIAN)
+
+#if defined(BYTE_ORDER) && defined(LITTLE_ENDIAN) && defined(BIG_ENDIAN)
+# if BYTE_ORDER == BIG_ENDIAN
+# define _BIG_ENDIAN
+# else
+# define _LITTLE_ENDIAN
+#endif
+
#endif
-#if !defined(__BYTE_ORDER)
+#if !defined(_BIG_ENDIAN) && !defined(_LITTLE_ENDIAN)
# error "Cannot determine endianness"
+#elif defined(_BIG_ENDIAN) && defined(_LITTLE_ENDIAN)
+# error "Your endianness is screwed up"
#endif
-#if __BYTE_ORDER == __BIG_ENDIAN
+#if defined (_BIG_ENDIAN)
# define ntohll(n) (n)
# define htonll(n) (n)
#else
next prev parent reply other threads:[~2014-05-01 19:18 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-01 7:43 [PATCH] Add extra logic required to detect endianness on Solaris Charles Bailey
2014-05-01 18:58 ` Junio C Hamano
2014-05-01 19:18 ` Junio C Hamano [this message]
2014-05-01 19:22 ` Junio C Hamano
2014-05-02 7:49 ` Charles Bailey
2014-05-02 7:55 ` [PATCH] Detect endianness on more platforms that don't use BYTE_ORDER Charles Bailey
2014-05-02 16:48 ` Junio C Hamano
2014-05-02 16:58 ` Charles Bailey
2014-05-02 19:34 ` Junio C Hamano
2014-05-02 19:43 ` Junio C Hamano
2014-05-02 20:02 ` Charles Bailey
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: http://vger.kernel.org/majordomo-info.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=xmqq61lpcnpd.fsf@gitster.dls.corp.google.com \
--to=gitster@pobox.com \
--cc=cbailey32@bloomberg.net \
--cc=git@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://80x24.org/mirrors/git.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).