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| From | Shao Miller <sha0.miller@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Union test for endianess |
| Date | 2011-06-17 00:50 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <iteme0$3va$1@dont-email.me> (permalink) |
| References | <f19541de-213c-4744-b712-92f20a3ad294@d26g2000prn.googlegroups.com> |
On 6/16/2011 10:16 PM, Bhasker Penta wrote:
> One way to test for endianess is to use a union:
>
> void endianTest()
> {
> union // sizeof(int) == 4
> {
> int i;
> char ch[4];
> } U;
>
> U.i=0x12345678; // writing to int member
> if ( U.ch[0]==0x78 ) // reading from char member
> puts("\nLittle endian");
> else
> puts("\nBig endian");
> }
>
Please note that
One possible way to help to ensure that 'sizeof (int) == 4' and that you
have 8-bit bytes is to:
#define TT_ASSERT(message, test) \
typedef char (message)[(test) ? 1 : -1]
TT_ASSERT(INT_IS_NOT_4_BYTES, sizeof (int) == 4);
TT_ASSERT(NOT_8_BIT_BYTE, CHAR_BIT == 8);
> Writing to one member of a union and reading from another member is
> implementation defined(K& R).
As far as I know, if 'sizeof (int) == 4' as shown, you can certainly
read from each element of the 'U.ch' array. C doesn't guarantee that
'sizeof (int) == 4', of course.
Combined with the 'TT_ASSERT's above, you could have your union as:
union {
unsigned int i;
unsigned char ch[sizeof (unsigned int)];
} U;
(Note that the use of 'unsigned' attempts to avoid any potential sign
bit complications; the 'TT_ASSERT' might be better off matching, too.)
> This example is used for testing
> endianess @ c-faq.com. I know that gcc allows this. Is the above
> snippet to test for endianess legal C or C++?
If you know that the implementation definitely uses an 8-bit byte, a
4-byte 'int', and that there are no padding bits and that '0x12345678'
is within the range of values for 'int', then I'd say yes for "legal C". :)
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-16 20:16 -0700
Re: Union test for endianess Ian Collins <ian-news@hotmail.com> - 2011-06-17 15:36 +1200
Re: Union test for endianess Willem <willem@toad.stack.nl> - 2011-06-17 14:13 +0000
Re: Union test for endianess Joe Wright <joewwright@comcast.net> - 2011-06-17 14:00 -0400
Re: Union test for endianess Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-06-19 19:25 +0000
Re: Union test for endianess China Blue Angels <chine.bleu@yahoo.com> - 2011-06-16 20:54 -0700
Re: Union test for endianess Willem <willem@toad.stack.nl> - 2011-06-17 14:14 +0000
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 10:28 -0400
Re: Union test for endianess Keith Thompson <kst-u@mib.org> - 2011-06-17 10:54 -0700
Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 10:35 -0500
Re: Union test for endianess "J. J. Farrell" <jjf@bcs.org.uk> - 2011-06-18 01:50 +0100
Re: Union test for endianess China Blue Angels <chine.bleu@yahoo.com> - 2011-06-16 21:00 -0700
Re: Union test for endianess Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-06-17 12:09 +0100
Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 00:50 -0500
Re: Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-16 23:17 -0700
Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 03:01 -0500
Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 13:33 -0500
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-18 09:18 -0400
Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-19 17:22 -0500
Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 16:24 -0700
Re: Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-18 01:08 -0700
Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 15:52 -0700
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 09:32 -0400
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 09:52 -0400
Re: Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-17 07:13 -0700
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 10:31 -0400
Re: Union test for endianess "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-06-17 15:57 -0700
Re: Union test for endianess Keith Thompson <kst-u@mib.org> - 2011-06-17 16:42 -0700
Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-18 02:53 +0000
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 23:59 -0400
Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-18 14:45 +0000
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-18 11:08 -0400
Re: Union test for endianess cri@tiac.net (Richard Harter) - 2011-06-18 19:47 +0000
Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-18 23:15 +0000
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-18 22:42 -0400
Re: Union test for endianess Jens <jens.gustedt@gmail.com> - 2011-06-19 07:41 -0700
Re: Union test for endianess Roberto Waltman <usenet@rwaltman.com> - 2011-06-22 11:25 -0400
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-22 11:40 -0400
Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-22 21:42 +0000
Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 21:18 -0400
Re: Union test for endianess falk@rahul.net (Edward A. Falk) - 2011-06-17 20:50 +0000
Re: Union test for endianess "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-06-18 10:42 -0400
Re: Union test for endianess Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-06-18 22:55 +0100
Re: Union test for endianess "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-06-19 21:14 -0400
Re: Union test for endianess Jens <jens.gustedt@gmail.com> - 2011-06-19 07:46 -0700
Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 16:14 -0700
Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-19 20:24 -0500
Re: Union test for endianess Jens <jens.gustedt@gmail.com> - 2011-06-20 00:31 -0700
Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-20 04:33 -0500
Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-20 12:22 -0700
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