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Full Discussion: Bit field issues in Linux
Top Forums Programming Bit field issues in Linux Post 302070817 by DreamWarrior on Friday 7th of April 2006 04:59:59 PM
Old 04-07-2006
OK...so, I did some more tinkering...here's some code and output and I'm very confused by what I'm seeing:

Code:
#include <stdio.h>
#include <stdlib.h>

typedef union
{
	unsigned char	raw;
	
	struct
	{
		unsigned char	high:4;
		unsigned char 	low:4;
	
	} data;
	
} bitwiseStruct;

static void prnBits(void *addr, size_t s);

void main()
{
	int		i;
	bitwiseStruct	bws;
	
	unsigned char	c;
	
	for (i = 0; i < 256; i++)
	{
		bws.raw = (unsigned char) i;
		
		printf("For value %3d:  bits [", (int) bws.raw);
		
		prnBits(&bws.raw, sizeof(char));
		
		c = bws.data.high;
		printf("]; high [");	
		prnBits(&c, sizeof(char));
		
		c = bws.data.low;		
		printf("]; low [");
		prnBits(&c, sizeof(char));
		
		printf("]\n");
	}
}

static void prnBits(void *addr, size_t s)
{
	int	i, j;
	
	unsigned char	*a = (unsigned char *) addr;
	
	for (i = 0; i < s; i++)
	{
		for (j = 7; j >= 0; j--)
		{
			printf("%c", *a & 1 << j ? '1' : '0');
		}

		a++;
	}
}

now, I'd expect this code to work identically on all machines. Why? I was under the impression that the endianess of a machine only affects byte ordering within a word *NOT* bit ordering within a byte. However, on a PA-Risc HP-UX box, for the value 0xF0 (240 decimal, 11110000 binary) I get the following output:

Code:
For value 240:  bits [11110000]; high [00001111]; low [00000000]

While, for my x86 based Linux box I get:

Code:
For value 240:  bits [11110000]; high [00000000]; low [00001111]

WHY?!?! This seems wrong!

edit: I guess it actually doesn't change the bit ordering within the byte. However, the HP compiler is extracting high and low order bits as I'd expect while the Linux compiler is swapping the extraction. Are they allowed to do this?
Last edited by DreamWarrior; 04-07-2006 at 06:37 PM..
 

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LEARN ABOUT DEBIAN

blaze-add

BLAZE-ADD(1)						    BlazeBlogger Documentation						      BLAZE-ADD(1)

NAME

       blaze-add - adds a blog post or a page to the BlazeBlogger repository

SYNOPSIS

       blaze-add [-pqCPV] [-b directory] [-E editor] [-a author] [-d date] [-t title] [-k keywords] [-T tags] [-u url] [file...]

       blaze-add -h|-v

DESCRIPTION

       blaze-add adds a blog post or a page to the BlazeBlogger repository. If a file is supplied, it adds the content of that file, otherwise an
       external text editor is opened for you. Note that there are several special forms and placeholders that can be used in the text, and that
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   Special Forms
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	   A mark to delimit a blog post synopsis.

   Placeholders
       %root%
	   A relative path to the root directory of the blog.

       %home%
	   A relative path to the index page of the blog.

       %page[id]%
	   A relative path to a page with the supplied id.

       %post[id]%
	   A relative path to a blog post with the supplied id.

       %tag[name]%
	   A relative path to a tag with the supplied name.

OPTIONS

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	   Allows you to specify a directory in which the BlazeBlogger repository is placed. The default option is a current working directory.

       -E editor, --editor editor
	   Allows you to specify an external text editor. When supplied, this option overrides the relevant configuration option.

       -t title, --title title
	   Allows you to specify the title of a blog post or page.

       -a author, --author author
	   Allows you to specify the author of a blog post or page.

       -d date, --date date
	   Allows you to specify the date of publishing of a blog post or page.

       -k keywords, --keywords keywords
	   Allows you to specify a comma-separated list of keywords attached to a blog post or page.

       -T tags, --tags tags
	   Allows you to supply a comma-separated list of tags attached to a blog post.

       -u url, --url url
	   Allows you to specify the url of a blog post or page. Allowed characters are letters, numbers, hyphens, and underscores.

       -p, --page, --pages
	   Tells blaze-add to add a page or pages.

       -P, --post, --posts
	   Tells blaze-add to add a blog post or blog posts. This is the default option.

       -C, --no-processor
	   Disables processing a blog post or page with an external application. For example, if you use Markdown to convert the lightweight
	   markup language to the valid HTML output, this will enable you to write this particular post in plain HTML directly.

       -q, --quiet
	   Disables displaying of unnecessary messages.

       -V, --verbose
	   Enables displaying of all messages. This is the default option.

       -h, --help
	   Displays usage information and exits.

       -v, --version
	   Displays version information and exits.

ENVIRONMENT

       EDITOR
	   Unless the core.editor option is set, BlazeBlogger tries to use system-wide settings to decide which editor to use.

EXAMPLE USAGE

       Write a new blog post in an external text editor:

	 ~]$ blaze-add

       Add a new blog post from a file:

	 ~]$ blaze-add new_packages.txt
	 Successfully added the post with ID 10.

       Write a new page in an external text editor:

	 ~]$ blaze-add -p

       Write a new page in nano:

	 ~]$ blaze-add -p -E nano

SEE ALSO

       blaze-init(1), blaze-config(1), blaze-edit(1), blaze-remove(1), blaze-make(1)

BUGS

       To report a bug or to send a patch, please, add a new issue to the bug tracker at <http://code.google.com/p/blazeblogger/issues/>, or visit
       the discussion group at <http://groups.google.com/group/blazeblogger/>.

COPYRIGHT

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       This program is free software; see the source for copying conditions. It is distributed in the hope that it will be useful, but WITHOUT ANY
       WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Version 1.2.0							    2012-03-05							      BLAZE-ADD(1)
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