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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 CENTOS

pam_issue

PAM_ISSUE(8)							 Linux-PAM Manual						      PAM_ISSUE(8)

NAME

       pam_issue - PAM module to add issue file to user prompt

SYNOPSIS

       pam_issue.so [noesc] [issue=issue-file-name]

DESCRIPTION

       pam_issue is a PAM module to prepend an issue file to the username prompt. It also by default parses escape codes in the issue file similar
       to some common getty's (using x format).

       Recognized escapes:

       d
	   current day

       l
	   name of this tty

       m
	   machine architecture (uname -m)

       

	   machine's network node hostname (uname -n)

       o
	   domain name of this system

       
	   release number of operating system (uname -r)

       	
	   current time

       s
	   operating system name (uname -s)

       u
	   number of users currently logged in

       U
	   same as u except it is suffixed with "user" or "users" (eg. "1 user" or "10 users")

       v
	   operating system version and build date (uname -v)

OPTIONS

       noesc
	   Turns off escape code parsing.

       issue=issue-file-name
	   The file to output if not using the default.

MODULE TYPES PROVIDED

       Only the auth module type is provided.

RETURN VALUES

       PAM_BUF_ERR
	   Memory buffer error.

       PAM_IGNORE
	   The prompt was already changed.

       PAM_SERVICE_ERR
	   A service module error occurred.

       PAM_SUCCESS
	   The new prompt was set successfully.

EXAMPLES

       Add the following line to /etc/pam.d/login to set the user specific issue at login:

		   auth optional pam_issue.so issue=/etc/issue

SEE ALSO

       pam.conf(5), pam.d(5), pam(8)

AUTHOR

       pam_issue was written by Ben Collins <bcollins@debian.org>.

Linux-PAM Manual						    09/19/2013							      PAM_ISSUE(8)
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