Byte Order

Byte Order

[r_zaca]

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  Hello all, 

  How can I say if the machine where I am working uses "Host Byte Order" or 
"Network Byte Order"? 
  Is it an obvius question? I mean, all hosts (machines) uses "Host Byte 
Order" and when it needs to send data through the network it uses "Network 
Byte Order". 
  Thanks. 

Re: Byte Order

[Eric Bambach]

On Wednesday 12 January 2005 11:03 am, you wrote:
>   Hello all,
>
>   How can I say if the machine where I am working uses "Host Byte Order" or
> "Network Byte Order"?
>   Is it an obvius question? I mean, all hosts (machines) uses "Host Byte
> Order" and when it needs to send data through the network it uses "Network
> Byte Order".
>   Thanks.

Well,

 Im not sure if this answers your question, but when doing network programming 
you should always use hton and ntoh conversions to make your program portable 
whether or not your machine uses the same byte order as the network.

 If you;re asking which architectures are Big Endian and little endian and how 
to tell..then im afraid I dont know. Google will tell you which architectures 
are which.

However, man pages also say "On machines which have a byte order which is the 
same as the network order, routines are defined as null macros." so its safe 
and desirable to use whether your machine uses the same of different ordering 
anyways.

P.S. If you want to check Endianess perhaps you can pass an int through htons 
or ntohs (etc.) and see if it changes the values. If it doesn't, then you're 
using the same ordering as the network. Perhaps someone can clarify if this 
will actually work its just a stab in the dark.
----------------------------------------
--EB

> All is fine except that I can reliably "oops" it simply by trying to read
> from /proc/apm (e.g. cat /proc/apm).
> oops output and ksymoops-2.3.4 output is attached.
> Is there anything else I can contribute?

The latitude and longtitude of the bios writers current position, and
a ballistic missile.

                --Alan Cox LKML-December 08,2000 

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Re: Byte Order

[Jan-Benedict Glaw]

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On Wed, 2005-01-12 15:03:57 -0200, r_zaca <r_zaca@ig.com.br>
wrote in message <20050112_170357_014648.r_zaca@ig.com.br>:
Content-Description: Mail message body
>   How can I say if the machine where I am working uses "Host Byte Order" or 
> "Network Byte Order"? 

Something like this should do the job:

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

int
main (int argc, char *argv[])
{
        uint32_t i = 0x44332211;
        unsigned char *c;

        c = (unsigned char *) &i;

        if (c[0] == 0x11 && c[1] == 0x22 && c[2] == 0x33 && c[3] == 0x44)
                printf ("This is a little-endian host\n");
        else if (c[0] == 0x44 && c[1] == 0x33 && c[2] == 0x22 && c[3] == 0x11)
                printf ("This is a big-endian host\n");
        else if (c[0] == 0x33 && c[1] == 0x44 && c[2] == 0x11 && c[3] == 0x22)
                printf ("This is a pdp-endian host\n");
        else {
                printf ("This host is broken:-)\n");
                return EXIT_FAILURE;
        }

        return EXIT_SUCCESS;
}


>   Is it an obvius question? I mean, all hosts (machines) uses "Host Byte 
> Order" and when it needs to send data through the network it uses "Network 
> Byte Order". 

The rule is easy: Just *always* either use htonl/htons/ntohl/ntohs or
transmit the number in readable format. The nice thing is that any sane
libc will implement those as macros, so if your host already works in
network byte order, the conversion will get optimized away.

MfG, JBG

-- 
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Re: Byte Order

[r_zaca]

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  Thanks a lot, that is what I was looking for. A simple test that I can do, 
using some C conde, to find out if the machine where the program is running 
on uses Big or Little endian "memory organization". 


On Wed, 2005-01-12 15:03:57 -0200, r_zaca <r_zaca@ig.com.br> 
wrote in message <20050112_170357_014648.r_zaca@ig.com.br>: 
Content-Description: Mail message body 
>   How can I say if the machine where I am working uses "Host Byte 
Order" or 
> "Network Byte Order"? 

Something like this should do the job: 

#include <stdint.h> 
#include <stdio.h> 
#include <stdlib.h> 

int 
main (int argc, char *argv[]) 
{ 
        uint32_t i = 0x44332211; 
        unsigned char *c; 

        c = (unsigned char *) &i; 

        if (c[0] == 0x11 && c[1] == 0x22 && c[2] == 0x33 && c[3] == 0x44) 
                printf ("This is a little-endian host\n"); 
        else if (c[0] == 0x44 && c[1] == 0x33 && c[2] == 0x22 && c[3] == 
0x11) 
                printf ("This is a big-endian host\n"); 
        else if (c[0] == 0x33 && c[1] == 0x44 && c[2] == 0x11 && c[3] == 
0x22) 
                printf ("This is a pdp-endian host\n"); 
        else { 
                printf ("This host is broken:-)\n"); 
                return EXIT_FAILURE; 
        } 

        return EXIT_SUCCESS; 
} 


>   Is it an obvius question? I mean, all hosts (machines) uses "Host 
Byte 
> Order" and when it needs to send data through the network it uses 
"Network 
> Byte Order". 

The rule is easy: Just *always* either use htonl/htons/ntohl/ntohs or 
transmit the number in readable format. The nice thing is that any sane 
libc will implement those as macros, so if your host already works in 
network byte order, the conversion will get optimized away. 

MfG, JBG 

Re: Byte Order

[Glynn Clements]

r_zaca wrote:

>   Thanks a lot, that is what I was looking for. A simple test that I can do, 
> using some C conde, to find out if the machine where the program is running 
> on uses Big or Little endian "memory organization". 

	#include <endian.h>
	
	#if __BYTE_ORDER == __LITTLE_ENDIAN
		...
	#elif __BYTE_ORDER == __BIG_ENDIAN
		...
	#elif __BYTE_ORDER == __PDP_ENDIAN
		...
	#endif

-- 
Glynn Clements <glynn@gclements.plus.com>