From mboxrd@z Thu Jan  1 00:00:00 1970
From: r_zaca <r_zaca@ig.com.br>
Subject: Re: Byte Order
Date: Thu, 13 Jan 2005 08:05:57 -0200
Message-ID: <20050113_100557_072441.r_zaca@ig.com.br>
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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 

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