Newsgroups: comp.lang.c
Path: utzoo!utgpu!cunews!dgbt!don
From: don@dgbt.doc.ca (Donald McLachlan)
Subject: RE: Portability vs Endianness
Message-ID: <1991Mar15.145526.12802@dgbt.doc.ca>
Sender: don@dgbt.doc.ca (Donald McLachlan)
Organization: The Communications Research Centre, Ottawa, Canada
Date: Fri, 15 Mar 91 14:55:26 GMT

>Sender: @dit.upm.es
>
>
>Given the following :
>
>long var;
>unsigned char Bytes[4];
>
>
>Is there a portable way to move the value held in var
>into the memory space pointed to by Bytes, with the restriction
>that the representation be in Most Significant Byte first
>format ?  I am well aware that sizeof(long) may not be 4.  I
>want the value contained in var converted to a 68000
>long word, and I want the code fragment to run on any
>machine.  The solution must be ANSI C.
>
>Feel free to suggest your own declarations
>to replace or augment mine.
>
>Thanks in advance for the input,
>
>Eric
>

Eric:

I can't be completely sure (I've only checked it on 2 different architectures)
and done dry runs in my head for 2 others, but I think whis should work.

Please don't flame me for posting this guys, because since I don't know
if this will work I want some input.

Don.

_______________________ cut ________________________________________
static int init = 1;
static char to_motorola[sizeof(long)];

main()
{
	extern long cvt_to_moto();

	char format_string[32];
	int i;
	long number;
	char *c = (char *)&number;

	number = 0x574319;
	sprintf(format_string, "number = 0x%%0%dlx\n", sizeof(number) * 2);
	printf(format_string, (unsigned long)number);

	printf("local order = ");
	for(i = 0; i < sizeof(number); ++i)
		printf("%02lx ", (unsigned long)c[i]);
	puts("");
	
	number = cvt_to_moto(number);

	printf("moto  order = ");
	for(i = 0; i < sizeof(number); ++i)
		printf("%02lx ", (unsigned long)c[i]);
	
	puts("");
}

long cvt_to_moto(host_num)
long host_num;
{
	char *native, *motorola;
	int i;
	long moto_num;

	if(init == 1)
		init_cvt_to_moto();

	native = (char *)&host_num;
	motorola = (char *)&moto_num;

	for(i = 0; i < sizeof(moto_num); ++i)
		motorola[i] = native[to_motorola[i]];
	
	return(moto_num);
}

init_cvt_to_moto()
{
	int i;
	long num;
	char *c = (char *)&num;

	for(i = 0; i < sizeof(num); ++i)	/* build moto-endian image */
		c[i] = i;			/* of 0x00010203... */

	for(--i; i >= 0; --i)			/* for each byte in num */
	{					/* read LSB bytes from num */
		to_motorola[i] = num & 0x00FFL;	/* and put in to_moto[LSB] */
		num >>=8;			/* get next byte in num */
	}
	init = 0;
}
_______________________ cut ________________________________________

here are the results from a Vax 750 running Ultrix

number = 0x00574319
local order = 19 43 57 00 
moto  order = 00 57 43 19 

and from a sun sparcstation 330 running Sun0/S

number = 0x00574319
local order = 00 57 43 19 
moto  order = 00 57 43 19