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From: wirzenius@cc.helsinki.fi (Lars Wirzenius)
Newsgroups: comp.lang.c
Subject: Re: Portability vs. Endianness
Message-ID: <1991Mar13.173420.5527@cc.helsinki.fi>
Date: 13 Mar 91 17:34:20 GMT
References: <1991Mar12.105451.19488@dit.upm.es>
Organization: University of Helsinki
Lines: 49

In article <1991Mar12.105451.19488@dit.upm.es>, esink@turia.dit.upm.es writes:
>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.

I think you're going to have difficulties on machines with non-32-bit
longs and non-8-bit chars. longs bigger than 32 bits won't fit into a
68000 longword (32 bits) without losing information.

On machines with 32-bit longs and 8-bit bytes, you could try the
following:
	
	#include <stdio.h>
	#define BITS_PER_CHAR 8
	
	int main() {
		unsigned long var;
		char Bytes[sizeof var];
		unsigned long mask;
		int i;
	
		var = 0x12345678;
		printf("var = %lx\n", var);
	
		mask = ~(~0 & (~0 << BITS_PER_CHAR));
			/* this creates a mask with only the lower
			 * BITS_PER_CHAR bits turned on
			 */

		for (i = 0; i < sizeof var; ++i) {
			Bytes[ (sizeof var) - i - 1] = 
				(var & mask);
			var >>= BITS_PER_CHAR;
		}
	
		for (i = 0; i < sizeof var; ++i) 
			printf("Byte %d: %02x\n", i, Bytes[i]);
		exit(0);
	}
	
This has been tested on a VAX (real good example, I know, but I can't
try it on anything else right now).

-- 
Lars Wirzenius    wirzenius@cc.helsinki.fi