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From: mbs@turing.acs.virginia.edu (Michael B. Smith)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: 4 bytes to a long?
Message-ID: <1991Jun13.220554.19518@murdoch.acc.Virginia.EDU>
Date: 13 Jun 91 22:05:54 GMT
References: <451.28517f01@beach.gal.utexas.edu> <22408@cbmvax.commodore.com>
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In article <22408@cbmvax.commodore.com> carolyn@cbmvax.commodore.com (Carolyn Scheppner - CATS) writes:
>In article <451.28517f01@beach.gal.utexas.edu> mrimages@beach.gal.utexas.edu writes:
>>
>>	Is there a fast, simple way to convert 4 bytes into a long?
>
>The IFF code has always done it this way:
>
>#define MAKE_ID(a,b,c,d)        \
>        ((ULONG) (a)<<24 | (ULONG) (b)<<16 | (ULONG) (c)<<8 | (ULONG) (d))
>
>where a,b,c,and d are characters and the result is a long
>-- 
>==========================================================================
B
> Carolyn Scheppner -- Tech. Mgr. CATS - Commodore Amiga Technical Support
> PHONE 215-431-9180 {uunet,rutgers}!cbmvax!carolyn  carolyn@commodore.com
>
> Calm down.  It's just ones and zeros.
>==========================================================================

I use code similar to this, but have found that when a, b, c, & d are pointers
that the lower 8 bits need to be masked. My example is the following code
fragment:  

long make_long (p)
char
	*p;
{
	return ( ((*(p + 0) & 0xff)<<24) +
		 ((*(p + 1) & 0xff)<<16) +
		 ((*(p + 2) & 0xff)<<8 ) +
		 ((*(p + 3) & 0xff)    )  );
}

I've always presumed (but never checked) that the compiler optmizes out the
+0 construct (there for readability). I use this when I am reconstructing
a file that I don't have a 'struct' for. And of course (since I'm new to this),
this code could be buggy (especially since I don't do the casts) -- but it
works for me.:)

Michael B. Smith
mbs@turing.acs.virginia.edu