Path: utzoo!utgpu!water!watmath!clyde!rutgers!psuvax1!gondor.cs.psu.edu!schwartz
From: schwartz@gondor.cs.psu.edu (Scott E. Schwartz)
Newsgroups: comp.arch
Subject: Re: RISC data alignment
Summary: you had better use XDR or something similar
Message-ID: <3246@psuvax1.psu.edu>
Date: 23 Jan 88 20:56:22 GMT
References: <2635@calmasd.GE.COM>
Sender: netnews@psuvax1.psu.edu
Reply-To: schwartz@gondor.cs.psu.edu (Scott E. Schwartz)
Organization: Penn State University
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In article <2635@calmasd.GE.COM> gjo@calmasd.UUCP (Glenn Olander) writes:
>machines?  In particular, I believe that many RISC machines require data
>to be aligned on a natural boundary, e.g. longwords must be referenced
>on a 4-byte boundary.  

This is true of sparc.

>If this is true, then it would seem to also be true that a C structure
>could have different lengths, depending on whether it was compiled 
>on a RISC or non-RISC machine.  Further, it would seem that
>if that C structure were written out to a file, it could only be read
>properly by a machine of the same type as that which wrote it.

This is exactly correct.

>Does such incompatibilty truly exist?  If I create a file on a Sun/4
>will I be able to read it on a Sun/3?

From "Porting Software to SPARC", Sun part no. 800-1596-10, Nov '87, p4:

	Because of the three considerations above, members of a given
	structure may have different offsets on SPARC machines than
	on the MC680x0, and the structure as a whole may have a different
	size.  Even though data representations identical on the MC680x0
	and the SPARC, binary files where raw structures have been
	written out may not be portable between processors.
	

-- Scott Schwartz            schwartz@gondor.cs.psu.edu