From:	SMTP%"smithdm@wmvx.lvs.dupont.com" 20-APR-1994 11:18:01.92
To:	EVERHART
CC:	
Subj:	Re: Problems with VAX C tools ported to DEC C?

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From: smithdm@wmvx.lvs.dupont.com (David M. Smith)
Subject: Re: Problems with VAX C tools ported to DEC C?
Message-ID: <1994Apr19.190457.16395@es.dupont.com>
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Organization: DuPont (Opinions are those of the writer only)
Date: Tue, 19 Apr 1994 19:04:57 GMT
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In article <1994Apr18.171403.2864@objy.com>, bruce@objy.com (Bruce Reed) writes:
>I'm looking for feedback from other users who have seen strange behavior
>from VAX C VAX/VMS compiled tools that were ported to Open/VMS AXP with
>DEC C. The few tools I've ported compiled rather easily using the 
>/STANDARD=VAXC mode of DEC C, but they just don't run right. 

In the past two weeks I have moved two .C programs from VAX to Alpha.  I
encountered a number of minor problems which showed up as compile-time errors. 
These were fairly easy to fix.  The most difficult problem I encountered was in
one of the programs after it compiled correctly it didn't work correctly.  I 
had not written the original program, so it took a fairly lengthy debugging
session to understand what was going on.

The general problem I had was the difference in VAX C and DEC C on Alpha with
regard to the default for #pragma member_alignment.  You can look this up in
the books, but the gist of it is that structures and/or unions compiled under
VAX C may have a different layout in when compiled on Alpha under DEC C, due to
DEC C "lining up" new elements of the structure on "natural" boundaries.  This
might be O.K. in general, but isn't if the structure happens to describe a
pre-existing disk file as it did in my case.  The program was accessing the
wrong bytes from records and producing bizarre results.

After seeing what was going on, I added code similar to the following to
several include files which define record layouts:

#pragma nostandard
#pragma member_alignment __save
#pragma nomember_alignment

	.
	.  (original definitions here)
	.

#pragma standard
#pragma member_alignment __restore

I found this sequence of preprocessor directives in some of the include modules
in ALPHA$LIBRARY:SYS$STARLET_C.TLB.
--------------------------------------------------------------------------------
David M. Smith  (302) 774-6914  smithdm@wmvx.dnet.dupont.com
DuPont                          (Opinions are those of the writer only)
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