Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.std.c
Subject: Re: Size of structure containing char fields
Message-ID: <3775@auspex.auspex.com>
Date: 28 Jul 90 17:36:50 GMT
References: <1030@lzaz.ATT.COM> <8631@cognos.UUCP>
Organization: Auspex Systems, Santa Clara
Lines: 25

>Sun's C compiler on a Sun 3 (with Sun OS 4.0) aligns structs on two
>byte boundaries. This is NOT an ANSI-compatible compiler, but I don't
>think that the ANSI standard will force them to change their ways. My
>guess is that the implementation was done this way simply because it
>was easier.

It was done that way because the Sun compiler was ultimately based on
the PCC port to the 68K done at MIT, and I think those porters decided
to impose only two-byte alignment on 4-byte data types - after all, it's
not as if Motorola was ever going to come out with a full-blown 32-bit
chip in the 68K family that would run faster with 4-byte alignment,
right? :-(

I think other 68K C compilers, at least for UNIX systems, do the same.

>Interestingly, with the Sun 4, your struct in fact has a size of 3,
>even though the Sun 4 generally has more strict alignment rules than
>the Sun 3.

Actually, there is one case where the Sun-4 compiler has *less* strict
alignment rules than the Sun-3 compiler.  The MIT compiler also made the
minimum alignment requirement for a structure 2-byte alignment, while
most other PCC-based compilers, including Sun's SPARC compiler, impose
no minimum alignment requirement on structures beyond the most
restrictive alignment requirement of the structure's members.