NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/lang/c/12274

Internet Message Format | 1992-08-13 | 2.6 KB

Path: sparky!uunet!usc!sdd.hp.com!swrinde!gatech!concert!sas!mozart.unx.sas.com!sasghm
From: [e-mail] (Gary Merrill)
Newsgroups: comp.lang.c
Subject: Re: Memory alignment of struct elements
Message-ID: <[e-mail]>
Date: 13 Aug 92 13:44:23 GMT
References: <[e-mail]>
Sender: [e-mail] (Noter of Newsworthy Events)
Organization: SAS Institute Inc.
Lines: 44
Originator: [e-mail]
Nntp-Posting-Host: theseus.unx.sas.com


In article <[e-mail]>, [e-mail] (Andrzej Udalski) writes:
|> I am writing the software for a big database and the space is very
|> important. If I try to use a
|>
|> struct { char ch ; float f }
|>
|> the compiler (both in SunOS 4.0.1 and Interactive SYSVR3) allocates
|> 8 bytes for it, starting the "f" element at fifth byte of the structure.
|> This makes both reading and writing such records from/to a binary database
|> file (consisting of 5-byte-long records) very inconvinient, requiring
|> a declaration of character array of 5 bytes and using "memcpy()". Is there
|> a way to force the compiler to align the struct elements following their
|> true length?

This depends upon the compiler, of course.  Many compilers have such
an option, but (depending upon the architecture, etc.) it may not
be a good idea to use it.  You might simply try:

    struct { float f; char ch; }

which will not require the internal padding you are encountering.
In general, to avoid such padding you should define your structure
members in decreasing order of alignment requirements.  Note that
the compiler will tend to add padding to the *end* of the structure
in order to achieve correct alignment in *arrays* of such structures.
Still, such an approach may help you.

I would be a little concerned about the maintainability of your code,
however. If things are done in a careful and modular manner (and,
for example, sizes of records, etc. are not "hardcoded"), I do not
see much inconvenience in going to/from the database via an intermediate
buffer.  The danger you need to be aware of, of course, is the
possibility that in the future someone decides to add a new field to
your structure, and either adds it in the "wrong" place (for padding
purposes) or forgets to update routines that access it.  Whatever you
do, you should choose safety and maintainability over any small
gain in what might be thought of as "convenience".


--
Gary H. Merrill  [Principal Systems Developer, C Compiler Development]
SAS Institute Inc. / SAS Campus Dr. / Cary, NC  27513 / [phone]
[e-mail] ... !mcnc!sas!sasghm