NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/lang/c/12345

Internet Message Format | 1992-08-14 | 1.8 KB

Path: sparky!uunet!ogicse!decwrl!world!ksr!jfw
From: [e-mail] (John F. Woods)
Newsgroups: comp.lang.c
Subject: Re: Memory alignment of struct elements
Message-ID: <[e-mail]>
Date: 14 Aug 92 17:15:30 GMT
Article-I.D.: ksr.14806
References: <[e-mail]>
Sender: [e-mail]
Lines: 24

[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?

It is possible that the architecture of the machine you are running on forbids
addressing multiple-byte quantities on non-aligned boundaries (I'm pretty
sure the SPARC forbids it).  You're almost certain to take a serious
performance penalty if it is permitted at all.  If the database has not
already been constructed, pad things to four-byte boundaries to make your life
easier, or (better yet) design your software to assume it is necessary to
unpack the on-disk structures into internal variables, as you're already doing.
Making that assumption will make it easier to change the internal layout (to
go from one machine to another, for example, or to change the internal float
to double precision).  In the modern world, where computer centers change
processor architectures frequently, writing in-core data directly out to disk
is asking for trouble.