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Path: utzoo!mnetor!seismo!mcvax!unido!iaoobelix!wagner
From: wagner@iaoobelix.UUCP
Newsgroups: comp.lang.c
Subject: Re: Byte vs. Word Alignment - (nf)
Message-ID: <6700007@iaoobelix.UUCP>
Date: Sat, 18-Apr-87 06:03:00 EST
Article-I.D.: iaoobeli.6700007
Posted: Sat Apr 18 06:03:00 1987
Date-Received: Tue, 21-Apr-87 02:18:59 EST
Lines: 39
Nf-ID: #N:iaoobelix:6700007:000:1605
Nf-From: iaoobelix!wagner    Apr 18 12:03:00 1987


Byte vs. word alignment is a problem I have encountered recently when I
tried to setup a simple server/client connection using sockets. The problem
was that you get very strange results on a VAX/uVAX/Sun if you print out
sizeof(struct foo) where struct foo is:

	struct foo {
		char frob_1;
		int frob_2;
	};

I intended to send something like the above structure as a packet over the
net. You know what happened? The VAX/uVAX produces *8* as sizeof struct foo,
whereas the Sun program yields *6*. The client program, run on the VAX sent
a packet of size 8, and the daemon on the Sun thought the packet size was 6.
Yet, things get even worse if you use something like

	struct packet {
		char packet_type;
		struct foo packet_data;
	}

Aarrgh! You get *8* on a Sun and *12* on a VAX/uVAX. The whole structure is
not being packed together into eight bytes (with the two chars in the first
word) but each char is placed in a 4-byte (four-byte!) word! BTW, the same
holds for shorts (2 bytes).

Maybe there should be a compiler switch -dont_align forcing the compiler to
take things literally, i.e. to leave these structs unchanged even if their
size is an odd number of bytes. The structures themselves could be word
aligned *in memory*, however, but the internal structure would be as the
programmer defined it.

Note: If you think of something like `unsigned frob: 8;' to get the desired
result... No chance! You will get exactly the same sizes for the structures on
both Sun and VAX/uVAX.

Juergen Wagner,		     (USENET) ...seismo!unido!iaoobel!wagner
("Gandalf")			 Fraunhofer Institute IAO, Stuttgart