Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!samsung!uunet!tdatirv!sarima
From: sarima@tdatirv.UUCP (Stanley Friesen)
Newsgroups: comp.lang.c++
Subject: Re: 64 bit architectures and C/C++
Message-ID: <230@tdatirv.UUCP>
Date: 30 Apr 91 15:56:08 GMT
References: <168@shasta.Stanford.EDU> <224@tdatirv.UUCP> <1991Apr29.140256.27605@cbnewsh.att.com>
Reply-To: sarima@tdatirv.UUCP (Stanley Friesen)
Organization: Teradata Corp., Irvine
Lines: 61

In article <1991Apr29.140256.27605@cbnewsh.att.com> warren@cbnewsh.att.com (warren.a.montgomery) writes:
<There are probably a lot of hidden assumptions in programs that
<sizeof(short)==2 and sizeof(long)==4 at this point, so any
<assignment of meanings on a 64 bit machine 

True, but such code is lkely to cause pain at any time anyway, Too many
currently *existing* machines violate this assumption.

<  I don't even know if it is possible in C to write a
<declaration guaranteed to produce an integer of a specific size
<(in bits or bytes) in a machine-independent way.

Well you can come rather close, though you need to do some special stuff
to do arithmetic on them:

struct {
	signed long value:SIZE;
};

will will give you SIZE bits on almost all machines.  
[To do arithmetic you must explicitely access the value member].

Remember if *bits* are important use bitfields

<There are lots of ways a programmer may want to declare an integer:
<
<1	Any convenient and reasonable size.

Declare this an int.

<2	Any convenient size large enough to represent X.

Use either short or long depending on whether 16 bits is sufficient or not.
[Note that a minimum requirement of a certain number of bits *is* portable,
short's must be at least 16 bits in size, though they may be larger].	

<3	The smallest convenient size large enough to represent X.

Harder to do portably.  I would suggest using a typedef and a machine-specific
header file for each architecture.

<4	Exactly Y bits or bytes long.

The only portable way to do this is bitfields.
[On a *given* machine you may use a basic integer type if it is the right size].

<5	Exactly the same size as datatype Z
<
<  2,3, and 5 aren't directly expressable in C, but people
<frequently mean this and use hidden assumptions about how big
<short and long are in order to do it.

But 2. *is* possible.  You can assume that a short is *at* *least* 16 bits,
and a long is *at* *least* 32 bit.  You just may get more than you absolutely
need.  (And of course char is at least 8 bits).

Just as long as you do not assume that any type is exactly some size you
are safe.
-- 
---------------
uunet!tdatirv!sarima				(Stanley Friesen)