Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!sdd.hp.com!hp-pcd!hpfcso!hpfcdc!rer
From: rer@hpfcdc.HP.COM (Rob Robason)
Newsgroups: comp.sys.hp
Subject: Re: alloca() on the HP9000/300
Message-ID: <5570467@hpfcdc.HP.COM>
Date: 9 Aug 90 20:39:31 GMT
References: <1455@tub.UUCP>
Organization: HP Fort Collins, Co.
Lines: 37

Oliver> Is it possible to write a "real" alloca() for the HP at all

I've seen at least one hack that claimed to be an alloca() for the 300.
The problem is that the author declined to guarantee that it would work
from one release to the next.  So it was of little use.  I think this is
an inherent problem with alloca()s because they rely on knowledge about
how the stack is layed out in a specific implementation, if that changes
due to a compiler improvement, for example, it's busted.

While I know there are some pretty significant applications that make
heavy use of alloca(), it has proved so troublesome that none of the
standards organizations have dared touch it.  Neither POSIX, SVID, XPG3,
OSF nor ANSI-C support alloca().  ANSI-C, in fact makes this point in the
rationale:

    "Some implementations provide a function (often called alloca) which
    allocates the requested object from automatic storage; the object is
    automatically freed when the calling function exits.  Such a
    function is not efficiently implementable in a variety of
    environments, so it was not adopted in the standard."

For this reason, most standards (and HP) are recommending that functions
and applications be modified to use malloc/free or some other standard
mechanism in order to be portable to the widest range of standards
conforming systems.

Many of the applications I've looked at that used alloca did so out of
laziness, rather than need:  It was a convenient way to be sure that
everything was freed without having to think.  In these cases the fix is
easy.  There are some applications where it will be hard, but possible,
to work around alloca being absent.

I know this isn't what you wanted to hear.  You may obtain an alloca
from another source and find it adequate for now.  You just need to know
that it's not necessarily the best way to solve the problem.

Rob "Don't kill the messenger" Robason