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From: mlandau@Diamond.BBN.COM (Matt Landau)
Newsgroups: comp.lang.c
Subject: Re: help needed about dynamic space allocation
Message-ID: <2431@slate.Diamond.BBN.COM>
Date: Wed, 26-Nov-86 21:10:51 EST
Article-I.D.: slate.2431
Posted: Wed Nov 26 21:10:51 1986
Date-Received: Wed, 26-Nov-86 22:59:10 EST
References: <13802@amdcad.UUCP> <359@prairie.UUCP> <1046@lifia.UUCP> <715@dg_rtp.UUCP> <332@apple.UUCP>
Reply-To: mlandau@Diamond.BBN.COM (Matt Landau)
Organization: BBN Laboratories, Inc., Cambridge, MA
Lines: 25
Summary: alloca is bad news

In newsgroup comp.lang.c (article <332@apple.UUCP>), turk@apple.UUCP 
(Ken "Turk" Turkowski) writes:
>
>There have been numerous responses to get the declarations consistent, so I
>won't belabor them here.
>
>However, I would like to point out the use of a function that can speed up the
>program considerably when you are only allocating temporary variables.  The
>function is alloca(), and allocates memory from the stack, rather than from
>the heap.  The result is that allocation is simpler, and vanishes automatically
>when you exit the routine.

Well, there are going to be a lot of people pointing this out, but I
thought I'd be the first :-)  Alloca is nonstandard, nonportable, and 
doesn't exist on a lot of machines.  In fact, the last time this subject
came up was when a lot of people were having trouble porting Gnu Emacs
bacause it used alloca().  At the time, I think the conclusion was that
there exist machine architectures for which it is practically impossible
to even implement it.

The bottom line: Don't use alloca if you expect your code to port.
-- 
 Matt Landau      	 		BBN Laboratories, Inc.
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