Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!vsi1!wyse!mips!sah@gumby
From: sah@gumby (Steve Hanson)
Newsgroups: comp.sys.mips
Subject: Re: alloca(3C)
Message-ID: <26677@gumby.mips.COM>
Date: 1 Sep 89 15:11:28 GMT
References: <11601@polya.Stanford.EDU>
Sender: sah@mips.COM
Reply-To: sah@gumby (Steve Hanson)
Organization: MIPS Computer Systems, Sunnyvale CA
Lines: 64
In-reply-to: ramani@modesty.Stanford.EDU (Ramani Pichumani)

In article <11601@polya.Stanford.EDU>, ramani@modesty (Ramani Pichumani) writes:
>
>Does anyone have a real working version of alloca(3C) for the MIPS
>compiler?  I'm using a Silicon Graphics 4D R2000 box which doesn't
>come with alloca() in its library.  Doug Gwen's semi-portable version
>won't do for my application since I truly need to extend the stack.
>I'm aware that the use of alloca() is discouraged because it is
>compiler and machine dependent.  Yet all the Berkeley UNIX machines
>I've used seem to have it.  And besides, allocating memory locally
>seems like a very convienent way of cleaning up garbage that is local
>to a procedure.  Furthermore, dynamically allocated local memory can
>ease fragmentation problems with the heap.
>
>I've been told by one source that alloca() can't be truly implemented
>on a MIPS compiler because of the way it handles it's stacks.  I hope
>this isn't the case.
>
>Ramani Pichumani                              Tel: (415) 723-2902 or 723-2437
>Department of Computer Science                            Fax: (415) 725-7411
>Margaret Jacks Hall, Room 308             email: ramani@patience.stanford.edu
>Stanford, CA 94305 USA                     uunet!patience.stanford.edu!ramani



	alloca has been implemented as a built-in operator in the upcoming
2.10 compiler when alloca.h is included, otherwise you will get the portable
version. The following is taken from alloca.h:
-------------------------------------------------------------------------------

/* --------------------------------------------------- */
/* | Copyright (c) 1986 MIPS Computer Systems, Inc.  | */
/* | All Rights Reserved.                            | */
/* --------------------------------------------------- */
/* $Header: alloca.h,v 1.1 89/07/19 17:29:15 lai Exp $ */
#ifndef __ALLOCA_H
#define __ALLOCA_H

/*
** Synopsis
**   #include <alloca.h>
**   void *alloca(integral_types);
**
** Description
**   This header is to be included if the built-in version
**   of alloca is desired. The built in version is more
**   efficient than the libc version, but it is less
**   flexible. The built-in version is implemented as an
**   operator and can only be applied to integral_types.
**   integral_types are comprised of type char, the signed
**   and unsigned integer types, and the enumerated types.
**   See also alloca(3).
*/


#define alloca  __builtin_alloca
#endif


----------------------------------------------------------------------------



Less flexible means it can't be assigned or its address can't be taken.
--