Xref: utzoo comp.os.msdos.programmer:548 alt.msdos.programmer:2050 comp.lang.c:31219
Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!zaphod.mps.ohio-state.edu!rpi!leah!bingvaxu!vu0310
From: vu0310@bingvaxu.cc.binghamton.edu (R. Kym Horsell)
Newsgroups: comp.os.msdos.programmer,alt.msdos.programmer,comp.lang.c
Subject: Re: alloca() for TurboC
Keywords: alloca, DOS, TurboC, assembly
Message-ID: <3860@bingvaxu.cc.binghamton.edu>
Date: 18 Aug 90 13:51:21 GMT
References: <2745@onion.reading.ac.uk>
Reply-To: vu0310@bingvaxu.cc.binghamton.edu.cc.binghamton.edu (R. Kym Horsell)
Organization: SUNY Binghamton, NY
Lines: 37

In article <2745@onion.reading.ac.uk> ssuhook@susssys1.cs.reading.ac.uk writes:
\\\
>In summary, I am mainly interested in finding out how to get an alloca() for
>the PC for Turbo C, and also what it is that alloca() actually does.
\\\

Alloca does something *like*

char *alloca(n) {
	n =			/* round up to words */;
	((char*)SP) += n;	/* bump up the stack ptr */
	return (char*)SP-n;	/* return address of stuff on stack */
	}

I.e. it just bumps up the stack by n bytes (usually word alignining
the result) and returns the address of the new stack section.
Since the SP is restore by C exit stuff from a `frame ptr register'
(typically, anyway) this memory is deallocated at the end of the function.

Note I've assumed the stack grows up (not a normal thing these days).
Also we can't *actually* write alloca() as above even when you can
refer to the SP directly since its exit will deallocate the
space just allocated -- you should think of alloca() as written
above as a macro.

To solve your problem with proting Bison -- why not use
(a)	malloc/alloc and just forget deallocating it, or
(b)	malloc/alloc and use an explicit free at the end of
	the function its used it (might as well catch *all*
	returns from the function while you're about it).

Ive written a fairly complete Yacc/Lex for AT's and didn't
both about memory leakage. After a PDP-11 there was more memory
than I new what to do with (ahhh...many years ago now).

Hope this's been some help,
-Kym Horsell