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From: trier@cwlim.CWRU.EDU (Stephen C. Trier)
Newsgroups: comp.os.msdos.programmer,alt.msdos.programmer,comp.lang.c
Subject: Re: alloca() for TurboC
Keywords: alloca, DOS, TurboC, assembly
Message-ID: <1990Aug22.032243.18214@usenet.ins.cwru.edu>
Date: 22 Aug 90 03:22:43 GMT
References: <2745@onion.reading.ac.uk> <3860@bingvaxu.cc.binghamton.edu>
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Reply-To: trier@po.CWRU.Edu
Organization: Case Western Reserve Univ. Cleveland, Ohio, (USA)
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Here's a version of alloca I use.  It's my clone of a version from some
piece of PD software I saw a year or two ago.

        #define alloca(A) ((_SP -= ((A)+1) & 0xFFFE), (void far *)(((unsigned long)_SS << 16) | _SP))

The idea is to subtract the size A from the stack pointer, making sure
it's word-aligned.  (This is the (_SP -= ((A)+1) & 0xFFFE) part.)  Then,
since alloca returns a pointer to the allocated space, the macro must
kludge together a far pointer from the stack segment and stack pointer.

I should add the disclaimer that I'm not entirely sure that this works.
It's my own version of someone else's, but I changed it a bit.  (I don't
remember what the original looked like.)

Hope this helps!

-- 
Stephen Trier                              Case Western Reserve University
Home: sct@seldon.clv.oh.us                 Information Network Services
Work: trier@cwlim.ins.cwru.edu
   I may work for the University, but that doesn't mean I speak for them.