Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!pilchuck!dataio!bright
From: bright@Data-IO.COM (Walter Bright)
Newsgroups: comp.lang.c++
Subject: Re: Variable sized objects
Message-ID: <2264@dataio.Data-IO.COM>
Date: 20 Dec 89 19:53:05 GMT
References: <1262@amethyst.math.arizona.edu> <1881@odin.SGI.COM> <5891@internal.Apple.COM>
Reply-To: bright@dataio.Data-IO.COM (Walter Bright)
Organization: Data I/O Corporation; Redmond, WA
Lines: 18

In article <5891@internal.Apple.COM> turk@Apple.COM (Ken "Turk" Turkowski) writes:
<My question is:  is there a way to determine whether the matrix is auto
<or "new"ed?  If it is possible to determine if it is auto, then
<alloca() could be used for allocating memory, without the tremendous
<overhead of new/malloc().

Yes, there definitely is a way. However, it is very compiler and machine
dependent. For instance, with ZTC in the C and L memory models, it is
sufficient to compare the segment of a pointer to the object with the
SS register. (ZTC large data models have a separate stack segment,
MSC doesn't, so you'll have to do it a different way for MSC).
For S and M models, you'll have to check the range of the
offset to see if it lies within the stack.

In general, the method would be to see if the pointer points into the
region of the stack. You could define a function:
	int isauto(void *p);
and then implement it for each machine you use.