scope of malloc

[Rahul Dhesi]

In <2182 at kraftbus.opal.cs.tu-berlin.de> net at opal.cs.tu-berlin.de
(Oliver Laumann) writes:

>I'm not aware of any plausible reason why vendors should omit something
>as useful and simple to implement as alloca() from their UNIX systems.

I too am not aware of one.

The designers of Ada assumed that such functionality would be available
in the language, and this is why they didn't worry too much about
requiring the equivalent of a free() function in the language.  (Well,
actually, they did worry about it, but in the reverse manner, arguing
that the availability of an unchecked way of freeing allocated memory
was dangerous, and it had better be indicated so in bold letters that
read something like UNCHECKED_DEALLOCATION.  Their preference was for
the memory allocated for objects in the heap to be freed automatically
when the scope of the objects expired at run-time, i.e., through a
mechanism equivalent to alloca().)

I think the problem, to quote Chris Torek somewhat out-of-context, is
that you're in a circle and it's hard to break out of it.  Since
alloca() must properly be designed into the compiler, it's hard to find
"prior art" in the form of user-written functions.  Since programmers
can't always assume the availability of alloca(), they find ways around
it -- and I suspect some of them, having worked hard to find ways
around it, aren't sympathetic to others who don't want to work so
hard.

I'm sure there are some very-CISC machine architectures in which
procedure exit is a single instruction, and adding alloca() would
result in less efficient procedure exit code that couldn't use this
single instruction.  But I don't see why alloca() should be impossible,
or even difficult, to implement on any machine.  Heck, if you can
implement a recursive language on a machine that doesn't have a stack
to begin with (e.g. IBM 360 onwards), and not complain about how
"impossible" it is to write a PL/1 compiler, you can surely implement
alloca() on every machine in the world.

There's enough prior art now, though, that perhaps the next team that
revises the C standard may be persuaded to add alloca() to it.
--
Rahul Dhesi <dhesi%cirrusl at oliveb.ATC.olivetti.com>
UUCP:  oliveb!cirrusl!dhesi