Re: What's new in C?

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: What's new in C?
Date	2012-11-30 16:41 +0000
Organization	Aioe.org NNTP Server
Message-ID	<k9anj1$iap$1@speranza.aioe.org> (permalink)
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Rui Maciel <rui.maciel@gmail.com> wrote:
> Malcolm McLean wrote:

>> With a non-recursive function, stack usage can be calculated at
>> compile time. Also, f the stack breaks, the sitation is likely to be
>> detected on the first run.

In the days of fixed sized small stacks, I suppose that is true.
Maybe it still is for embedded systems. I remember dynamic stacks
back to OS/2 1.2, though. The linker required one to set a stack
size if larger than the default 4K, but I could make it really big
(more than available virtual memory) and it worked fine.

>> That's not the case with vlas. They can only be coded safely by imposing
>> a maximum. But if you set the maximum so that the stack cannot overflow
>> on any input, normally you must hardcode the maxmium. So there's actually
>> not much advantage in a vla.

Is it required that VLAs be stack allocated?

Stack allocation of large arrays has been a Fortran problem for
a long time. There are assumptions, mostly from C, that automatic
arrays should be small, which isn't always true. Also, Fortran can
sometimes allocate temporary arrays of unknown (at compile time) size.

> That's exactly why VLAs, when used properly, can be an improvement over 
> fixed-length arrays (FLAs).  

That seems useful to me. Even more, in the case of recursion, you 
can size each instance as appropriate. 

> VLAs tend to be depicted as a way to get arrays larger than the one needed 
> by FLAs, but it's actually the other way around.  With VLAs, it's possible 
> to calll functions which declare arrays that are smaller than what would 
> otherwise be required by a FLA solution.  Therefore, if VLAs are allocated 
> in the stack, the worst case scenario is that they require as much stack 
> space as the FLA solution, and in any other case they will take over less 
> memory than what would otherwise be allocated.  This is an improvement, and 
> can be a significant one, as it reduces the chances of overflowing the call 
> stack.  This might look like nit picking, but high availability is achieved 
> through nit picking.

Well, in the case of recursion, you can never be sure how big the
stack needs to be, in cases where recursion depth (and array size)
depends on input data. But also, most systems allow changing the 
stack size without recompiling.

> So, along with the syntactic sugar side of VLAs, using them does have its 
> advantages, and depending on the application and how they are used, they 
> might actually be significant.

Note that PL/I has had variable length AUTO arrays since day zero.
(Sometime in the mid 1960's.)  

(Well, OS/360 doesn't have a stack, so everything was heap allocated. 
To speed thing up, it would allocate a large (maybe 4K) block and then
divide that up among the different DSAs.)

In the days of small computers, it may have made sense to allocate small
local arrays (of fixed size) on the stack, and large ones on the heap.
Other languages never had the same distinction.

-- glen

Thread

What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-11-28 19:41 -0700
  Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-11-28 19:15 -0800
    Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-06 02:43 -0700
      Re: What's new in C? James Kuyper <jameskuyper@verizon.net> - 2012-12-06 07:45 -0500
  Re: What's new in C? Noob <root@127.0.0.1> - 2012-11-29 10:36 +0100
    Re: What's new in C? Weland <weland@sdf.lonestar.org> - 2012-11-29 10:17 +0000
    Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-04 01:45 -0700
  Re: What's new in C? Weland <weland@sdf.lonestar.org> - 2012-11-29 09:48 +0000
    Re: What's new in C? David Brown <david@westcontrol.removethisbit.com> - 2012-11-29 13:50 +0100
  Re: What's new in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-11-29 05:55 -0800
    Re: What's new in C? Les Cargill <lcargill99@comcast.com> - 2012-11-29 12:48 -0600
      Re: What's new in C? Rui Maciel <rui.maciel@gmail.com> - 2012-11-30 10:46 +0000
        Re: What's new in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-11-30 05:31 -0800
          Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-11-30 14:22 +0000
          Re: What's new in C? Rui Maciel <rui.maciel@gmail.com> - 2012-11-30 15:04 +0000
            Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-11-30 16:41 +0000
              Re: What's new in C? James Kuyper <jameskuyper@verizon.net> - 2012-11-30 11:49 -0500
              Re: What's new in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-11-30 10:12 -0800
              Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-11-30 11:12 -0800
                Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-11-30 22:26 +0000
                Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-11-30 22:36 +0000
                Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-11-30 23:27 +0000
                Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-04 01:39 -0700
                Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-12-04 00:43 -0800
                Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-04 03:14 -0700
                Re: What's new in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-04 12:50 +0000
                Re: What's new in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-12-04 05:54 -0800
                Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-04 20:51 +0000
                Re: What's new in C? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-04 16:09 -0500
                Re: What's new in C? James Kuyper <jameskuyper@verizon.net> - 2012-12-04 16:19 -0500
                Re: What's new in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-04 21:45 +0000
                Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-05 01:13 +0000
                Re: What's new in C? James Kuyper <jameskuyper@verizon.net> - 2012-12-04 20:36 -0500
                Re: What's new in C? Robert Wessel <robertwessel2@yahoo.com> - 2012-12-04 20:26 -0600
                Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-12-04 18:48 -0800
                Re: What's new in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-05 02:57 +0000
                Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-12-05 01:42 +0000
                Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-06 02:06 -0700
                Re: What's new in C? James Kuyper <jameskuyper@verizon.net> - 2012-12-06 07:50 -0500
                Re: What's new in C? JohnF <john@please.see.sig.for.email.com> - 2012-12-06 13:46 +0000
                Re: What's new in C? James Kuyper <jameskuyper@verizon.net> - 2012-12-06 09:39 -0500
              Re: What's new in C? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-11-30 15:52 -0500
                Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-04 01:53 -0700
                Re: What's new in C? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-04 08:16 -0500
                Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-04 21:18 +0000
                Re: What's new in C? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-04 18:04 -0500
                Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-05 01:19 +0000
                Re: What's new in C? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-04 21:05 -0500
                Re: What's new in C? Noob <root@127.0.0.1> - 2012-12-05 16:45 +0100
                Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-06 01:45 -0700
                Re: What's new in C? James Kuyper <jameskuyper@verizon.net> - 2012-12-06 08:12 -0500
                Re: What's new in C? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-06 21:13 +0000
                Re: What's new in C? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-06 09:15 -0500
                Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-12 18:28 -0700
        Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-11-30 11:22 -0800
          Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-04 03:04 -0700
  Re: What's new in C? Rui Maciel <rui.maciel@gmail.com> - 2012-11-29 18:19 +0000
    Re: What's new in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-11-29 11:41 -0800
  Re: What's new in C? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-11-30 20:50 +0000
    Re: What's new in C? Jens Gustedt <jens.gustedt@loria.fr> - 2012-11-30 23:35 +0100
      Re: What's new in C? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-12-05 03:26 +0000
        Re: What's new in C? gwowen <gwowen@gmail.com> - 2012-12-05 23:49 -0800
          Re: What's new in C? Cal Dershowitz <cal@example.invalid> - 2012-12-06 01:49 -0700
          Re: What's new in C? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-12-08 07:55 +0000
            Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-12-08 13:29 +0000
              Re: What's new in C? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-12-08 15:23 +0000
                Re: What's new in C? Martin Shobe <martin.shobe@yahoo.com> - 2012-12-08 17:46 +0000
                Re: What's new in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-12-08 10:45 -0800
                Re: What's new in C? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-12-08 19:32 +0000
                Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-12-08 18:55 +0000
                Re: What's new in C? Ian Collins <ian-news@hotmail.com> - 2012-12-09 08:26 +1300
                Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-12-08 20:12 +0000
                Re: What's new in C? Ian Collins <ian-news@hotmail.com> - 2012-12-09 09:18 +1300
                Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-12-08 21:21 +0000
                Re: What's new in C? Ian Collins <ian-news@hotmail.com> - 2012-12-09 10:27 +1300
                Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-12-08 13:36 -0800
                Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-12-08 12:59 -0800
                Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-12-08 22:10 +0000
                Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-12-08 16:15 -0800
                Re: What's new in C? Ian Collins <ian-news@hotmail.com> - 2012-12-09 14:23 +1300
                Re: What's new in C? Ian Collins <ian-news@hotmail.com> - 2012-12-09 14:20 +1300
                Re: What's new in C? Keith Thompson <kst-u@mib.org> - 2012-12-08 11:32 -0800
                Re: What's new in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-09 02:52 +0000
                Re: What's new in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-09 02:53 +0000
                Re: What's new in C? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-12-08 20:08 +0000
                Re: What's new in C? "BartC" <bc@freeuk.com> - 2012-12-08 20:25 +0000

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