Re: Array allocated on stack or heap

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: Array allocated on stack or heap
Date	2017-03-20 12:24 +0100
Organization	A noiseless patient Spider
Message-ID	<oaoe00$c5o$1@dont-email.me> (permalink)
References	(6 earlier) <f9f50c16-af70-4b1a-b8db-bfcddcd3c914@googlegroups.com> <oami10$539$1@dont-email.me> <01036705-6fe5-4262-8fd0-86eb5ef5f449@googlegroups.com> <oamvq8$mec$1@dont-email.me> <oan28a$tm6$1@dont-email.me>

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On 19/03/17 23:57, jacobnavia wrote:
> Le 19/03/2017 à 23:16, BartC a écrit :
>> He mentioned a register-based stack, which doesn't really mean anything,
>> except suggesting the entire stack is contained within registers.
>>
>> That seems extraordinarily unlikely for a processor powering a
>> smartphone.
> 
> Yes. :-)
> 
> Unix like Android variety of smart phone...
> 
> Why this eternal discussion?
> 
> Again:
> 
> In most CPUs there is a stack pointer and a hardware stack, that is
> mapped by the language to stacked function scopes.

Yes.  For some cpus, like ARM (other than Thumb) and PPC, there is no
dedicated stack pointer - there is merely the convention that one of the
registers is usually used for a stack, and there are no dedicated PUSH,
POP, CALL, RETURN instructions that use a specific register.  But in
practice, they are used in much the same way as a cpu with a dedicated
stack pointer (like x86).  This is something the compiler implementer
must understand, but makes little difference to the user.

> 
> Except small micro-computers and weird old hardware, most
> microprocessors have a hardware stack.

Agreed.  But it is still an important exception, and it is the reason
that the C standards do not mention a stack.

> 
> This allows to teach newcomers simple rules for developing software like:
> 
> Automatic variables are in the stack and get freed automatically with
> just a machine instruction.

A good many automatic variables are /not/ on the stack - they are in
registers, or optimised away entirely.  And how and when they get freed
is dependent on the implementation and the code in question.

Simplifications can be very useful for teaching - but it makes most
sense to stick to /accurate/ simplifications.  "Automatic variables are
allocated when they are defined within the function, and deallocated at
the end of their scope".  There is no need to mention a stack at all.

> All the temporary workspace we declare in
> local variables need not be freed after usage.
> 
> Stack space is not unlimited however, and use with care. But with modern
> CPUs, this just doesn't matter.

Stack space is usually much more limited than dynamic storage ("heap"
storage - though again, the standards don't use that word).  Depending
on the OS and the settings, the stack can be easily overrun by big local
arrays.  Unlike dynamic storage, there is no standard way to determine
when there is space on the stack - it depends on the OS and possibly
compiler flags or extensions.

So I would be very wary of suggesting to a beginner that stack space
limitations do not matter - even when programming for PC's.

> 
> All depends on the application and some applications are constrained by
> RAM usage, for instance, and stack variables are to be avoided. By
> following simple rules you can avoid them, or indicate the compiler how
> to avoid them. But that is the exception to the rule:
> 
> Local variables are thread safe by design and allocation/deallocation is
> not needed.

Be careful there too - local variables are only thread safe when used
within the same thread.  If you export them (by passing their address)
to code that runs from a different thread, you can quickly have a great
deal of trouble.

> They are used in a stack-like fashion, what is easy to
> conceptualize.
> 

True - though often it is easy to conceptualise them without referring
to a stack at all.  But I don't think it is unreasonable to talk about
"in a stack-like fashion" or a "logical stack".

Thread

Array allocated on stack or heap Prashant Shah <pshah.mumbai@gmail.com> - 2017-03-16 22:58 -0700
  Re: Array allocated on stack or heap Barry Schwarz <schwarzb@dqel.com> - 2017-03-16 23:57 -0700
    Re: Array allocated on stack or heap Mark Storkamp <mstorkamp@yahoo.com> - 2017-03-17 12:44 -0500
      Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-17 11:26 -0700
        Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-17 20:38 +0100
          Re: Array allocated on stack or heap Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-17 16:08 -0400
            Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-18 00:03 +0100
              Re: Array allocated on stack or heap Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-17 20:54 -0400
  Re: Array allocated on stack or heap John Bode <jfbode1029@gmail.com> - 2017-03-17 12:15 -0700
    Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-17 20:22 +0100
    Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-17 20:45 +0100
      Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-17 12:59 -0700
      Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-17 21:01 +0100
        Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-18 00:12 +0100
          Re: Array allocated on stack or heap jameskuyper@verizon.net - 2017-03-17 16:25 -0700
            Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-17 17:08 -0700
              Re: Array allocated on stack or heap Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-03-17 17:37 -0700
          Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-19 12:30 +0100
            Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 12:12 +0000
              Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-19 13:52 +0100
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 16:51 +0000
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-20 10:44 +0100
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-20 12:01 +0000
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-20 06:02 -0700
                Re: Array allocated on stack or heap GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-03-20 14:54 +0100
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-20 07:28 -0700
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-20 16:43 +0100
                Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-20 09:02 -0700
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-21 10:11 +0100
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-21 02:48 -0700
                Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-21 08:19 -0700
                Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-21 08:17 -0700
                Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-21 09:06 -0700
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-21 17:32 +0100
                Re: Array allocated on stack or heap Les Cargill <lcargill99@comcast.com> - 2017-03-25 15:14 -0500
                Re: Array allocated on stack or heap Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-03-25 16:05 -0700
                Re: Array allocated on stack or heap Les Cargill <lcargill99@comcast.com> - 2017-03-25 23:53 -0500
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-26 12:30 +0100
                Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-26 14:09 -0700
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-26 22:17 +0100
                Re: Array allocated on stack or heap Robert Wessel <robertwessel2@yahoo.com> - 2017-03-26 21:52 -0500
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-27 13:06 +0100
                Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-27 10:10 -0700
                Re: Array allocated on stack or heap Robert Wessel <robertwessel2@yahoo.com> - 2017-03-27 15:56 -0500
                Re: Array allocated on stack or heap bartc <bc@freeuk.com> - 2017-03-27 22:48 +0100
                Re: Array allocated on stack or heap Robert Wessel <robertwessel2@yahoo.com> - 2017-03-27 19:58 -0500
                Re: Array allocated on stack or heap GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-03-20 14:56 +0100
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-20 15:34 +0100
                Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-20 09:28 -0700
                Re: Array allocated on stack or heap bart4858@gmail.com - 2017-03-20 13:05 -0700
                Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-20 13:53 -0700
                Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-20 14:26 -0700
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-20 22:24 +0000
                Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-20 15:44 -0700
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-21 02:44 -0700
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-21 11:15 +0100
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-21 11:12 +0000
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-21 04:23 -0700
                Re: Array allocated on stack or heap scott@slp53.sl.home (Scott Lurndal) - 2017-03-21 13:13 +0000
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-21 13:38 +0000
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-21 15:33 +0100
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-21 07:38 -0700
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-21 07:35 -0700
                Re: Array allocated on stack or heap scott@slp53.sl.home (Scott Lurndal) - 2017-03-21 15:09 +0000
              Re: Array allocated on stack or heap Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-19 13:05 +0000
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 13:27 +0000
                Re: Array allocated on stack or heap GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-03-19 14:26 +0100
                Re: Array allocated on stack or heap Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-19 13:34 +0000
                Re: Array allocated on stack or heap GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-03-19 14:35 +0100
                Re: Array allocated on stack or heap Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-19 13:36 +0000
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 14:00 +0000
                Re: Array allocated on stack or heap Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-19 19:00 +0000
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 19:24 +0000
                Re: Array allocated on stack or heap Ian Collins <ian-news@hotmail.com> - 2017-03-20 08:45 +1300
                Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-19 14:32 -0700
                Re: Array allocated on stack or heap Ian Collins <ian-news@hotmail.com> - 2017-03-20 16:16 +1300
                Re: Array allocated on stack or heap Gareth Owen <gwowen@gmail.com> - 2017-03-20 18:58 +0000
                Re: Array allocated on stack or heap raltbos@xs4all.nl (Richard Bos) - 2017-03-24 09:17 +0000
                Re: Array allocated on stack or heap Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-19 20:50 +0000
              Re: Array allocated on stack or heap GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-03-19 14:10 +0100
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 13:38 +0000
                Re: Array allocated on stack or heap GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-03-19 14:49 +0100
                Re: Array allocated on stack or heap luser droog <luser.droog@gmail.com> - 2017-03-20 07:59 -0700
              Re: Array allocated on stack or heap Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-19 10:57 -0400
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 16:14 +0000
                Re: Array allocated on stack or heap Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-19 15:53 -0400
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 20:37 +0000
                Re: Array allocated on stack or heap Jean-Marc Bourguet <jm@bourguet.org> - 2017-03-19 23:09 +0100
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 22:38 +0000
                Re: Array allocated on stack or heap Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-03-19 16:21 -0700
                Re: Array allocated on stack or heap Ian Collins <ian-news@hotmail.com> - 2017-03-20 16:18 +1300
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-20 11:06 +0100
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-20 11:00 +0100
              Re: Array allocated on stack or heap supercat@casperkitty.com - 2017-03-19 08:53 -0700
      Re: Array allocated on stack or heap Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-17 16:11 -0400
      Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-17 13:35 -0700
        Re: Array allocated on stack or heap Chad <cdalten@gmail.com> - 2017-03-17 14:00 -0700
          Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-17 23:57 +0100
            Re: Array allocated on stack or heap Ian Collins <ian-news@hotmail.com> - 2017-03-18 12:42 +1300
              Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-18 00:45 +0100
            Re: Array allocated on stack or heap Chad <cdalten@gmail.com> - 2017-03-18 14:17 -0700
              Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-19 19:21 +0100
                Re: Array allocated on stack or heap already5chosen@yahoo.com - 2017-03-19 15:00 -0700
                Re: Array allocated on stack or heap BartC <bc@freeuk.com> - 2017-03-19 22:16 +0000
                Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-19 23:57 +0100
                Re: Array allocated on stack or heap Keith Thompson <kst-u@mib.org> - 2017-03-19 16:15 -0700
                Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-20 12:24 +0100
                Re: Array allocated on stack or heap scott@slp53.sl.home (Scott Lurndal) - 2017-03-20 13:24 +0000
            Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-19 13:21 +0100
      Re: Array allocated on stack or heap John Bode <jfbode1029@gmail.com> - 2017-03-17 14:30 -0700
        Re: Array allocated on stack or heap jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-17 23:59 +0100
        Re: Array allocated on stack or heap Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-17 21:02 -0400
        Re: Array allocated on stack or heap David Brown <david.brown@hesbynett.no> - 2017-03-19 13:26 +0100

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