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Groups > comp.lang.c++ > #68812

Re: alloca()-support

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c++, comp.lang.c
Subject Re: alloca()-support
Date 2019-09-30 11:13 -0700
Organization None to speak of
Message-ID <lneezx3b6b.fsf@kst-u.example.com> (permalink)
References (1 earlier) <qmtbkq$pmp$1@dont-email.me> <qmtbqv$ttm$1@news.albasani.net> <qmtc2o$pmp$4@dont-email.me> <qmtc6v$uk5$1@news.albasani.net> <qmtfeh$gno$1@dont-email.me>

Cross-posted to 2 groups.

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Soviet_Mario <SovietMario@CCCP.MIR> writes:
> On 30/09/2019 18:58, Bonita Montero wrote:
>>>>> Since when (if at any time) was variable size automatic 
>>>>> array supported ?
>> 
>>>> Since C99.
>> 
>>> for curiosity, this feature has one or more of the 
>>> limitations I feared ... or even none of them ?
>> 
>> Yes, you could run out of stack-space, maybe hitting a 
>> guard-page
>> in the best case so that you wouldn't corrupt any other data.
>
> uhm, I didn't mean THAT kind of risk (unavoidable), but the 
> possible difficulties in defining variables beyond the point 
> of declaration of the variable-size array.
>
> I can't figure out how the compiler would reference them in 
> standard efficient ways (use of registers).
>
> Anyway, the difference with alloca () would be in a safer 
> management of possible collisions and running out of stack 
> space ? (compared with the plain declarations) ?

No.  Neither VLAs nor alloca() provide any protection against running
out of space.  Both have undefined behavior if there isn't enough memory
for the allocation.

alloca() (which, again, is non-standard) is a function that returns a
pointer.  It has no provision to return a null pointer on failure.

Defining a VLA, on the other hand, creates an array object to which you
can meaningfully apply sizeof.

Also, space allocated by alloca() is deallocated on return from the
enclosing function, not at the end of the enclosing block.

Since alloca() is a function, it can be invoked in contexts where a VLA
can't be created.  If you call alloca() as part of a function argument,
it's likely to corrupt the stack.

VLAs were introduced in C99 and made optional in C11 and are not
supported, except perhaps as an extension, in C++.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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Thread

alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 14:02 +0200
  Re: alloca()-support Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-09-30 07:02 -0700
    Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 16:09 +0200
      Re: alloca()-support Bo Persson <bo@bo-persson.se> - 2019-09-30 17:29 +0200
        Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 17:33 +0200
    Re: alloca()-support gazelle@shell.xmission.com (Kenny McCormack) - 2019-09-30 16:26 +0000
  Re: alloca()-support Anton Shepelev <anton.txt@g{oogle}mail.com> - 2019-09-30 17:12 +0300
    Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 16:16 +0200
      Re: alloca()-support Anton Shepelev <anton.txt@g{oogle}mail.com> - 2019-09-30 17:30 +0300
        Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 16:44 +0200
          Re: alloca()-support Anton Shepelev <anton.txt@gmail.com> - 2019-10-02 00:36 +0300
            Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-02 06:28 +0200
              Re: alloca()-support Anton Shepelev <anton.txt@gmail.com> - 2019-10-02 16:08 +0300
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-02 15:42 +0200
    Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-09-30 14:30 +0000
      Re: alloca()-support Anton Shepelev <anton.txt@g{oogle}mail.com> - 2019-09-30 17:33 +0300
        Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-09-30 16:56 +0200
    Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-09-30 15:29 +0000
      Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 17:31 +0200
        Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-01 07:03 +0000
          Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 10:57 +0200
            Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-01 18:49 +0000
              Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 21:20 +0200
                Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-10-01 20:39 +0000
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-02 04:15 +0000
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-02 06:28 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-02 04:29 +0000
    Re: alloca()-support Keith Thompson <kst-u@mib.org> - 2019-09-30 11:14 -0700
  Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 18:04 +0200
    Re: alloca()-support William Ahern <william@25thandClement.com> - 2019-09-30 11:01 -0700
      Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 07:19 +0200
    Re: alloca()-support Paavo Helde <myfirstname@osa.pri.ee> - 2019-09-30 22:26 +0300
      Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-09-30 19:46 +0000
    Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 07:32 +0200
      Re: alloca()-support Paavo Helde <myfirstname@osa.pri.ee> - 2019-10-01 09:08 +0300
        Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 08:35 +0200
  Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-09-30 18:48 +0200
    Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 18:51 +0200
      Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-09-30 18:55 +0200
        Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 18:58 +0200
          Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-09-30 19:53 +0200
            Re: alloca()-support Keith Thompson <kst-u@mib.org> - 2019-09-30 11:13 -0700
            Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-09-30 19:38 +0000
              Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 07:41 +0200
                Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-10-01 15:19 +0000
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 17:31 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-01 18:58 +0000
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 21:24 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-02 04:28 +0000
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-02 06:30 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-02 04:36 +0000
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-02 10:01 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-02 04:42 +0000
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-02 10:10 +0200
                Re: alloca()-support Mel <mel@zzzzz.com> - 2019-10-02 15:13 +0200
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-01 21:10 +0200
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 21:25 +0200
                Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-10-01 20:44 +0000
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-01 23:26 +0200
                Re: alloca()-support Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-10-01 21:08 -0600
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-02 10:19 +0200
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-02 06:39 +0200
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-02 10:34 +0200
                Re: alloca()-support Mel <mel@zzzzz.com> - 2019-10-02 15:14 +0200
                Re: alloca()-support Mel <mel@zzzzz.com> - 2019-10-02 15:18 +0200
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-02 15:21 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-03 04:09 +0000
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-03 07:48 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-03 05:49 +0000
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-03 07:52 +0200
                Re: alloca()-support Melzzzzz <Melzzzzz@zzzzz.com> - 2019-10-03 06:46 +0000
                Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-03 10:59 +0200
              Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-10-03 13:39 +0200
                Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-10-03 13:23 +0000
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-03 15:38 +0200
                Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-10-03 14:51 +0000
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-04 09:49 +0200
                Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-10-03 21:47 +0200
                Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-04 09:53 +0200
                Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-10-03 21:52 +0200
            Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 07:39 +0200
        Re: alloca()-support James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-09-30 21:36 -0400
      Re: alloca()-support Philipp Klaus Krause <pkk@spth.de> - 2019-09-30 19:14 +0200
        Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 19:16 +0200
          Re: alloca()-support Robert Wessel <robertwessel2@yahoo.com> - 2019-10-01 00:18 -0500
            Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 07:43 +0200
            Re: alloca()-support Keith Thompson <kst-u@mib.org> - 2019-10-01 02:42 -0700
              Re: alloca()-support James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-10-01 08:11 -0400
              Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-01 14:53 +0200
      Re: alloca()-support scott@slp53.sl.home (Scott Lurndal) - 2019-09-30 17:52 +0000
    Re: alloca()-support Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-30 11:20 -0600
      Re: alloca()-support Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-30 19:42 +0200
        Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-09-30 20:02 +0200
      Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-09-30 19:54 +0200
        Re: alloca()-support Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-30 14:28 -0600
          Re: alloca()-support David Brown <david.brown@hesbynett.no> - 2019-10-01 09:38 +0200
      Re: alloca()-support Soviet_Mario <SovietMario@CCCP.MIR> - 2019-09-30 20:01 +0200
        Re: alloca()-support Manfred <noname@add.invalid> - 2019-10-01 17:45 +0200
  Re: alloca()-support Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-10-02 00:03 -0700
    Re: alloca()-support Paavo Helde <myfirstname@osa.pri.ee> - 2019-10-02 11:30 +0300
    Re: alloca()-support Ian Collins <ian-news@hotmail.com> - 2019-10-02 21:34 +1300
      Re: alloca()-support Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-10-02 01:36 -0700
        Re: alloca()-support Ian Collins <ian-news@hotmail.com> - 2019-10-02 21:41 +1300
          Re: alloca()-support Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-10-02 02:28 -0700
  Re: alloca()-support "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-10-02 02:35 -0700
    Re: alloca()-support Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-10-02 02:43 -0700
    Re: alloca()-support Anton Shepelev <anton.txt@gmail.com> - 2019-10-02 16:26 +0300
      Re: alloca()-support "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-10-02 12:33 -0700
        Re: alloca()-support Anton Shepelev <anton.txt@gmail.com> - 2019-10-03 01:47 +0300
          Re: alloca()-support "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-10-05 14:08 -0700

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