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Re: Limitations and workarounds to expressions defining size of an array

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On 09/04/2012 01:14 PM, Francois Grieu wrote:
> I'm trying to figure out the exact limitations that (possibly:
> various versions of ISO/IEC 9899) C puts to expressions defining
> the size of a plain array at compile time, and how to workaround
> that.


In C99:
6.7.5.2p1: "... the expression shall have an integer type. If the
expression is a constant expression, it shall have a value greater than
zero."
6.7.5.2p2: "... If an identifier is declared to be an object with static
storage duration, it shall not have a variable length array type."
6.7.5.2p4: "... If the size is an integer constant expression and the
element type has a known constant size, the array type is not a variable
length array type; otherwise, the array type is a variable length array
type."
6.7.5.2p5: "If the size is an expression that is not an integer constant
expression expression: if it occurs in a declaration at function
prototype scope, it is treated as if it were replaced by *; otherwise,
each time it is evaluated it shall have a value greater than zero."

C2011:
Change 6.7.5 => 6.7.6
Add "thread storage duration" as another context where VLAs are prohibited.
Implementation support for VLAs is now optional.

C90:
I don't have a copy of C90 to confirm the exact details of the wording,
but VLAs were added in C99, and were therefore not an option in C90.

> An application would be, given EXPECTED, to define at compile
> time an array with EXPECTED + 12*sqrt(EXPECTED) elements, or
> some even more complex formula, as required by
> http://stats.stackexchange.com/q/35658/8469
> 
> 
> A tentative (not tested)
> 
> #define EXPECTED 15000
> 
> /* define an array of EXPECTED + 12*sqrt(EXPECTED)
>    pointers to struct foo (or slightly more) */
> struct foo * bar[
> #if (EXPECTED)<900
>   #error "unsupported EXPECTED value"
>   -1
> #elif (EXPECTED)<3600
>   (EXPECTED)+360+((EXPECTED)-896)/5
> #elif (EXPECTED)<14400
>   (EXPECTED)+720+((EXPECTED)-3591)/10
> #else
>   (EXPECTED)+1440ul+((EXPECTED)-14381ul)/20
> #endif
>   ];
> 
> I think the above works for any EXPECTED>=900 up to
> some large value like ULONG_MAX-ULONG_MAX/19 and then some,

Correct. All of your expressions have integer type and a constant value,
and except for the first one, have a positive value. Unless I missed
something, they should be acceptable in all versions of the standard.

> and errs on the safe side, with at worse 5% wasted memory;
> but this is
> - unclear at best;
> - painful to extend to lower bounds or less wasted memory;
> - hard to generalize to more complex formula involving
>   for example log or exp.
> 
> Can I
> - use floating point at all ?
> - if yes: use sqrt, log, exp ?

In C90, "No" to both questions. For arrays with static or thread storage
duration, no. However, in C99, for arrays with automatic storage
duration, this is permitted, so long as the entire expression containing
those sub-expressions has an integer type and a positive value. In
C2011, it depends upon whether or not __STDC_NO_VLA__ is pre-#defined by
the implementation.

> - or at least, use intermediate values (other than
>   macro identifiers) to clarify things and remain below
>   the "4095 characters in a logical source line" limit?

Whether or not that's permitted depends upon precisely what you mean by
that phrase - it's not clear from context.

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Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-04 19:14 +0200
  Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-04 14:08 -0400
    Re: Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-05 09:56 +0200
      Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 07:33 -0400
        Re: Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-05 23:39 +0200
          Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-05 18:11 -0400
    Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 07:17 -0400
  Re: Limitations and workarounds to expressions defining size of an array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-04 19:18 +0100
  Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 18:41 +0000
    Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-04 20:22 +0100
      Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-04 17:07 -0400
        Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-04 17:25 -0400
          Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-04 18:24 -0400
            Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-04 17:29 -0700
        Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-04 21:26 +0000
          Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-04 21:31 +0000
            Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-04 17:41 -0400
              Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-05 20:45 +0000
                Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 17:23 -0400
                Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-05 21:30 +0000
                Re: Limitations and workarounds to expressions defining size of an array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-05 23:04 +0100
                Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-05 18:17 -0400
                Re: Limitations and workarounds to expressions defining size of an array James Kuyper <jameskuyper@verizon.net> - 2012-09-05 18:29 -0400
            Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 21:44 +0000
      Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 21:39 +0000
        Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-04 22:58 +0100
          Re: Limitations and workarounds to expressions defining size of an array Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 22:12 +0000
          Re: Limitations and workarounds to expressions defining size of an array Lowell Gilbert <lgusenet@be-well.ilk.org> - 2012-09-04 18:55 -0400
      Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-04 16:51 -0700
        Re: Limitations and workarounds to expressions defining size of an array Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-09-04 20:58 -0400
          Re: Limitations and workarounds to expressions defining size of an array Francois Grieu <fgrieu@gmail.com> - 2012-09-05 10:04 +0200
          Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 04:47 -0700
            Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-07 14:04 -0700
              Re: Limitations and workarounds to expressions defining size of an array Casey Carter <casey_at_carter_dot_net@nospam.invalid> - 2012-09-07 16:40 -0500
              Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 19:54 -0700
                Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-08 14:15 -0700
                Re: Limitations and workarounds to expressions defining size of an array lawrence.jones@siemens.com - 2012-09-17 12:09 -0400
        Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 04:53 -0700
          Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-07 13:07 +0100
            Re: Limitations and workarounds to expressions defining size of an array Keith Thompson <kst-u@mib.org> - 2012-09-07 14:08 -0700
            Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 19:41 -0700
  Re: Limitations and workarounds to expressions defining size of an array "BartC" <bc@freeuk.com> - 2012-09-04 20:26 +0100
  Re: Limitations and workarounds to expressions defining size of an array Barry Schwarz <schwarzb@dqel.com> - 2012-09-04 14:14 -0700
  Re: Limitations and workarounds to expressions defining size of an array Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 04:41 -0700
  Re: Limitations and workarounds to expressions defining size of an array army1987 <army1987@ask-for-it.invalid> - 2012-09-07 13:32 +0000

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