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Re: sizeof on an uninitialized pointer to a VLA

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James Kuyper <jameskuyper@verizon.net> writes:
> On 11/19/2015 02:39 PM, John Bode wrote:
>> I've been using the following idiom a lot to dynamically allocate
>> 2D arrays:
>> 
>>     T (*arr)[N] = malloc( sizeof *arr * M );
>
> M=>N?
>
>> Works great where N is known at compile time.  However, I've also been
>> applying it to VLAs, like so:
>> 
>>     size_t rows;
>>     size_t cols;
>>     ...
>>     T (*arr)[rows] = malloc( sizeof *arr * cols );
>> 
>> I've never had any issues, but...does this count as undefined behavior?  
>> `sizeof` evaluates the operand when it's a VLA, but what does "evaluate"
>> mean in this context?  Is it trying to dereference the pointer, or is it
>> simply determining the size of *any* object the pointer may refer to?  
>
> The key thing about VLA types is that their sized depends upon the value
> of one or more expressions, in this case the relevant expression is
> "rows". The evaluation it's talking about is the evaluation of that
> expression. Unless the expression has side-effects (for example,
> rows++), the possibility of it being evaluated is not an issue you need
> to worry about.

No, the expression `rows` is not evaluated.

Consider:

    size_t rows = 10;
    char vla[rows];
    rows = 20;
    sizeof vla;

`sizeof vla` is 10, not 20.  Evaluating `rows` is not just
unnecessary; it gives the wrong result.

I think the standard is just being sloppy to the point of being
incorrect here.  For non-VLA types, `sizeof expr` and `sizeof
(typename)` are evaluated at compile time.

For an expression of a VLA type, `sizeof expr` has to be determined
at run time, but the only thing that actually needs to be evaluated
is the stored value of whatever anonymous object the compiler
uses to remember the size (with perhaps some other computation if,
for example, the stored value is an element count).  Neither the
expression that determined the size (whose value may not be the same)
nor the value of the array object (which consists of the values of
its elements) actually needs to be evaluated.

For `sizeof` applied to a parenthesized VLA type, any expressions
that are part of the type name have to be evaluated.  For example,
in `sizeof (char[func()])`, the function has to be called and any
side effects must occur; similarly for `sizeof (char[n++])`.

The standard doesn't actually manage to say all this, but I think
that's the only reasonable interpretation of the intent.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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sizeof on an uninitialized pointer to a VLA John Bode <jfbode1029@gmail.com> - 2015-11-19 11:39 -0800
  Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 12:09 -0800
  Re: sizeof on an uninitialized pointer to a VLA "A. Bolmarcich" <aggedor@earl-grey.cloud9.net> - 2015-11-19 21:40 +0000
    Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 13:50 -0800
  Re: sizeof on an uninitialized pointer to a VLA James Kuyper <jameskuyper@verizon.net> - 2015-11-19 17:50 -0500
    Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 15:31 -0800
    Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-20 01:09 +0000
      Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 19:10 -0800
    Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 03:52 -0800
      Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-20 14:24 +0000
        Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-20 10:25 -0800
        Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 18:29 -0800
          Re: sizeof on an uninitialized pointer to a VLA Ian Collins <ian-news@hotmail.com> - 2015-11-21 16:35 +1300
            Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 20:24 -0800
              Re: sizeof on an uninitialized pointer to a VLA Ian Collins <ian-news@hotmail.com> - 2015-11-21 17:38 +1300
                Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 21:08 -0800
          Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 10:46 +0000
            Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-21 03:26 -0800
              Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 12:46 +0000
                Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 14:01 +0000
              Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-21 09:02 -0800
                Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-21 12:09 -0800

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