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Detecting Incomplete Array Initialization at Compile Time?

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  Detecting Incomplete Array Initialization at Compile Time? "David T. Ashley" <dashley@gmail.com> - 2016-07-21 11:39 -0700
    Re: Detecting Incomplete Array Initialization at Compile Time? Ken Brody <kenbrody@spamcop.net> - 2016-07-21 14:47 -0400
      Re: Detecting Incomplete Array Initialization at Compile Time? Keith Thompson <kst-u@mib.org> - 2016-07-21 12:42 -0700
    Re: Detecting Incomplete Array Initialization at Compile Time? Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-21 20:07 -0700

#90735 — Detecting Incomplete Array Initialization at Compile Time?

Found a bug related to incomplete array initialization yesterday.  The form of the bug was:

enum 
  {
  Apples,
  Oranges,
  Grapefruits,
  MaxElements
  };

int myarray[MaxElements] =
   {
   429,
   258
   //Oops, missing initializer--not intended.
   //C-language defaults to 0.
   };

Question:  Is there any way to detect incomplete initialization at compile-time?  (Any obscure feature of the language I'm not aware of?)

Caveat:  I've oversimplified the problem for the purposes of example.  Also, this occurred in an embedded system, and doing anything at runtime is off the table.

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#90736

On 7/21/2016 2:39 PM, David T. Ashley wrote:
> Found a bug related to incomplete array initialization yesterday.  The form of the bug was:
>
> enum
>   {
>   Apples,
>   Oranges,
>   Grapefruits,
>   MaxElements
>   };
>
> int myarray[MaxElements] =
>    {
>    429,
>    258
>    //Oops, missing initializer--not intended.
>    //C-language defaults to 0.
>    };
>
> Question:  Is there any way to detect incomplete initialization at compile-time?  (Any obscure feature of the language I'm not aware of?)
>
> Caveat:  I've oversimplified the problem for the purposes of example.  Also, this occurred in an embedded system, and doing anything at runtime is off the table.
>

This might do what you want:

enum
   {
   Apples,
   Oranges,
   Grapefruits,
   MaxElements
   };

int myarray[] = /* Leave off the array size here. */
    {
    429,
    258
    //Oops, missing initializer--not intended.
    //C-language defaults to 0.
    };

static char MyArraySizeCheck[ ( sizeof(myarray)/sizeof(myarray[0]) )
                               == MaxEmements ? 1 : -1 ];

If the size is "wrong", then the compiler will complain about the "-1" array 
size on "MyArraySizeCheck".

-- 
Kenneth Brody

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#90738

Ken Brody <kenbrody@spamcop.net> writes:
> On 7/21/2016 2:39 PM, David T. Ashley wrote:
>> Found a bug related to incomplete array initialization yesterday.  The form of the bug was:
>>
>> enum
>>   {
>>   Apples,
>>   Oranges,
>>   Grapefruits,
>>   MaxElements
>>   };
>>
>> int myarray[MaxElements] =
>>    {
>>    429,
>>    258
>>    //Oops, missing initializer--not intended.
>>    //C-language defaults to 0.
>>    };
>>
>> Question:  Is there any way to detect incomplete initialization at compile-time?  (Any obscure feature of the language I'm not aware of?)
>>
>> Caveat:  I've oversimplified the problem for the purposes of example.  Also, this occurred in an embedded system, and doing anything at runtime is off the table.
>>
>
> This might do what you want:
>
> enum
>    {
>    Apples,
>    Oranges,
>    Grapefruits,
>    MaxElements
>    };
>
> int myarray[] = /* Leave off the array size here. */
>     {
>     429,
>     258
>     //Oops, missing initializer--not intended.
>     //C-language defaults to 0.
>     };
>
> static char MyArraySizeCheck[ ( sizeof(myarray)/sizeof(myarray[0]) )
>                                == MaxEmements ? 1 : -1 ];
>
> If the size is "wrong", then the compiler will complain about the "-1" array 
> size on "MyArraySizeCheck".

That's a hack to implement a static assert.  (I'm not using the word
"hack" negatively.)

If you have C11 support, or a compiler that implements it as an
extension, you can just use static assert:

    _Static_assert(sizeof myarray / sizeof myarray[0] == MaxElements,
                   "Extra or missing elements");

(I'd suggest using #if and #error, but sizeof can't be evaluated in
preprocessor expressions.)

-- 
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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#90750

"David T. Ashley" <dashley@gmail.com> writes:

> Found a bug related to incomplete array initialization yesterday.
>  {} The form of the bug was:
>
> enum
>   {
>   Apples,
>   Oranges,
>   Grapefruits,
>   MaxElements
>   };
>
> int myarray[MaxElements] =
>    {
>    429,
>    258
>    //Oops, missing initializer--not intended.
>    //C-language defaults to 0.
>    };
>
> Question:  Is there any way to detect incomplete initialization at
> compile-time?  (Any obscure feature of the language I'm not aware
> of?)
>
> Caveat:  I've oversimplified the problem for the purposes of example.
> Also, this occurred in an embedded system, and doing anything at
> runtime is off the table.

If the code is being (or can be) compiled under C99 or later,
there is a way that's a little bit clunky but probably not so bad
as to be unusable.  First assume there is a STATIC_ASSERT() macro
of a form like those seen as examples from time to time (eg,
using an array bound that is either 1 or -1 depending).  Define
the set of initial array values as a macro, eg (disclaimer:
all the code below just typed in by hand, not compiled)

  #define myarray_INITIALIZER { \
     429, \
     258  \
  }

Next define the array using the initial values macro to do the
initialization:

  int myarray[MaxElements] = myarray_INITIALIZER;

Next do a static assertion to check the initializer count, using
sizeof with a compound literal to see how many there are:

  STATIC_ASSERT(
    sizeof myarray == sizeof ( (int[]) myarray_INITIALIZER )
  );

This technique gives a compile-time check with no run-time cost.
It does require the element type to be supplied a second time,
but that seems unlikely to cause any serious problems.

If the pattern seems burdensome, probably a macro can be defined
that both defines the array and does the size check, without too
much difficulty.  Maybe a PITA to write, but once written should
be usable enough to be worth the effort.

I know this is just a rough sketch, hopefully the details
are simple enough so they can be filled in with no problem.

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