Cliff Jansen commented on PROTON-57:

Mary, to make your question more clear, could you please attach a small 
representative code snippet from proton, showing the existing code and what it 
would look like after your proposed changes?  Then people can more easily 
visualize the impact your changes would have on the existing code.

I detect two separate questions that you are mingling:

  - How best to mimic VLAs in proton?

  - Are VLAs inherently too dangerous for proton to use, at least sometimes?

The former is the one I would like to focus on.  Presumably, if there is a 
design flaw of using stack (instead of heap/malloc) for temporary storage, that 
affects the existing proton C99 code as much as a C++ implementation.  I would 
prefer that gets a separate JIRA and doesn't cloud the compiler related issue.

Assuming the use of the VLA is appropriate in a particular instance, I believe 
an alloca from within C++ is appropriate.  This has the benefit of requiring no 
associated free().  It has the downside that it doesn't support a realloc (at 
least on all platforms) so code that reuses a VLA with a new size is 
problematic.  Hopefully those cases are few and can be reworked.

> Proton porting problems between current codebase and Visual Studio 2010 
> toolset
> -------------------------------------------------------------------------------
>                 Key: PROTON-57
>                 URL: https://issues.apache.org/jira/browse/PROTON-57
>             Project: Qpid Proton
>          Issue Type: Improvement
>          Components: proton-c
>         Environment: Windows using Visual Studio 2010
>            Reporter: Mary hinton
>              Labels: build
> This thread will be used to discuss the porting problems encountered using 
> Visual Studio 2010
> Here’s the first one to discuss:
> 1. Visual Studio doesn’t support variable length arrays. 
>     a.  Currently using malloc()/realloc() in my port just to get it to 
> compile and be able to report memory allocation errors. This is not what I 
> want to submit to the proton group for memory allocation.
>     b.  Cliff had a good method that included  setting up macros and replace 
> the VLAs with  alloca() in the Windows version, but it could still cause 
> problems when the stack overflowed. VLAs can also run out of stack space.
>     c.  _malloca() should be a better way than _alloca() on Visual Studio. 
> Any messages under 1K would be allocated out of the stack. Over 1K will use 
> heap. If the average messages are under 1K, this may be the most efficient 
> way in Visual Studio. _malloca() also has new security enhancements. 
>         1.  Using _malloca() in the Windows version and VLA in Linux would 
> require two macros. The major difference for the Linux version would be to 
> use the new macro for VLA and to include the new free macro even though there 
> is nothing to free using VLA.  In Visual Studio, _freea(buf) will not free 
> anything if it is allocating off the stack.
> Linux can continue to use VLAs.
> #ifdef C99                    
>         #define PN_VLA(TYPE, buf, size)     TYPE buf[size]
>         #define PN_VLA_FREE
> #else
>         #define PN_VLA(TYPE, buf, size)     TYPE *buf = (TYPE*)  
> _malloca(size)
>        #define PN_VLA_FREE(buf)              _freea(buf)      
> #endif
>     d. If the average size messages to allocate out of the stack needs to be 
> increased for performance reasons, we can set up a new memory model. The 1K 
> is not adjustable for _malloca().
> We can set up new macros along the lines of Microsoft’s suggestion below.
>  “I would suggest something similar to what ATL does.  Use alloca for small 
> (where you define what that is) sizes and use heap allocation for larger 
> ones.  You can wrap the logic inside of a class or macros.  To work around 
> the fact that alloca isn't cleaned up at block scope, rewrite the block into 
> functions or use lambdas.  (I think alloca works inside of lambdas.)”

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