Cliff Jansen commented on PROTON-57:
I regret the title of this JIRA, as it unintentionally implies a narrow scope
and may not attract the full intended audience. Visual Studio on Windows is
indeed the working platform but the questions are really about building proton
using C++ compilers in general.
The underlying question is: how hard would it be to make proton-c work in C++
as an additional target language? Proton is written in C99, which has a large,
but not perfect overlap with C++. So perhaps ultimately, the question boils
down to: is the intersection of C++ and C99 a rich enough subset of C for long
term proton-c work?
For some perspective on Mary's work, she has been investigating this in detail
(using VS2010) and she has found that the overall impact looks promisingly
manageable. But some intrusive changes are required. Variable length arrays
are one such issue and show up frequently in the code. It would be nice to
have some community feedback on this issue in advance of an actual patch.
> Proton porting problems between current codebase and Visual Studio 2010
> 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
> #define PN_VLA(TYPE, buf, size) TYPE *buf = (TYPE*)
> #define PN_VLA_FREE(buf) _freea(buf)
> 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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