Re: avr-crypto-lib

[David Brown]

On 29/05/2021 14:37, BERTRAND Joël wrote:
> Nigel Winterbottom a écrit :
>> I took a quick look and attempted to compile your Crypto library in
>> Visual Studio.
>>
>> I got nowhere: Probably for good reasons; Microsoft C doesn't allow
>> dynamic array size like this:
>>
>>   bigint_word_t d_b[a->length_W + b->length_W];
>>
>> This is about as far as I'm going to get on this, there is just far too
>> much work to get it to compile to run on the Desktop.
>> Wouldn't it be funny if thiat line was the reason for your crash.
> 
>       Do you think I have to replace these allocations by malloc()/free() or
> alloca() ? I haven't written theses routines, but I can try to fix them.
> But I would be sure my issue comes from this kind of allocation.
> 

A VLA like the one above is fine when using a decent C compiler - which
MSVC is not.  (It's an okay C++ compiler, but has traditionally failed
to support C99.  I gather it's a bit better with the most recent
versions, but it still has its own silly ideas about some features and
library functions.)

You do have to be aware that a VLA like this gets allocated on the data
stack.  If you are using an RTOS and have limited stack size, that can
quickly be an issue.  (alloca also allocates on the stack.)

If you can, you should try to allocate this array statically, using a
maximum size for the array.  That gives you a much clearer idea of the
sizes you need, and whether or not you have space for everything on the
device.  It avoids run-time complications or allocation failures, and
the risk of memory fragmentation (malloc/free should be avoided in
embedded systems whenever possible).  And it will be noticeably more
efficient on an AVR.

David