Making alloca more safe

[Denis Koroskin]

On Mon, 16 Nov 2009 20:39:57 +0300, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> Denis Koroskin wrote:
>> On Mon, 16 Nov 2009 19:27:41 +0300, Andrei Alexandrescu  
>> <SeeWebsiteForEmail at erdani.org> wrote:
>>
>>> bearophile wrote:
>>>> Walter Bright:
>>>>
>>>>> A person using alloca is expecting stack allocation, and that it  
>>>>> goes away after the function exits. Switching arbitrarily to the gc  
>>>>> will not be detected and may hide a programming error (asking for a  
>>>>> gigantic piece of memory is not anticipated for alloca, and could be  
>>>>> caused by an overflow or logic error in calculating its size).
>>>>  There's another solution, that I'd like to see more often used in  
>>>> Phobos: you can add another function to Phobos, let's call it salloca  
>>>> (safe alloca) that does what Denis Koroskin asks for (it's a very  
>>>> simple function).
>>>
>>> Can't be written. Try it.
>>>
>>> Andrei
>>  It's tricky. It can't be written *without a compiler support*, because  
>> it is considered special for a compiler (it always inlines the call to  
>> it). It could be written otherwise.
>>  I was thinking about proposing either an inline keyword in a language  
>> (one that would enforce function inlining, rather than suggesting it to  
>> compiler), or allways inline all the functions that make use of alloca.  
>> Without either of them, it is impossible to create wrappers around  
>> alloca (for example, one that create arrays on stack type-safely and  
>> without casts):
>>  T[] array_alloca(T)(size_t size) { ... }
>>  or one that would return GC-allocated memory when stack allocation  
>> fails:
>>  void* salloca(size_t size) {
>>     void* ptr = alloca(size);
>>     if (ptr is null) return (new void[size]).ptr;
>>      return ptr;
>> }
>
> The problem of salloca is that alloca's memory gets released when  
> salloca returns.
>
> Andrei

You missed the point of my post. I know it can't be implemented, and I  
told just that. I also told about 2 possible solutions to this issue.