Making alloca more safe

[Andrei Alexandrescu]

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