Re: [Chicken-users] Stack space allocation

[John Lenz]

> > What I am concerned about is alloca calls inside a C block.  I am   
> > unsure if the compiler might be optimizing stack space.  That is,  
> > since  I create three blocks, it would assume that stack space can  
> > be shared  between them and then the alloca call would return the  
> > same space to  all three blocks.
>
> Yes, I agree that this might be dangerous. It's preferable to alloc  
> all
> the storage at once. C_pair will update the known_space pointer
> accordingly on every initialization.

Well, after a bunch of searching, I found this
http://www.gnu.org/software/libc/manual/html_node/GNU-C-Variable-Size-Arrays.html

which seems to imply that alloca persists until the function returns,  
unlike arrays which last until the end of the block.  The man page for  
alloca also states that the memory persists until the end of the  
function call.  So I will write the SWIG interface functions to assume  
that alloca will always be unique.


> It usually is (even though it doesn't _have_ to be). If you can be  
> sure that
> the compiler wont optimize things away (and I don't know right now),  
> then
> the former approach might be ok.
> Are the lists potentially very long?

Well, this is for the SWIG wrapper, so the list will be as long as  
whoever is using SWIG inputs.  Ideally, we would not like to place  
restrictions on the size, so that SWIG will work for whatever purpose.   
Is there a better way to allocate data space?  alloca works for things  
like return values and a few paramaters, but for very long lists, it is  
probably a bad idea to stick all that allocation on the stack.  (maybe  
it is a good idea...?)

John