[cffi-devel] Re: WITH-FOREIGN-STRING allocates on heap?

[Chun Tian (binghe)]

Hi, cffi-devel

As an addition, I tested your code on LispWorks 5.1.1 (64-bit Linux  
Edition), seem there's no limit on stack size, though LispWorks has a  
default value, when it reached, I got a prompt to decide whether  
increase it (by 50%/300%).

Some outputs when I break the run manually (and LispWorks fine, no  
error):

[60114] #<Pointer to type :BYTE = #x2AAB276F38C0>: total allocated  
3,077,836,800
[60115] #<Pointer to type :BYTE = #x2AAB277000D0>: total allocated  
3,077,888,000
[60116] #<Pointer to type :BYTE = #x2AAB2770C8E0>: total allocated  
3,077,939,200

P.S. By looking at LispWorks FLI, I cannot find any option in FLI:WITH- 
FOREIGN-STRING. Don't know how to control a foreign string be  
allocated in heap or stack...

--binghe

> Many Lisps seem to operate with much smaller stacks.  Here's the  
> test I
> used:
>
>  (defconstant +size+ (* 50 (expt 2 10)))        ; 50 KB
>
>  (defun test ()
>    (labels ((ek (n)
>               (cffi:with-foreign-pointer (p #.+size+)
>                 (loop for i below +size+
>                       do (setf (cffi:mem-ref p :char i) 0))
>                 (format t "[~A] ~A: total allocated ~:D~%"
>                         n p (* +size+ n))
>                 (ek (1+ n)))))
>      (ek 1)))
>
>  (compile 'test)
>
>
> SBCL: ~2 MB followed by "the party is over."
> CCL: ~2 MB followed by a segfault.
> CLISP: 8 MB followed by a graceful stack overflow.
> Allegro: ~4 MB followed by a graceful stack overflow.
> (used 40 KB chunks otherwise Allegro refused to do stack allocation)
>
> This is on linux/amd64 (but I think my copy of Allegro is a 32-bit
> version).  Hopefully my test is not (too) bogus.
>
>
>> Given this, I'd think that making stack allocation be the default and
>> heap allocation an option would be beneficial.  If you really don't
>> like it, making the default a compile-time option would be better  
>> than
>> mandating an allocation policy, I think.
>
> Since the size argument has to be known at compile-time (IIRC), maybe
> WITH-FOREIGN-POINTER can look at that size and decide if it should
> fallback to heap allocation.
>
> CCL and Allegro do this.  SBCL and CLISP don't.  If we did that check
> ourselves we'd get more consistent behaviour across the various  
> Lisps we
> support and that might reduce the likelihood of overflowing the stack
> with big objects, particularly strings.  Also, it would be nice if  
> SBCL
> and CCL could handle stack overflows without crashing.
>
> -- 
> Luís Oliveira
> http://student.dei.uc.pt/~lmoliv/
>
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