[gambit-list] os_dyn

Marc Feeley feeley at iro.umontreal.ca
Tue Oct 6 10:51:36 EDT 2009


On 6-Oct-09, at 8:37 AM, Michele Zaffalon wrote:

>> It is quite probable that OpenBSD forbids executing code in the C  
>> heap, for
>> security reasons I guess.  Usually there is a way to tell the  
>> operating
>> system that a certain range of addresses contains executable code.   
>> I have
>> no clue what this is on OpenBSD.  If you do find out or someone on  
>> the list
>> knows, please let me know.
>
> Is this the only way to have a Scheme defined function passed on the
> fly to C? Short of using global variables as in here
> https://mercure.iro.umontreal.ca/pipermail/gambit-list/2009-January/002939.html

Well, the only solutions I find acceptable.  There might be some (more  
or less portable) library to implement C closures that could be used  
(libffi?).  You have to understand what the fundamental problem is.   
In C, functions pointers are implemented (usually) as the address of  
the code which implements the function.  The C compiler generates a  
"call" instruction to that address.  Scheme closures carry with them  
an environment... that is what makes them more useful than plain  
functions.  So to convert a Scheme closure to a C function pointer, in  
general it is necessary to allocate a fresh code address that will  
correspond to the Scheme closure.  The technique shown in the message  
linked above is based on the idea of allocating at compile time a pool  
of C functions, and then to assign them on demand to Scheme closures.   
It is 100% portable, but it places a static limit on the number of  
Scheme closures that can be converted (the code could be improved to  
un-assign the Scheme closure when it is no longer live using wills for  
example).  The other solution, based on dynamic code generation, does  
not place a limit on the number of converted Scheme closures, but it  
is not portable (it is CPU and OS dependent).  There is no portable  
solution I know of that solves both issues (portable here means it  
respects the C standard and only relies on the C standard).

Marc



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