mallocForeignPtr vs. C

[Axel Simon]

Hi Evan, Ed,

On Jul 12, 2010, at 22:53, Edward Z. Yang wrote:

> Excerpts from Evan Laforge's message of Mon Jul 12 16:43:45 -0400  
> 2010:
>> Yeah, that's definitely the safest and simplest.  But the copying
>> defeats the purpose of passing a pointer in the first place, which  
>> was
>> to not have to copy the giant array just to pass it.
>
> Well, if your C code wasn't squirreling away the pointer, you could  
> have
> avoided the copy.  Memory copies in C are suprisingly cheap; I'm  
> really
> surprised how often I see memcpy() in high performance code (though,  
> it may be
> the case that the details of the algorithm are a much more important  
> influence
> on performance.)

If your C code has a way to properly unref a pointer then you could  
wrap your ForeignPtr in a StablePtr and pass that to C land. Once C  
has freed the StablePtr the ForeignPtr can become dead when Haskell  
has dropped all references and it will be garbage collected.

The downside is that your C code needs to hold the StablePtr and do  
the accesses through the Ptr in the ForeignPtr, so it needs to be able  
to store two pointers at once.

Axel