FFI Ptr question
Thu, 3 Jul 2003 11:26:07 +0100
> > The wierd thing is, I cannot use the the function
> > Foreign.Marshal.Alloc.free to free the ptr. I must
> > use my own C function to do a simple free() call.
Derek Elkins wrote:
> You didn't allocate it with the Foreign library, why would you expect to
> free it with it? Actually, I'm kind of surprised it doesn't work,
> nevertheless I don't believe that the intent of the Alloc module is to
> free memory allocated by a foreign language, or allocate memory that
> will be freed by a foreign language. [...]
Foreign.Marshal.Alloc almost certainly uses either the allocator that is
native to the language used to implement the Haskell runtime system. This is
usually C so using the C 'free' function will often work but if your Haskell
code were translated to Java instead, it would probably be allocated by a
Java library and should be freed by the same Java library.
Or Foreign.Marshal.Alloc might be optimized by exploiting the Haskell garbage
collector in some way and so the memory has to be released in a way that
informs the Haskell GC.
In general (not just in Haskell's FFI), any time you allocate memory from a
given memory pool or library, you should free it into the same memory pool/
library. (We tend not to be aware of this rule because most programs have
just one allocator.)
We could have avoided this problem by defining Foreign.Marshal.Alloc as a
frontend for malloc/free. We didn't do so mostly because
Foreign.Marshal.Alloc is intended to be language independent (i.e., your code
should still work if interfacing to Java or if your Haskell code is compiled
to/ interpreted by a Java program) and, in small part, because malloc/free
implementations are so damn inefficient.