HEADS UP: finalizer changes coming in GHC 6.10.2

Simon Marlow marlowsd at gmail.com
Wed Jan 14 07:14:18 EST 2009

By popular demand, GHC 6.10.2 will support finalizers that are actually 
guaranteed to run, and run promptly.  There aren't any API changes: this 
happens for finalizers created using newForeignPtr as normal.

However, there's a catch.  Previously it was possible to call back into 
Haskell from a finalizer (finalizers are C functions), by using foreign 
import "wrapper" or foreign export.  According to the FFI spec, whether 
this is allowed or not is "system dependent".  In GHC 6.10.1 and earlier it 
was allowed, but in 6.10.2 and later it is not.  The reason being that C 
finalizers are now executed directly by the GC, when the runtime is not in 
a position to handle callbacks.

You can still have fully-fledged Haskell finalizers if you want: we have 
Foreign.Concurrent.newForeignPtr for that purpose.  But those finalizers 
are not subject to the same promptness guarantees that you get with C 
finalizers - they run in a separate thread, and are not guaranteed to be 
run at program exit.

We'll make sure this is documented prominently in the 6.10.2 release notes, 
but I thought a heads-up would be a good idea right now as it turns out 
that there are existing libraries (e.g. LLVM) that will be affected.


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