Foreign.StablePtr: nullPtr & double-free questions.

Edward Z. Yang ezyang at MIT.EDU
Sat Mar 9 06:23:16 CET 2013

Excerpts from Remi Turk's message of Fri Mar 08 18:28:56 -0800 2013:
> Good night everyone,
> I have two questions with regards to some details of the
> Foreign.StablePtr module. [1]
> 1) The documentation suggests, but does not explicitly state, that
>   castStablePtrToPtr `liftM` newStablePtr x
> will never yield a nullPtr. Is this guaranteed to be the case or not?
> It would conveniently allow me to store a Maybe "for free", using
> nullPtr for Nothing, but I am hesitant about relying on something that
> isn't actually guaranteed by the documentation.

No, you cannot assume that.  In fact, stable pointer zero is

    ezyang at javelin:~/Dev/haskell$ cat sptr.hs
    import Foreign.StablePtr
    import Foreign.Ptr

    main = do
        let x = castPtrToStablePtr nullPtr
        freeStablePtr x
    ezyang at javelin:~/Dev/haskell$ ~/Dev/ghc-build-tick/inplace/bin/ghc-stage2 --make sptr.hs -debug 
    [1 of 1] Compiling Main             ( sptr.hs, sptr.o )
    Linking sptr ...
    ezyang at javelin:~/Dev/haskell$ gdb ./sptr
    GNU gdb (GDB) 7.5-ubuntu
    Copyright (C) 2012 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <>
    This is free software: you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
    and "show warranty" for details.
    This GDB was configured as "x86_64-linux-gnu".
    For bug reporting instructions, please see:
    Reading symbols from /srv/code/haskell/sptr...done.
    (gdb) b freeStablePtrUnsafe
    Breakpoint 1 at 0x73f8a7: file rts/Stable.c, line 263.
    (gdb) r
    Starting program: /srv/code/haskell/sptr 
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/lib/x86_64-linux-gnu/".

    Breakpoint 1, freeStablePtrUnsafe (sp=0x0) at rts/Stable.c:263
    263	    ASSERT((StgWord)sp < SPT_size);
    (gdb) list 
    258	}
    260	void
    261	freeStablePtrUnsafe(StgStablePtr sp)
    262	{
    263	    ASSERT((StgWord)sp < SPT_size);
    264	    freeSpEntry(&stable_ptr_table[(StgWord)sp]);
    265	}
    267	void
    (gdb) p stable_ptr_table[(StgWord)sp]
    $1 = {addr = 0x9d38e0}
    (gdb) p *(StgClosure*)stable_ptr_table[(StgWord)sp]
    $2 = {header = {info = 0x4e89c8 <base_GHCziTopHandler_runIO_info>}, payload = 0x9d38e8}

Regardless, you don't want to do that anyway, because stable pointers
have a bit of overhead.

> 2) If I read the documentation correctly, when using StablePtr it is
> actually quite difficult to avoid undefined behaviour, at least in
> GHC(i). In particular, a double-free on a StablePtr yields undefined
> behaviour. However, when called twice on the same value, newStablePtr
> yields the same StablePtr in GHC(i).
> E.g.:
> module Main where
> import Foreign
> foo x y = do
>     p1 <- newStablePtr x
>     p2 <- newStablePtr y
>     print $ castStablePtrToPtr p1 == castStablePtrToPtr p2
>     freeStablePtr p1
>     freeStablePtr p2 -- potential double free!
> main = let x = "Hello, world!" in foo x x -- undefined behaviour!
> prints "True" under GHC(i), "False" from Hugs. Considering that foo
> and main might be in different packages written by different authors,
> this makes correct use rather complicated. Is this behaviour (and the
> consequential undefinedness) intentional?

I think this bug was inadvertently fixed in the latest version of GHC;

    commit 7e7a4e4d7e9e84b2c57d3d55e372e738b5f8dbf5
    Author: Simon Marlow <marlowsd at>
    Date:   Thu Feb 14 08:46:55 2013 +0000

        Separate StablePtr and StableName tables (#7674)

        To improve performance of StablePtr.


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