Re: Need for scm_remember_upto_here_* in guile-2.0

[Ludovic Courtès]

Mark H Weaver <address@hidden> skribis:

> David Kastrup <address@hidden> writes:
>
>> address@hidden (Ludovic Courtès) writes:
>>
>>> In practice scm_remember_upto_here_* is useless with libgc, since libgc
>>> also tracks pointers in registers.
>>
>> Sounds like a big non-sequitur.  scm_remember_upto_here_* is for keeping
>> SCM values active (including in registers).  Any conversion of an SCM
>> value to some other data depending on it will not protect the SCM value
>> as such and may lead to collection/finalization of the SCM cell (calling
>> guardians, removing from weak hashtables and so on).
>
> I think David is right.  Consider the following case: a SMOB contains a
> pointer to a block allocated by 'malloc' and freed by the SMOB freeing
> procedure.  Some C code starts with a SCM value for that SMOB, uses it
> to obtain a pointer to the malloc'd block, and then discards the SCM
> value and proceeds to do a long-running job on the malloc'd block.  If
> there are no other references to the SCM value, Boehm GC can free the
> SMOB and call its finalizer, which frees the malloc'd block.  The
> long-running job in C continues to access the freed block.

Indeed, that’s a valid scenario.

My “in practice” assumed no finalizers and only GC-managed memory,
I guess.

Ludo’.