Re: Need for scm_remember_upto_here_* in guile-2.0

[Mark H Weaver]

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.

Am I missing something?

      Mark