Re: [Caml-list] Garbage collector woes

[Jeremy Yallop <yallop@gmail.com>]

On 4 July 2016 at 18:06, Christoph Höger <christoph.hoeger@tu-berlin.de> wrote:
> Am 04.07.2016 um 18:21 schrieb Jeremy Yallop:
>> I'm guessing that 'desc' points to a block and N, MFIT and ICONV are
>> field accessors.
>
> That is correct. N, MFIT and ICONV are macros defined as Field(.., ..).
>
>> If so, you need to register 'desc' as a root here, since
>> caml_copy_double or caml_ba_alloc_dims could trigger a gc, moving desc
>> before it's next used.
>
> By registering as a root you mean to also declare it via CAMLlocal?

Yes, that's a reasonable way to do it.

> Is this necessary/appropriate, since &desc is already a registered global root.

In fact, &desc is not a registered global root, since desc is a local
variable, with its own fresh address.  It's user_data that's a global
root, and desc is simply a copy of the contents of user_data.  So when
the GC updates *user_data, desc is left unchanged.

> Do you have any clue how that moving of desc could trigger the
> overwriting of x and cond?

If the GC replaces the value that desc points to with some other value
that's then invoked as a callback, pretty much anything can happen.
(Of course, it's also possible that the problem lies somewhere else
entirely.)