Re: Dynamic loading progress

[Philipp Stephani <p.stephani2@gmail.com>]

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Stephen J. Turnbull <turnbull@sk.tsukuba.ac.jp> schrieb am Mo., 14. Sep.
2015 um 05:43 Uhr:

> Daniel Colascione writes:
>  > On 09/13/2015 01:31 PM, Stefan Monnier wrote:
>
>  > >> It's not possible to skip frames in module code using longjmp, so
>  > >
>  > > Why not?
>  >
>  > Because most C code isn't expecting to be unwound. Forcing non-local
>  > flow control on module code is completely unacceptable.
>
> "Completely unacceptable" is nonsense.  Module code needs to be
> written the same way any other Emacs code is written,


Why? The module interface defines an interface, not an implementation
strategy. The observable behavior of Emacs is described in the Emacs
manual, and that contains nothing about longjmp (unsurprisingly, given that
it's an implementation detail).


> using various
> unwind-protect constructs.


It is not possible to write such constructs in general. Stack frames that
don't originate from C programs simply can't be part of a non-local jump.
unwind-protect doesn't help at all because it doesn't stop non-local jumps.


>
>

> And I'm sure that there's non-robust code in libraries out there that
> will be totally horked on a non-local exit.  But AFAICS you have the
> same problem there if you link statically or at Emacs initialization
> -- that code just shouldn't be linked into Emacs period.
>
>
>
Such a decision would massively reduce the usefulness of modules. It would
prevent dynamic modules in any language other than C, and even for C
modules implementors would need to be careful not to write longjmp-hosting
code. This is indeed unacceptable.

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