Calling Lisp in C functions

Calling Lisp in C functions

[Yuan Fu]

What’s the best way to evaluate Lisp or call a Lisp function that could signal in a C function and then free allocated memory? I assume that I can’t just call the Lisp function and hope it doesn’t signal.

Yuan

Re: Calling Lisp in C functions

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Tue, 21 Sep 2021 12:38:21 -0700
> 
> What’s the best way to evaluate Lisp or call a Lisp function that could signal in a C function and then free allocated memory? I assume that I can’t just call the Lisp function and hope it doesn’t signal.

You want to catch the errors in Lisp that you call from C?  If so, use
the safe_call functions we already have.

If all you want is make sure memory will be allocated, but don't mind
if the signal thrown by Lisp gets back to command loop, use
record_unwind_protect to register a function which will release the
memory.

Re: Calling Lisp in C functions

[Yuan Fu]

> On Sep 21, 2021, at 10:45 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Tue, 21 Sep 2021 12:38:21 -0700
>> 
>> What’s the best way to evaluate Lisp or call a Lisp function that could signal in a C function and then free allocated memory? I assume that I can’t just call the Lisp function and hope it doesn’t signal.
> 
> You want to catch the errors in Lisp that you call from C?  If so, use
> the safe_call functions we already have.
> 
> If all you want is make sure memory will be allocated, but don't mind
> if the signal thrown by Lisp gets back to command loop, use
> record_unwind_protect to register a function which will release the
> memory.

Thanks, I still don’t know how does stack works so here’s some naive questions:

I tried to write

ptrdiff_t count = SPECPDL_INDEX ();
record_unwind_protect_ptr (ts_delete_cursor, cursor);
...
return unbind_to (count, xxx);

But it give a type error, because ts_delete_cursor doesn’t take a void*. I guess I need to define a wrapper function to avoid that error? Can I use record_unwind_protect_ptr more than once? What does that count mean? What is specpdl?

Yuan

Re: Calling Lisp in C functions

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Wed, 22 Sep 2021 20:59:00 -0700
> Cc: Emacs developers <emacs-devel@gnu.org>
> 
> I tried to write
> 
> ptrdiff_t count = SPECPDL_INDEX ();
> record_unwind_protect_ptr (ts_delete_cursor, cursor);
> ...
> return unbind_to (count, xxx);
> 
> But it give a type error, because ts_delete_cursor doesn’t take a void*. I guess I need to define a wrapper function to avoid that error?

Yes, see for example directory_files_internal_unwind and its usage in
dired.c.

> Can I use record_unwind_protect_ptr more than once?

Yes.

> What does that count mean?

It tells unbind_to how much of the unwind-protect stack to unwind.
See the definition of unbind_to in eval.c.

> What is specpdl?

It's an array used for manually managed stack for tracking unwinds and
other stuff by the Emacs Lisp machine.  See the comments for 'union
specbinding', starting at line 3135 of lisp.h.

Re: Calling Lisp in C functions

[Andreas Schwab]

On Sep 23 2021, Eli Zaretskii wrote:

>> What is specpdl?
>
> It's an array used for manually managed stack for tracking unwinds and
> other stuff by the Emacs Lisp machine.  See the comments for 'union
> specbinding', starting at line 3135 of lisp.h.

pdl = push-down list.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

Re: Calling Lisp in C functions

[Yuan Fu]

> On Sep 23, 2021, at 12:47 AM, Andreas Schwab <schwab@linux-m68k.org> wrote:
> 
> On Sep 23 2021, Eli Zaretskii wrote:
> 
>>> What is specpdl?
>> 
>> It's an array used for manually managed stack for tracking unwinds and
>> other stuff by the Emacs Lisp machine.  See the comments for 'union
>> specbinding', starting at line 3135 of lisp.h.
> 
> pdl = push-down list.

Thanks!

Yuan

Re: Calling Lisp in C functions

[Yuan Fu]

> On Sep 22, 2021, at 11:59 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Wed, 22 Sep 2021 20:59:00 -0700
>> Cc: Emacs developers <emacs-devel@gnu.org>
>> 
>> I tried to write
>> 
>> ptrdiff_t count = SPECPDL_INDEX ();
>> record_unwind_protect_ptr (ts_delete_cursor, cursor);
>> ...
>> return unbind_to (count, xxx);
>> 
>> But it give a type error, because ts_delete_cursor doesn’t take a void*. I guess I need to define a wrapper function to avoid that error?
> 
> Yes, see for example directory_files_internal_unwind and its usage in
> dired.c.
> 
>> Can I use record_unwind_protect_ptr more than once?
> 
> Yes.
> 
>> What does that count mean?
> 
> It tells unbind_to how much of the unwind-protect stack to unwind.
> See the definition of unbind_to in eval.c.
> 
>> What is specpdl?
> 
> It's an array used for manually managed stack for tracking unwinds and
> other stuff by the Emacs Lisp machine.  See the comments for 'union
> specbinding', starting at line 3135 of lisp.h.

Thanks, I think I know how it works now.

Yuan