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Re: Catch arbitrary signal and throw it later in C

From: Po Lu
Subject: Re: Catch arbitrary signal and throw it later in C
Date: Thu, 13 Apr 2023 16:22:47 +0800
User-agent: Gnus/5.13 (Gnus v5.13)

Yuan Fu <casouri@gmail.com> writes:

> unwind-protect (and its internal C counterpart) pushes the cleanup
> form onto specpdl. Supposingly that form is later eval’ed? I want to
> run some C code which obviously it doesn’t support. I need to
> understand how this whole unwinding thing work before I can maybe
> modify or write something that does what I want. Hopefully some one
> can enlighten me :-)

Typically, you do this:

  specpdl_ref count;
  struct localcontext context;

  count = SPECPDL_INDEX ();
  record_unwind_protect_ptr (cleanup_func, &context);

  /* Code which may signal.  */

  unbind_to (count, Qnil);

where cleanup_func is:

  cleanup_func (void *arg)

Once a signal happens between the `record_unwind_protect_ptr', and
`unbind_to', `cleanup_func' will be called with `&context' as an
argument.  This happens before any longjmp, so it is safe to use
references to variables with automatic storage duration within unwind
protects you record.

Note that `record_unwind_protect_ptr' itself may also signal if the
specpdl is full, calling the unwind function in the process.

If your cleanup function needs arguments that aren't pointers, you can
also use `record_unwind_protect_int', `record_unwind_protect', or

Saving a signal, only to throw it again later, is not something we do in
Emacs.  I suggest you make your cleanup code work as described above.

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