Re: pending signal after rl_callback_read_char

[Chet Ramey]

On 9/20/22 4:17 PM, Simon Marchi wrote:

> > In the meantime, you could look at rl_persistent_signal_handlers,
> > which, in theory, should work on readline versions >= 7.0.
>
> Ok, however GDB has its own signal handlers that it relies on, so I'm
> not sure how it would work if readline's handlers were installed all the
> time.

"All the time" means from the time readline prints a prompt until it
executes rl_accept_line or otherwise sets rl_done, presumably over a series
of calls to rl_callback_read_char(). It's the equivalent of having 
readline's signal handlers installed for the duration of a call to
readline().

The default is to have the signal handlers installed only for the duration
of a call to rl_callback_read_char().

Readline's signal handler calls any application-installed handler (well,
it resets the signal dispositions and resends the signal). That's how gdb's
SIGINT handler gets called when readline's signal handlers are installed.

> Just a note, earlier you said that GDB is using readline without any
> callback and that it blocks into rl_callback_read_char, I don't think
> that's true.  We have an event loop and only call into readline when
> input is available.

You haven't executed a readline command that would cause a function pointer 
to be assigned to _rl_callback_func (private and internal). That's another 
code path that executes _rl_internal_char_cleanup() (there are several).


> So, I'm not entirely sure how things work when you are sitting at the
> GDB prompt and press ^C.  Does readline get involved at all in that
> case? 

If readline is in the middle of a call to rl_callback_read_char, then yes.
If not, then no.

> What are readline signals useful for? 

Readline changes the terminal settings and other user state, and allocates
per-call memory and other resources, and has to catch signals so it can
clean that up.

> I see we can also disable
> readline's signal handling (set rl_catch_signals to 0), I'm wondering if
> that would work for us.

Sure, if you want to do all that work for yourself.

Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/