Re: [PATCH/RFA] Introduce new $_isvoid() convenience function

[Eli Zaretskii <eliz at gnu dot org>]

> From: Sergio Durigan Junior <sergiodj@redhat.com>
> Cc: GDB Patches <gdb-patches@sourceware.org>
> Date: Fri, 13 Sep 2013 03:42:35 -0300
> 
> On Thursday, September 12 2013, Doug Evans wrote:
> 
> > Hi.  Thanks for persevering!
> 
> Thank you for reviewing :-).
> 
> > While I often follow "it's easier to relax restrictions than impose
> > them after the fact",
> > in this case I wonder if there's any reason to restrict $_isvoid() to
> > convenience variables.
> >
> > Thoughts?
> >
> > For "void foo(){}", is there any real difference to:
> > (gdb) set $foo = foo()
> > (gdb) p $_isvoid($foo)
> > (gdb) p $_isvoid(foo())
> >
> > [Haven't tried it, but I'm guessing removing the internalva4 check in
> > your patch would make that work.]
> 
> Yes, I agree.  I have changed the code to accept not only convenience
> variables, but other kinds of expressions (it was just a matter of
> removing the check for internalvar, really).  BTW, I have reworded the
> help message and the documentation and did s/convenience
> variable/expression/g.
> 
> > Also, you've put  a fair bit of time into this, and I'm happy with
> > what you've got.  And thanks!
> > The high order bit here for me is being able to script $_exitcode + $exitsignal.
> > If we're going with checking for a void value, one could just leave it
> > to be handled in Python.
> 
> Yeah, I thought about it too, but then I thought that maybe we shouldn't
> always depend on Python.  This was easy to do in C, and has the
> advantage that it will be available everywhere.  But I guess you said
> something along those lines below...
> 
> > OTOH, this patch is really simple, and IMO it is *nice* to be able to
> > do this from just a plain gdb script,
> > so I'm advocating accepting this patch (modulo maybe handling any void
> > value, not just conv vars).
> 
> Nice.  I hope you like the new version of the patch.
> 
> > btw, I noticed that if I run a program to exit, and then rerun to
> > main, $_exitcode has the last exit code - I would have expected it to
> > get reset to void upon rerun.  After all, if I stop at main the first
> > time through, $_exitcode is void.
> > [I'll file a bug, no need to address this in your patch series.]
> 
> Yes, this is a bug indeed.  Thanks for filing it.  I may even have a fix
> hanging around, will look.
> 
> -- 
> Sergio
> 
> gdb/
> 2013-09-13  Sergio Durigan Junior  <sergiodj@redhat.com>
> 
> 	* NEWS: Mention new convenience function $_isvoid.
> 	* value.c (isvoid_internal_fn): New function.
> 	(_initialize_values): Add new convenience function $_isvoid.
> 
> gdb/doc/
> 2013-09-13  Sergio Durigan Junior  <sergiodj@redhat.com>
> 
> 	* gdb.texinfo (Convenience Functions): Mention new convenience
> 	function $_isvoid.
> 
> gdb/testsuite/
> 2013-09-13  Sergio Durigan Junior  <sergiodj@redhat.com>
> 
> 	* gdb.base/gdbvars.c (foo_void): New function.
> 	(foo_int): Likewise.
> 	* gdb.base/gdbvars.exp (test_convenience_functions): New
> 	function.  Call it.
> 
> diff --git a/gdb/NEWS b/gdb/NEWS
> index ad97f6f..e58d6ce 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -3,6 +3,9 @@
>  
>  *** Changes since GDB 7.6
>  
> +* New convenience function "$_isvoid", to check whether an expression
> +  is void.

This is OK, but please add a sentence explaining what is a "void
expression".  This is not something GDB users will automatically know
or understand.

> +
>  * The "maintenance print objfiles" command now takes an optional regexp.
>  
>  * The "catch syscall" command now works on arm*-linux* targets.
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index b6ba239..70d580f 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -9829,6 +9829,18 @@ Returns the length of string @var{str}.
>  
>  @end table
>  
> +These functions do not require @value{GDBN} to be configured with
> +@code{Python} support, which means that they are always available.

I'd prefer to describe functions that don't need Python before those
that do, not after.

> +@table @code
> +
> +@item $_isvoid (@var{expr})
> +@findex $_isvoid@r{, convenience function}
> +Return one if the expression @code{expr} is void.  Otherwise it
                                ^^^^^^^^^^^
@var{expr}.

And please put here an explanation of what is a void expression,
preferably more detailed than what you will have in NEWS.  AFAICS, we
never explain that anywhere in the manual.

And now a question about this feature: what will use of this function
do in versions of GDB before 7.7?

Thanks.