Based on the discussion at:

<https://sourceware.org/ml/gdb-patches/2013-09/msg00301.html>
<https://sourceware.org/ml/gdb-patches/2013-09/msg00383.html>

This patch adds a new convenience function called $_isvoid, whose
only purpose is to check whether an expression is void or not.
This became necessary because the new convenience variable
$_exitsignal (not yet approved) has a mutual exclusive behavior
with $_exitcode, i.e., when one is "defined" (i.e., non-void),
the other is cleared (i.e., becomes void).  Doug wanted a way to
identify which variable to use, and checking for voidness is the
obvious solution.

It is worth mentioning that my first attempt, after a conversation with
Doug, was to actually implement a new $_isdefined() convenience
function.  I would do that (for convenience variables) by calling
lookup_only_internalvar.  However, I found a few problems:

- Whenever I called $_isdefined ($variable), $variable became defined
  (with a void value), and $_isdefined always returned true.

- Then, I tried to implement $_isdefined ("variable"), and do the "$" +
  "variable" inside GDB, thus making it impossible for GDB to create the
  convenience variable.  However, it was hard to extract the string
  without having to mess with values and their idiossincrasies.
  Therefore, I decided to abandon this attempt (specially because I
  didn't want to spend too much time struggling with it).

Anyway, after talking to Doug again we decided that it would be easier
to implement $_isvoid, and this will probably help in cases like
<http://stackoverflow.com/questions/3744554/testing-if-a-gdb-convenience-variable-is-defined>.

I wrote a NEWS entry for it, and some new lines on the documentation.

gdb/
2013-09-16  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/
2013-09-16  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (Convenience Functions): Mention new convenience
	function $_isvoid.

gdb/testsuite/
2013-09-16  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.

1 files changed, 8 insertions, 0 deletions

diff --git a/gdb/NEWS b/gdb/NEWS
index 5eb046b..af06a21 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -3,6 +3,14 @@
 
 *** Changes since GDB 7.6
 
+* New convenience function "$_isvoid", to check whether an expression
+  is void.  A void expression is an expression where the type of the
+  result is "void".  For example, some convenience variables may be
+  "void" when evaluated (e.g., "$_exitcode" before the execution of
+  the program being debugged; or an undefined convenience variable).
+  Another example, when calling a function whose return type is
+  "void".
+
 * The "maintenance print objfiles" command now takes an optional regexp.
 
 * The "catch syscall" command now works on arm*-linux* targets.