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


On Friday, September 13 2013, Eli Zaretskii wrote:

>> From: Sergio Durigan Junior <sergiodj@redhat.com>
>> Cc: dje@google.com, gdb-patches@sourceware.org
>> Date: Fri, 13 Sep 2013 16:40:52 -0300
>> 
>> > This is fine, but I would also include an example with $_exitcode.
>> 
>> I intend to expand this explanation when I resubmit the patch for
>> $_exitsignal.  In fact, I will add another example showing the behavior
>> of $_exitsignal and $_exitcode.  But if you wish, I can add $_exitcode
>> here as well.
>
> It would be best to have the explanation in both places, or at least a
> cross-reference from one to the other.

WDYT of this version?

-- 
Sergio

diff --git a/gdb/NEWS b/gdb/NEWS
index ad97f6f..13bb647 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.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index b6ba239..33318b8 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -9800,6 +9800,69 @@ function can be used in an expression just like an ordinary function;
 however, a convenience function is implemented internally to
 @value{GDBN}.
 
+These functions do not require @value{GDBN} to be configured with
+@code{Python} support, which means that they are always available.
+
+@table @code
+
+@item $_isvoid (@var{expr})
+@findex $_isvoid@r{, convenience function}
+Return one if the expression @var{expr} is @code{void}.  Otherwise it
+returns zero.
+
+A @code{void} expression is an expression where the type of the result
+is @code{void}.  For example, you can examine a convenience variable
+(see @ref{Convenience Vars,, Convenience Variables}) to check whether
+it is @code{void}:
+
+@smallexample
+(@value{GDBP}) print $_exitcode
+$1 = void
+(@value{GDBP}) print $_isvoid ($_exitcode)
+$2 = 1
+(@value{GDBP}) run
+Starting program: ./a.out
+[Inferior 1 (process 29572) exited normally]
+(@value{GDBP}) print $_exitcode
+$3 = 0
+(@value{GDBP}) print $_isvoid ($_exitcode)
+$4 = 0
+@end smallexample
+
+In the example above, we used @code{$_isvoid} to check whether
+@code{$_exitcode} is @code{void} before and after the execution of the
+program being debugged.  Before the execution there is no exit code to
+be examined, therefore @code{$_exitcode} is @code{void}.  After the
+execution the program being debugged returned zero, therefore
+@code{$_exitcode} is zero, which means that it is not @code{void}
+anymore.
+
+The @code{void} expression can also be a call of a function from the
+program being debugged.  For example, given the following function:
+
+@smallexample
+void
+foo (void)
+@{
+@}
+@end smallexample
+
+The result of calling it inside @value{GDBN} is @code{void}:
+
+@smallexample
+(@value{GDBP}) print foo ()
+$1 = void
+(@value{GDBP}) print $_isvoid (foo ())
+$2 = 1
+(@value{GDBP}) set $v = foo ()
+(@value{GDBP}) print $v
+$3 = void
+(@value{GDBP}) print $_isvoid ($v)
+$4 = 1
+@end smallexample
+
+@end table
+
 These functions require @value{GDBN} to be configured with
 @code{Python} support.
 
diff --git a/gdb/testsuite/gdb.base/gdbvars.c b/gdb/testsuite/gdb.base/gdbvars.c
index aa3b4d8..352a76b 100644
--- a/gdb/testsuite/gdb.base/gdbvars.c
+++ b/gdb/testsuite/gdb.base/gdbvars.c
@@ -4,6 +4,17 @@ typedef void *ptr;
 
 ptr p = &p;
 
+static void
+foo_void (void)
+{
+}
+
+static int
+foo_int (void)
+{
+  return 0;
+}
+
 int
 main ()
 {
diff --git a/gdb/testsuite/gdb.base/gdbvars.exp b/gdb/testsuite/gdb.base/gdbvars.exp
index 23a6758..85aaca0 100644
--- a/gdb/testsuite/gdb.base/gdbvars.exp
+++ b/gdb/testsuite/gdb.base/gdbvars.exp
@@ -54,6 +54,34 @@ proc test_convenience_variables {} {
 	"Print contents of uninitialized convenience variable"
 }
 
+proc test_convenience_functions {} {
+    gdb_test "print \$_isvoid" " = <internal function _isvoid>" \
+	"Print internal function \$_isvoid"
+
+    gdb_test "print \$isvoid_foo" " = void" \
+	"Print void convenience variable"
+
+    gdb_test "print \$_isvoid (\$isvoid_foo)" " = 1" \
+	"Check whether void convenience variable is void"
+
+    gdb_test_no_output "set \$isvoid_foo = 1" \
+	"Set void convenience variable to 1"
+
+    gdb_test "print \$_isvoid (\$isvoid_foo)" " = 0" \
+	"Check whether non-void convenience variable is void"
+
+    # For the next test, we need the inferior to be running.
+    if { ![runto_main] } {
+	return -1
+    }
+
+    gdb_test "print \$_isvoid (foo_void ())" " = 1" \
+	"Check whether void function is void"
+
+    gdb_test "print \$_isvoid (foo_int ())" " = 0" \
+	"Check whether non-void function is void"
+}
+
 proc test_value_history {} {
     global gdb_prompt
 
@@ -114,4 +142,5 @@ gdb_test_no_output "set print sevenbit-strings"
 
 test_value_history
 test_convenience_variables
+test_convenience_functions
 test_with_program
diff --git a/gdb/value.c b/gdb/value.c
index 42a8d2f..edbfc70 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -3584,6 +3584,23 @@ value_fetch_lazy (struct value *val)
   return 0;
 }
 
+/* Implementation of the convenience function $_isvoid.  */
+
+static struct value *
+isvoid_internal_fn (struct gdbarch *gdbarch,
+		    const struct language_defn *language,
+		    void *cookie, int argc, struct value **argv)
+{
+  int ret;
+
+  if (argc != 1)
+    error (_("You must provide one parameter for $_isvoid."));
+
+  ret = TYPE_CODE (value_type (argv[0])) == TYPE_CODE_VOID;
+
+  return value_from_longest (builtin_type (gdbarch)->builtin_int, ret);
+}
+
 void
 _initialize_values (void)
 {
@@ -3616,4 +3633,10 @@ VARIABLE is already initialized."));
   add_prefix_cmd ("function", no_class, function_command, _("\
 Placeholder command for showing help on convenience functions."),
 		  &functionlist, "function ", 0, &cmdlist);
+
+  add_internal_function ("_isvoid", _("\
+Check whether an expression is void.\n\
+Usage: $_isvoid (expression)\n\
+Return 1 if the expression is void, zero otherwise."),
+			 isvoid_internal_fn, NULL);
 }


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