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

[Sergio Durigan Junior <sergiodj at redhat dot com>]

On Friday, September 13 2013, Eli Zaretskii wrote:

>> 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.

Right, I have added a sentence explaining both on the NEWS and on
gdb.texinfo.  Please take a look and see if it is OK.

>> +
>>  * 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.

Reordered.

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

Fixed.

> 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.

Done that.  Please tell me what you think.

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

I am not sure I understood your question.  Do you want to know what will
happen when the user calls a non-existent $_isvoid convenience function?

  (gdb) p $_isvoid ($var)
  You can't do that without a process to debug.
  (gdb) p $_isvoid (a)
  You can't do that without a process to debug.
  (gdb) start
  Temporary breakpoint 1 at 0x4004a7: file 1.c, line 12.
  Starting program: /home/sergio/work/src/git/isvoid/build-64/a.out 

  Temporary breakpoint 1, main () at 1.c:12
  12          return 0;
  (gdb) p $_isvoid ($var)
  Invalid data type for function to be called.
  (gdb) p $_isvoid (a)
  Invalid data type for function to be called.
  (gdb) p $_isvoid (1)
  Invalid data type for function to be called.

Is this what you wanted to know?  IOW, the user won't be able to call
this function from inside his/her script on a GDB prior to 7.7.

Thanks,

-- 
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..35aaf41 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -3,6 +3,11 @@
 
 *** 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, 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..a5705e4 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -9800,6 +9800,42 @@ 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 void.  Otherwise it
+returns zero.
+
+A void expression is an expression where the type of the result is
+`void'.  For example, given the following function:
+
+@smallexample
+void
+foo (void)
+@{
+@}
+@end smallexample
+
+The result of calling it inside @value{GDBN} is `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);
 }