Re: Restored Objective-C language support

[Giah de Barag <gdb at crelg dot com>]

GDB and Objective C
-------------------

1. GDB Objective C Language Support
2. How GDB Creates Objective C NSString Objects
3. How GDB Looks Up Objective C Selectors
4. GNUstep Source & Build Scripts
5. GNUstep and GDB Test
6. Questions Regarding the Patch


1. GDB Objective C Language Support
-----------------------------------

GDB support for Objective C (GCC libobjc and GNUstep) is deep,
powerful, and way beyond mature. The transition from Object to
NSObject was made somewhere around 1990.

GNUstep is *the* (the) GNU library of Objective C high-level types
(string, array, dictionary, set, process, thread, file, etc).

GCC libobjc is one of the two (2) low-level implementations of the
object-oriented runtime design (object, method, selector, etc.) that
work with GCC/GDB.

GNUstep is a part of the GNU software collection, is trivial to build
and package, and is a package on GNU-type system distributions.

GDB’s Objective C language support code has interacted with GNUstep
and the NSObject hierarchy for decades. That code is simple, easy to
understand, and easy to maintain.


2. How GDB Creates Objective C NSString Objects
-----------------------------------------------

Here is a look at part of the GDB/GNUstep debugger API.

In this example, GDB creates a GNUstep NSString when the user types
@"foo". It looks for a convenience function _NSNewStringFromCString,
and if not found, it sends the message stringWithCString to the
GNUstep class NSString.

  if (lookup_minimal_symbol("_NSNewStringFromCString", 0, 0).minsym)
    {
      function = find_function_in_inferior("_NSNewStringFromCString", NULL);
      nsstringValue = call_function_by_hand(function, 1, &stringValue[2]);
    }
  else if (lookup_minimal_symbol("+[NSString stringWithCString:]", 0, 0).minsym)
    {
      function
	= find_function_in_inferior("+[NSString stringWithCString:]", NULL);
      type = builtin_type (gdbarch)->builtin_long;
      stringValue[0] = value_from_longest 
	(type, lookup_objc_class (gdbarch, "NSString"));
      stringValue[1] = value_from_longest 
	(type, lookup_child_selector (gdbarch, "stringWithCString:"));
      nsstringValue = call_function_by_hand(function, 3, &stringValue[0]);
    }
  else
    error (_("NSString: internal error -- no way to create new NSString"));

In http://svn.gna.org/svn/gnustep/libs/base/trunk/Source/NSDebug.m,
which is in the GNUstep repository, at the very end of the file, you
can see the implementation of _NSNewStringFromCString. Also there is
the function _NSPrintForDebugger which handles the “po” (print object)
command of GDB.

    // GNUstep debugger support functions
    NSString *_NSNewStringFromCString(const char *cstring);
    const char *_NSPrintForDebugger(id object);


3. How GDB Looks Up Objective C Selectors
-----------------------------------------

Here is GDB’s code that looks up a selector.

In the previous example, GDB addressed GNUstep, the high-level type
library, to create an Objective C string (NSString), and here we are
interacting with the low-level runtime library, GCC libobjc, and its
function sel_getUid.

  if (lookup_minimal_symbol("sel_getUid", 0, 0).minsym)
    function = find_function_in_inferior("sel_getUid", NULL);
  else
    {
      complaint (&symfile_complaints,
		 _("no way to lookup Objective-C selectors"));
      return 0;
    }

In https://github.com/gcc-mirror/gcc/blob/master/libobjc/selector.c at the
very end you can see sel_getUid, which takes the string name representing a
method and returns the selector for it, SEL sel_getUid (const char *name).


4. GNUstep Source & Build Scripts
---------------------------------

GNUstep is trivial to install. If it is not a package, you can check
it out and build it.

You only need the “base” library which implements the basic high-level
types like NSString and has the GDB debugger support functions.

Source:
  svn co http://svn.gna.org/svn/gnustep/modules/core

  (For GDB Objective C language support,
  only “make” and “base” are needed.)

Build Scripts:
  http://svn.gna.org/svn/gnustep/tools/scripts/trunk/

It would be good for the teams (GCC/GDB/GNUstep) to be more in step
with each other. What do you think?


5. GNUstep and GDB Test
-----------------------

This is a test of:

    a) printing an object with po
    b) sending a message with [obj msg:arg]
    c) creating an NSString with @"foo"
    d) looking up a selector with @selector(msg)

GDB console session:

    (gdb) po [@"foo" performSelector:@selector(uppercaseString)]
    FOO
    (gdb) 


6. Questions Regarding the Patch
--------------------------------

FAOD:

1. About the Contribution Checklist. Do you need me to update this
   patch to conform to the Contribution Checklist?

2. About tests. Are there in fact no automated tests for objective-c
   support, and do you need me to create them?

3. Are there any barriers to this patch getting into 7.11, 7.12, and
   master (other than the above two questions)?

Thank you.