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Re: Restored Objective-C language support

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)
	= 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]);
    error (_("NSString: internal error -- no way to create new NSString"));

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);
      complaint (&symfile_complaints,
		 _("no way to lookup Objective-C selectors"));
      return 0;

In 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.

  svn co

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

Build Scripts:

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)]

6. Questions Regarding the Patch


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.

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