Weak-import more symbols in closure_blocks_leopard_compat

Weak-import more symbols in closure_blocks_leopard_compat:

__Block_copy, __Block_release, __Block_object_assign, and __Block_object_dispose

BUG=91978
TEST=Included
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=96979

[Nico, 2011-08-16 17:18:14]

http://codereview.chromium.org/7657014/diff/1/chrome/browser/mac/closure_bloc...
File chrome/browser/mac/closure_blocks_leopard_compat.h (right):

http://codereview.chromium.org/7657014/diff/1/chrome/browser/mac/closure_bloc...
chrome/browser/mac/closure_blocks_leopard_compat.h:84: MAYBE_WEAK_IMPORT extern
void _Block_object_dispose(const void*, const int);
since you declare these here, why do you need the delcaration in the .S as well?

[Mark Mentovai, 2011-08-16 17:54:07]

http://codereview.chromium.org/7657014/diff/1/chrome/browser/mac/closure_bloc...
File chrome/browser/mac/closure_blocks_leopard_compat.h (right):

http://codereview.chromium.org/7657014/diff/1/chrome/browser/mac/closure_bloc...
chrome/browser/mac/closure_blocks_leopard_compat.h:84: MAYBE_WEAK_IMPORT extern
void _Block_object_dispose(const void*, const int);
Nico wrote:
> since you declare these here, why do you need the delcaration in the .S as
well?

These are declarations. The .S file has definitions.

The .h and .S files do two different, but related, things.

The declarations are necessary to tell anything that references these symbols
that they’re supposed to be weakly imported. Weak-import is a property of the
thing that references the symbol, not a property of the thing that defines the
symbol. The .h is needed when targeting 10.5, even if a newer SDK is in use, to
get the references to the symbols marked as weak imports.

The .S file exists to provide definitions so that the linker is able to resolve
the symbols. There’s some added magic to make it appear that the definitions
came from libSystem, so that dyld will look in libSystem for them at load time.
The .S is only needed when using the 10.5 SDK. Otherwise, the SDK’s libSystem
already has definitions.

[Nico, 2011-08-16 18:07:18]

LGTM