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Re: weak for Darwin



On 01/11/2004, at 1:37 PM, Aaron W. LaFramboise wrote:


Geoff Keating wrote:

On 01/11/2004, at 11:21 AM, Aaron W. LaFramboise wrote:

2. Applied to a symbol that isn't defined in the current translation
unit it makes a weak undefined symbol (i.e. one whose address must be
tested before the symbol is used, because at runtime it may or may
not have a definition).

How can something be 'not external' and yet not 'defined in the current
translation unit' either? Is weak on PECOFF just a way of saying "this
symbol is NULL"?

Quick map of PECOFF weak symantics:


If symbol is weak 'w' in a particular object

If symbol is defined 'D' in any translation unit

    References to that symbol are resolved normally to the
    defined symbol (weak has no effect in this case).

If symbol is not defined in any other translation unit

    References within the object that has the weak symbol
    are resolved to the default value for the weak symbol
    (which may or may not be 0, depending on whether it
    has been specified a value or alias locally.)

In particular, 'U' undefined symbols in obj1.o will never be resolved to
'w' symbols in obj2.o.


This still preserves the normal usage and semantics we expect of weak
symbols. For example, the example from
<http://gcc.gnu.org/onlinedocs/libstdc++/debug.html#mem> works as expected:


      extern void* __dso_handle __attribute__ ((__weak__));
      __cxa_atexit((void (*) (void *)) __libc_freeres, NULL,
                   &__dso_handle ? __dso_handle : NULL);

Aah, I see. You have both #2 and #1 but with the extra semantic that a weak symbol never satisfies any reference.


I really do want the semantics of 'weak' as on ELF.  In particular, I
want the property that:

1. Applied to a symbol that's defined in the current translation
unit, it makes it a weak definition. (i.e. one that can be overridden
by a strong definition elsewhere).

This is the part that confuses me. I thought it was the usual case that
symbols in libraries such as libsupc++ are always overridable (certainly
they are on ELF and COFF) by user code. Is this different with Darwin's
linker or loader?

Yes, it's different. It's a very long explanation as to why this particular approach seems best.


It's not true that they're *always* overridable; it depends on the link ordering.

It would be possible to have PECOFF disable the weak-ness using a macro.

OK. What do you think about adding a macro named something like
_GLIBCXX_WEAK_DEFINITION that is __attribute__((weak)) for Darwin,
nothing for PE targets, and whatever the concensus is for other targets?

Sure. Will you make a patch, or should I?

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