RE: Add STB_SECONDARY to gABI

["Suprateeka R Hegde" <hegdesmailbox at gmail dot com>]

> -----Original Message-----
> From: generic-abi@googlegroups.com [mailto:generic-
> abi@googlegroups.com] On Behalf Of Lowell, Randy
> Sent: 14 May 2012 07:12 PM
> To: generic-abi@googlegroups.com; GCC Development; Binutils; GNU C
> Library; Ansari, Zia
> Subject: RE: Add STB_SECONDARY to gABI
> 
> This looks good.  I just want to check one thing with you.  In point
> 5
> you state that unresolved secondary symbols have a zero value.  Are
> you implying that unresolved secondary symbols should not result in
> a
> link or load-time error?  If that's the case, you should also make
> it
> clear that a secondary reference (STB_SECONDARY/SHN_UNDEF) has lower
> precedence than a global or weak reference.

In this case we should just mention that "secondary reference (STB_SECONDARY/SHN_UNDEF) has lower precedence than a global or weak reference". We cannot keep point number 5 (as it appears now) under differences because gABI already says "Unresolved weak symbols
have a zero value". So either remove point number 5 under differences or change it to something discussed above.

--
Supra

> 
> Randy
> 
> > -----Original Message-----
> > From: generic-abi@googlegroups.com [mailto:generic-
> > abi@googlegroups.com] On Behalf Of H.J. Lu
> > Sent: Saturday, May 12, 2012 9:53 AM
> > To: Generic System V Application Binary Interface; GCC
> Development;
> > Binutils; GNU C Library; Ansari, Zia
> > Subject: Add STB_SECONDARY to gABI
> >
> > Here is the final proposal to add STB_SECONDARY to gABI.
> > Any comments?
> >
> > Thanks.
> >
> > --
> > H.J.
> > ---
> > We want to provide a relocatable object which can take advantage
> of all
> > versions of a supported OS.  For a function, foo, in the C
> library, we
> > can use it only if it is available on all versions of the C
> library or
> > we provide our own implementation of foo.  With our own foo, the
> one in
> > the C library will never be used.  Here is a proposal to add
> > STB_SECONDARY
> > to gABI to support the secondary definition so that a software
> vendor
> > can provide an alternative implementation in case it isn't
> available
> > in the C library.
> >
> > STB_SECONDARY
> >
> >       Secondary symbols are similar to weak symbols, but their
> definitions
> >       have even lower precedence.  The difference between
> secondary symbols
> >       and weak symbols are
> >
> > 	1. The link editor must search archive library and extract
> > 	archive members to resolve defined and undefined secondary
> symbol.
> > 	2.  When the link editor searches a shared object, it must
> honor
> > 	the global or weak definition in the shared object and ignore
> the
> > 	secondary one with the same name.
> > 	3. The link editor ignores the secondary definition if there is
> > 	a global, weak or common definition with the same name.
> Multiple
> > 	secondary definitions with the same name will not cause an
> error.
> > 	The first appearance of the secondary definition should be
> honored
> > 	and the rest are ignored.
> > 	4. The link editor may treat the secondary definition in the
> > 	shared object as a global definition.
> > 	5. Unresolved secondary symbols have a zero value.
> >
> >       The purpose of this symbol binding is to provide the primary
> >       definition as a global, weak or common symbol in an archive
> library
> >       or a shared object while keeping a secondary definition in a
> >       relocatable object.  If there is no primary definition, the
> >       secondary definition will be used.
> >
> >       When secondary definitions become part of an executable or
> shared
> >       object, linker may convert them to global or local
> definitions.
> >
> >       At run-time, when resolving a symbol, after seeing a
> secondary
> >       definition, the dynamic linker must keep searching until a
> >       global or weak definition is found.  If a global or weak
> >       definition is found, it will be used to satisfy the symbol
> lookup.
> >       Otherwise, the secondary definition will be used.
> >
> >       If the dlopen loads a global or weak definition after the
> program
> >       has already resolved references to a secondary definition,
> those
> >       references remain bound to the secondary definition.  Any
> >       references resolved after the dlopen, for which the dlopened
> >       module is included in the module search list, would be
> resolved
> >       to the global or weak definition.
> >
> > STB_SECONDARY is defined as:
> >
> > #define STB_SECONDARY	3	/* Secondary symbol */
> >
> > --
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