RE: Add STB_SECONDARY to gABI

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

The write-up looks good. 

Two typos:
1. An instance of "week" instead of "weak"
2. Some text mention "linker" and some other mention "link editor".

Question: will this be part of the current gABI draft soon?

--
Supra

> -----Original Message-----
> From: generic-abi@googlegroups.com [mailto:generic-
> abi@googlegroups.com] On Behalf Of H.J. Lu
> Sent: 28 June 2012 07:30 PM
> To: generic-abi@googlegroups.com
> Cc: GCC Development; Binutils; GNU C Library; Ansari, Zia
> Subject: Re: Add STB_SECONDARY to gABI
> 
> On Thu, Jun 28, 2012 at 6:43 AM, Lowell, Randy <Randy.Lowell@hp.com>
> wrote:
> > This looks good.  I just have one wordsmithing comment.
> >
> > I would have listed the STB_SECONDARY differences in a different
> > order -- maybe (3 1 2 4).  That puts the most general difference
> > first, and it matches the order used for the description of
> > STB_WEAK.
> 
> Here is the revised proposal.
> 
> 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 lower precedence than global and week symbols.  The
> difference
>       between secondary symbols and weak symbols are
> 
> 	1. 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.
> 	2. The link editor must search archive library and extract
> 	archive members to resolve defined and undefined secondary
> symbol.
> 	3.  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.
> 	4. The link editor may treat the secondary definition in the
> 	shared object as a global definition.
> 
>       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 */
> 
> NOTE:
>       The behavior of secondary symbols in areas not specified by
> this
>       proposal is implementation defined.
> 
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