Add STB_SECONDARY to gABI

Lowell, Randy Randy.Lowell@hp.com
Mon May 14 13:43:00 GMT 2012


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.

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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