Add STB_SECONDARY to gABI
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.
> -----Original Message-----
> From: email@example.com [mailto:generic-
> firstname.lastname@example.org] 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?
> 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
> 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.
> 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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