On Wed, Apr 18, 2012 at 01:36:26PM +0300, Andriy Gapon wrote: > > I just would like to share something that I stumbled upon. > Maybe this is something well known, then forgive me for the noise. > > When ld combines multiple object files it overrides weak symbol definitions > with > a strong definition (if any). There are many examples/demonstrations on the > Internet on how this works, e.g.: > http://winfred-lu.blogspot.com/2009/11/understand-weak-symbols-by-examples.html > > But when the object files are spread across multiple archives, the there could > be some surprises. > My understanding is that there are two big rules that linker follows with > respect to archive libraries: > - linker extracts an object file from a library only if according to a symbol > table of the library the object file contains some interesting symbols > - if linker extracts an object file then it processes all symbols in it > > And now the following observation: if linker has already seen a weak > definition > for a symbol, then it will not actively seek any other definitions for it. > But > it will take into account the definitions that it stumbles upon. > For example, if an object file in an archive library contains only (strong) > definitions for some symbols that already have weak definitions, the linker > will > not extract that object file and will not look into it. And thus the weak > definition will not be overridden. OTOH, if that object file contains a > definition for at least one still undefined symbol, then the file will be > interesting to linker, it will extract the file, process _all_ symbols in it > and > thus will override the weak definitions with the strong ones. > > This is something that was unexpected to me. > > Some references: > http://webpages.charter.net/ppluzhnikov/linker.html > http://glandium.org/blog/?p=2388
This is from the ELF standard version 1.2 PDF, page 1-5: When the link editor searches archive libraries, it extracts archive members that contain definitions of undefined global symbols. The member's definition may be either a global or a weak symbol. The link editor does not extract archive members to resolve undefined weak symbols. Unresolved weak symbols have a zero value.
pgpGyuE7zLysR.pgp
Description: PGP signature
