Date: Wed, 18 Apr 2012 13:49:55 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Andriy Gapon <avg@freebsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: weak symbols vs archive libraries Message-ID: <20120418104955.GV2358@deviant.kiev.zoral.com.ua> In-Reply-To: <4F8E992A.2090705@FreeBSD.org> References: <4F8E992A.2090705@FreeBSD.org>
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--bvzltMstmN4o5fnw Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Apr 18, 2012 at 01:36:26PM +0300, Andriy Gapon wrote: >=20 > I just would like to share something that I stumbled upon. > Maybe this is something well known, then forgive me for the noise. >=20 > When ld combines multiple object files it overrides weak symbol definitio= ns 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-example= s.html >=20 > 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 sy= mbol > table of the library the object file contains some interesting symbols > - if linker extracts an object file then it processes all symbols in it >=20 > And now the following observation: if linker has already seen a weak defi= nition > 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 (stron= g) > definitions for some symbols that already have weak definitions, the link= er 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. >=20 > This is something that was unexpected to me. >=20 > Some references: > http://webpages.charter.net/ppluzhnikov/linker.html > http://glandium.org/blog/?p=3D2388 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. --bvzltMstmN4o5fnw Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk+OnFMACgkQC3+MBN1Mb4gR9gCfWKSt7zwtmutep4JKV15Tv0F0 xEMAoIaOLawLAjA1k4N5Dsbc4fVRwQPg =CCfN -----END PGP SIGNATURE----- --bvzltMstmN4o5fnw--
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