Date: Fri, 04 Apr 1997 10:35:07 -0800 From: John Polstra <jdp@polstra.com> To: smc@servtech.com Cc: hackers@freebsd.org Subject: Re: FreeBSD Elf-Kit and dynamic loading Message-ID: <199704041835.KAA24555@austin.polstra.com> In-Reply-To: <334525B9.167EB0E7@servtech.com> References: <5i0j1d$jtk@news.itfs.nsk.su> <19970403191209.52889@keltia.freenix.fr> <5i2k5h$4jb@news.itfs.nsk.su> <334525B9.167EB0E7@servtech.com>
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In article <334525B9.167EB0E7@servtech.com>, Shawn Carey <smc@servtech.com> wrote: > Nickolay N. Dudorov wrote: > > Yes - it turns out that because I build and install elf-libc > > from (copy of) FreeBSD-2.2 sources there was no libc.so --> libc.so.3.0 > > link. After I make all libXX.so --> libXX.so.M.N links my test > > programs builds as dynamic. > > > > As an ELF newbie, it seems curious to me that the ELF linker requires a > symlink to the shared lib that does not have the version in its name. > Does ELF keep track of the shared lib versions? It's kind of a screwy system, but it's the way ELF does things. Here's how it works. Say you've got three versions of libc: libc.so.3 (the newest) libc.so.2 libc.so.1 (the oldest) (ELF doesn't use minor version numbers.) You also arrange to have a symlink pointing to the newest version: libc.so -> libc.so.3 The static linker "ld" does not do any searching based on version numbers. It always opens its libraries under the unversioned name, e.g., "libc.so". If the link is set up right, that means that "ld" always uses the newest version of the library. Recorded inside each shared library is its fully-versioned name, e.g., "libc.so.3". That name is stored into the executable file that "ld" produces, as a "shared object dependency." When you run the program, the dynamic linker uses the shared object dependencies to form a list of libraries that it has to open and link in. So in this case, it will always look for "libc.so.3", and won't use the symbolic link. Now suppose you add a new version of the library, "libc.so.4". You change the symlink "libc.so" to point to the new version. Things work out the way they should. Existing executables that were linked against libc.so.3 still will use that same library, because the versioned name is recorded in them. When you build new programs, though, they'll use the newest library, because that's where the unversioned symlink points now. I didn't invent it. I just implemented it. :-) -- John Polstra jdp@polstra.com John D. Polstra & Co., Inc. Seattle, Washington USA "Self-knowledge is always bad news." -- John Barth
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