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From: schorr@ead.dsa.com (Andrew J. Schorr)
Newsgroups: comp.unix.aix
Subject: AIX 3.1 ld symbol resolution
Message-ID: <1990Oct18.140050@ead.dsa.com>
Date: 18 Oct 90 18:00:50 GMT
Sender: news@ead.dsa.com
Reply-To: schorr@ead.dsa.com (Andrew J. Schorr)
Organization: Daiwa Securities America, New York
Lines: 56

According to the man page for ld,

      In this version of ld, the first definition of each symbol in
  the link always takes precedence and is  used  even  if the first
  reference follows the definition.

However, this does not appear to be completely true when shared
libraries are involved.

Here is an example: I have a shared library; some of the routines
exported by this library include calls to printf.  For some
applications, I want to substitute my own version of printf for the
one in libc.a.  To do this, I simply link in printf.o (containing
my version of printf) when I compile.  This works fine if I use
a static version of my library.  However, if I use the shared
version, the library routines will use the printf defined in libc.a.
I think I understand why this happens: the printf in the shared
library is resolved when the shared library is linked, not when
the application program is linked.  

This behavior is very problematic.  I normally use SunOS, and Sun's
dynamic linking behaves normally: the references in the shared library
are resolved when the library is linked at run-time.  The only
possible work-around that I can see is a painful one: have multiple
versions of the shared library, one for each version of printf that
I want to use.

Is there any reasonable way to get around this problem?
If not, will it be fixed?

-Andy

P.S.  For anyone who is still confused about how to make shared
  objects under AIX 3.1, here is my understanding of the process:

First, make the following definitions:
   LIBOBJS = {object files containing routines to be exported}
         Note that these are just normal object files (no special
		compilation flags required; nothing analogous to
		SunOS's -pic option).
   SHDEPLIBS = {libraries containing routines that are referenced
		by exported routines}
   Typically, SHDEPLIBS = -lc

1. Create a list of symbols to be exported.  The following command
   will create a list containing all externally defined symbols:
     nm -g $(LIBOBJS) | awk '$(NF-1) == "D" {print $NF}' > shlib.exp

2. Create the shared object:
     ld -o shared_object.o $(LIBOBJS) -bE:shlib.exp -bM:SRE \
		-T512 -H512 $(SHDEPLIBS)
3. Insert shared_object.o into an archive (if so desired).

N.B. It seems tempting to leave out the SHDEPLIBS and use the -r flag
     in step 2.  This will appear to work, but will give you an error
     at run-time.