[Bug 945927] Re: ld incorrectly gives 'undefined reference'

[CF]

Hi Matthias,

thanks for your patience. First of all, I just noticed a mistake in my
simple example linkage of libfoo - I forgot to link libfoo with libm at
creation time, which is crucial for this bug. The commands to use are

gcc -fPIC -c a.c
gcc -shared -fPIC a.o -o libfoo.so -lm
gcc -fPIC -c b.c
gcc -shared -fPIC b.o -o libbar.so

When I now type

gcc c.c -L . -lbar -lfoo -lm -Wl,--trace-symbol=sinh

I get

./libbar.so: reference to sinh
/usr/lib/gcc/x86_64-linux-gnu/4.6.1/../../../x86_64-linux-gnu/libm.so: definition of sinh
./libbar.so: undefined reference to `sinh'
collect2: ld gab 1 als Ende-Status zurück

(in case you are not German-speaking: collect2: ld returned exit code 1)

The same happens if I change the order into -lfoo -lbar -lm
What works, however, is to leave out -lfoo
gcc c.c -L . -lbar -lm -Wl,--trace-symbol=sinh

Now, according to my understanding of the linker, the presence of
superfluous -lfoo should not be able to break the link. Please correct
me, if I am wrong.

The point is that -lm gets very early into LIBS in the configure (which used to be very reasonable). If I use AC_CHECK_LIB (bar, ...)  even with the extra-libs option), it will always test for
-lbar $LIBS extralibs
so I have no chance to squeeze the -lm between the tested -lbar and $(LIBS). And even if I would manage to, it is not sure whether libtool would not remove double -lm's when we do the real linking in the Makefile. When I said that this bug breaks AC_CHECK_LIBS, I meant exactly this: there is no easy way to check for multiple libs, without introducing non-standard dependencies on the libraries like "please test lbar only in the absence of lfoo, even if you may want to link them botht a later stage".


Christoph

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https://bugs.launchpad.net/bugs/945927

Title:
  ld incorrectly gives 'undefined reference'

Status in “binutils” package in Ubuntu:
  Invalid

Bug description:
  I had an unexpected link problem while trying to probe libraries in a
  configure script.

  I am using the latest update (2012-03-03 21:00 MEZ) of kubuntu 11.10, the binutils package is 2.21.53.20110810-0ubuntu5.1
  My machine is a  Intel(R) Core(TM)2 Duo CPU     E8400  @ 3.00GHz, but I doubt that matters.

   I was able to cook the problem down to the following.

  Situation:
  libfoo.so needs libm, and was created with -lm (ldd shows the dependency)
  libbar.so needs libm, but was not created with -lm (no ldd dependency)
  My program only needs libbar.so

  Problem:

  gcc prog.c -L. -lbar -lfoo -lm

  yields an 'undefined reference to sinh'
  If I run the link with -Wl,--trace-symbol=sinh, the linker actually reports that -lm contains sinh, but fails nevertheless.

  What works is
  gcc prog.c -L. -lbar -lm
  gcc prog.c -L. -lbar -lm -lfoo

  [The example of course cries for a simple workaround by reordering the
  libs, but this is a mess when you try it in the configure. So just
  take the lib ordering for granted (and this configure works on dozens
  of other systems). I also know that libbar isn't up to the standards -
  but, in real life, I won't change the way how other packages create
  their shared libs.]

  Here's some simple C files to reproduce the bug
  --- a.c:
  #include <math.h>
  double foo (double x) { return exp(x); }
  ---- b.c:
  #include <math.h>
  double bar(double x) { return sinh(x); }
  ---- c.c:
  double bar (double);
  int main ()
  {
     double x = bar (1.);
  }
  ---
  gcc -fPIC -c a.c
  gcc -shared -fPIC a.o -o libfoo.so
  gcc -fPIC -c b.c
  gcc -shared -fPIC b.o -o libbar.so

  gcc c.c -L. -lbar -lfoo -lm

  ProblemType: Bug
  DistroRelease: Ubuntu 11.10
  Package: binutils 2.21.53.20110810-0ubuntu5.1
  ProcVersionSignature: Ubuntu 3.0.0-16.28-generic 3.0.17
  Uname: Linux 3.0.0-16-generic x86_64
  ApportVersion: 1.23-0ubuntu4
  Architecture: amd64
  Date: Sat Mar  3 23:08:39 2012
  InstallationMedia: Kubuntu 10.04.1 LTS "Lucid Lynx" - Release amd64 (20100816.2)
  ProcEnviron:
   LANGUAGE=
   PATH=(custom, user)
   LANG=de_DE.UTF-8
   SHELL=/bin/tcsh
  SourcePackage: binutils
  UpgradeStatus: Upgraded to oneiric on 2012-02-29 (3 days ago)

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