Question

Short question:
How does llvm-ld locate libstdc++?

Details:

I am getting the following error message:

llvm-ld: error: Cannot find library 'stdc++'

while running llvm-ld. I am trying to understand how llvm-ld searches for libstdc++.

I am setting up a new system and following compilation steps that work on a different system. Eventually I noticed a difference with the LD_LIBRARY_PATH that was set in my .bashrc on the old system that included a large number of directories including for Cadence and other miscellaneous software. I don’t want to use the LD_LIBRARY_PATH, I want to be able to link against libstdc++ by supplying the appropriate command line parameters to llvm-ld.

The command I am running is:

llvm-ld -disable-internalize -native -o foo foo.bc4 -L/usr/lib/x86_64-linux-gnu -lpthread -lrt  -lstdc++ -lm -v

which results in the following output:

  Linking bitcode file 'foo.bc4'
  Linked in file 'foo.bc4'
  Linking archive file '/usr/lib/x86_64-linux-gnu/libpthread.a'
  Linking archive file '/usr/lib/x86_64-linux-gnu/librt.a'
llvm-ld: error: Cannot find library 'stdc++'

However running ls -l /usr/lib/x86_64-linux-gnu/libstdc++* results in:

lrwxrwxrwx 1 root root     19 Apr 15 16:34 /usr/lib/x86_64-linux-gnu/libstdc++.so.6 ->    libstdc++.so.6.0.16
-rw-r--r-- 1 root root 962656 Apr 15 16:36 /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16

So I don’t understand why llvm-ld is not finding this file? Especially since when I compile with with the LD_LIBRARY_PATH set and then run ldd on the resulting executable I get the following output:

linux-vdso.so.1 =>  (0x00007ffff7ffe000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007ffff7dc1000)
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007ffff7ac0000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007ffff77c6000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007ffff75b0000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ffff71f0000)
/lib64/ld-linux-x86-64.so.2 (0x0000555555554000)

Which seems to indicate that the version of libstdc++ that I want is /usr/lib/x86_64-linux-gnu/libstdc++.so.6 but I can’t figure out why llvm-ld is not locating it with the search path -L/usr/lib/x86_64-linux-gnu.

For reference: uname -a results in: Linux FOO 3.2.0-30-generic #48-Ubuntu SMP Fri Aug 24 16:52:48 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

And llvm-ld --version:

LLVM (http://llvm.org/):
  LLVM version 3.1svn
  Optimized build.
  Built Sep 14 2012 (13:22:38).
  Default target: x86_64-unknown-linux-gnu
  Host CPU: core2

Answer 1

It looks like llvm-ld isn’t looking for .so.#. According to the man page:

When looking for a library specified with the -l option, llvm-ld first attempts to load a file with that name from the current directory. If that fails, it looks for liblibrary.bc, liblibrary.a, or liblibrary.shared library extension, in that order, in each directory added to the library search path with the -L option. These directories are searched in the order they are specified. If the library cannot
be located, then llvm-ld looks in the directory specified by the LLVM_LIB_SEARCH_PATH environment variable. If it does not find a library there, it fails.

You can make this work by creating a symlink /usr/lib/x86_64-linux-gnu/libstdc++.so -> libstdc++.so.6.

I usually link with clang directly, since it understands searching for C++ library things better.