Missing a library in ldd after using gcc -l

Question

I'm compiling an executable called "interrogate" with:

g++ -o built/bin/interrogate -Lbuilt/lib -Lbuilt/tmp -L/usr/X11R6/lib \
built/tmp/interrogate_composite1.o built/tmp/interrogate_composite2.o \
-lp3cppParser -lp3dtool -lp3dtoolconfig -lp3pystub -pthread -ldl

After the compilation, when i try to execute the executable:

$ LD_LIBRARY_PATH=built/lib built/bin/interrogate
built/bin/interrogate: symbol lookup error: built/lib/libp3dtool.so.1.8: undefined symbol: _Py_NoneStruct

This symbol is provided by the libp3pystub.so, but the interrogate executable doesn't have any reference to this library (I did used the -lp3pystub):

$ LD_LIBRARY_PATH=built/lib ldd built/bin/interrogate
    linux-vdso.so.1 =>  (0x00007fff2016a000)
    libp3dtool.so.1.8 => built/lib/libp3dtool.so.1.8 (0x00007f498d57a000)
    libp3dtoolconfig.so.1.8 => built/lib/libp3dtoolconfig.so.1.8 (0x00007f498d51b000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f498d1f2000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f498cfdc000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f498cdbf000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f498c9ff000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f498c7fb000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f498c4ff000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f498d5bc000)

If i manually load the libp3pystub.so, the executable works:

$ LD_PRELOAD=built/lib/libp3pystub.so LD_LIBRARY_PATH=built/lib built/bin/interrogate

Usage:
  interrogate [opts] file.C [file.C ...]
  interrogate -h

My question is: why the library i've added with -lp3pystub is not referenced by the interrogate executable?

Answer 1

I actually find the answer myself. I was compiling on Ubuntu, and they added a default optimization flags: -Wl,--as-needed. This optimization check if no symbol from the libraries passed with -l are used in the main executable, they will be removed.

And that's where my error was: _Py_NoneStruct is not directly used by interrogate, but by another shared library. So i must manually specify that the p3pystub is needed.

One possible fix would be:

$ g++ -o built/bin/interrogate -Lbuilt/lib -Lbuilt/tmp -L/usr/X11R6/lib \
built/tmp/interrogate_composite1.o built/tmp/interrogate_composite2.o \
-Wl,--no-as-needed -lp3cppParser -lp3dtool -lp3dtoolconfig -lp3pystub \
-pthread -ldl

And then i correctly got the library in the ldd output:

$ LD_LIBRARY_PATH=built/lib ldd built/bin/interrogate
    linux-vdso.so.1 =>  (0x00007fff0edff000)
    libp3dtool.so.1.8 => built/lib/libp3dtool.so.1.8 (0x00007fa1c36be000)
    libp3dtoolconfig.so.1.8 => built/lib/libp3dtoolconfig.so.1.8 (0x00007fa1c365f000)
>>> libp3pystub.so.1.8 => built/lib/libp3pystub.so.1.8 (0x00007fa1c3658000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fa1c342f000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fa1c312c000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fa1c2e2f000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fa1c2c19000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fa1c29fc000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa1c263c000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fa1c3700000)

Reference: https://wiki.ubuntu.com/NattyNarwhal/ToolchainTransition