linux linker/loader search order

This question is related to the way libraries are looked up during compilation and dynamic linking.

Consider this small project:

• project
  • liba
    • a.hpp
    • a.cpp
  • libb
    • b.hpp
    • b.cpp
  • main.cpp

a.hpp:

int AAA();

a.cpp:

#include <b.hpp>

int AAA()
{
    return BBB();
}

b.hpp:

int BBB();

b.cpp:

int BBB()
{
    return 3;
}

main.cpp:

#include "liba/a.hpp"
#include "libb/b.hpp"

int main()
{
    AAA();
    BBB();
}

libb is comp开发者_开发问答iled with:

cd libb; g++ -shared -o libb.so b.cpp

liba is compiled with:

cd liba; g++ -I../libb/ -L../libb/ -lb -shared -o liba.so -Wl,-rpath /full/path/to/project/libb/ a.cpp

and main is compiled with:

g++ -Lliba -la -Wl,-rpath /full/path/to/project/liba/ main.cpp

The compilation finishes without error, but when executing a.out, libb.so can't be found.

ldd outputs:

ldd ./a.out 
    linux-gate.so.1 =>  (0xffffe000)
    liba.so => /full/path/to/project/liba/liba.so (0xb780a000)
    libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0xb76ba000)
    libm.so.6 => /lib/libm.so.6 (0xb7692000)
    libgcc_s.so.1 => /lib/libgcc_s.so.1 (0xb7672000)
    libc.so.6 => /lib/libc.so.6 (0xb750f000)
    libb.so => not found
    libb.so => /full/path/to/project/libb/libb.so (0xb750c000)
    /lib/ld-linux.so.2 (0xb780e000)

Note, that libb.so occurs twice. One time, the dynamic linker is not able to find the library and one time it is. I assume, that in the second case it uses the rpath embedded in liba.so.

Of course, there are multiple ways to fix that problem (e.g. LD_LIBRARY_PATH, embed the correct rpath when compiling main.cpp...), but I'm more interested in why the compilation of main.cpp works, while the dynamic linking doesn't.

So far I assumed that the same procedure was used for searching the needed libraries. Is there something I'm missing, or some hidden magic I don't know of? :)

(Tested on a SLED11 box with gcc 4.3.4.)

---

There's no hidden magic but you've basically promised the compiler that you're going to tell the loader, at run-time, where to find the functions but you haven't. The BBB called from AAA knows how to get there but the BBB called from main does not.