An executable and a shared library dependent on a same statically linked library

Question

Suppose you're developing a shared library libshared.so.

And you have a static library libstatic.a with some internal classes and functionality you need. You'd like to link it to your .so like this:

g++ -o libshared.so -shared myObj.o -lstatic

Also you have an executable.sh which will use your .so and dynamically open it in the runtime

dlopen("libshared.so", RTLD_NOW)

You know this executable was as well statically linked against libstatic.a (but you're not sure the version of the library is exactly the same as yours).

So the question is:

Is it safe and correct to statically link your libshared.so against libstatic.a when you know the same library is already used in executable.sh?

Answer 1

You should avoid linking a static library into a shared one.

Because a shared library should have position independent code (otherwise, the dynamic linker has to do too much relocation, and you lose the benefits of shared libraries), but a static library usually does not have PIC.

Read Drepper's paper: How to write a shared library

You build your library with

  g++ -Wall -O -fPIC mySrc.cc -c -o myObj.pic.o
  g++ -o libshared.so -shared myObj.pic.o -lotherlib