Extraneous Library Linkage

Question

I have a question which may be somewhat silly because I'm pretty sure I may know the answer already.

Suppose you have static library A, and dynamic shared object library B and your program C under linux. Suppose that library A calls functions from library B and your program calls functions from library A. Now suppose that all functions that C calls in A make no use of functions in B.

To compile C will it be enough to link just A and omit B and furthermore can your program C be run on a system without library B installed?

Answer 1

If your program calls functions in A that don't reference B then B is not required either at link or load time, assuming that the functions in A are in separate compilation units, which is usually the case for a library.

The linker will pull the functions from the library that C uses and since none of them call functions in B, B will not be needed.

Answer 2

Holy placeholder name overload, batman. Let's first replace A, B, and C, with libstatic, libshared, and myapp to make things a little more legible:

Suppose you have static library libstatic, and dynamic shared object library libshared and your program myapp under linux. Suppose that library libstatic calls functions from library libshared and your program (myapp) calls functions from library libstatic. Now suppose that all functions that myapp calls in libstatic make no use of functions in libshared.

To compile myapp will it be enough to link just libstatic and omit libshared and furthermore can your program myapp be run on a system without library libshared installed?

So the way I understand your question, there is a library libstatic, some functions in which make use of libshared. You want to know: if I don't use any of the libstatic functions that are dependent on libshared, will myapp link and run without libshared?

The answer is yes, so long as two things are true:

1. The calls you make into libstatic do not depend on libshared directly or indirectly. Meaning that if myapp calls a function in libstatic which calls another function in libstatic which calls a function in libshared, then myapp is now dependent on libshared.

2. The calls you make into libstatic do not depend on any function in libstatic whose implementation appears in the same compilation unit (object file) with a call to libshared. The linker brings in code from the static library at the level of object files, not at the level of individual functions. And remember, this dependency is similarly chained, so if you call a function in foo.o, and something else in foo.o calls a function in bar.o, and something in bar.o depends on libshared, you're toast.

When you link in a static library into an application, only the object files that contain the symbols used (directly or indirectly) are linked. So if it turns out that none of the object files that myapp ends up needing from libstatic depend on libshared, then myapp doesn't depend on libshared.