CODEWITHSUNDEEP
cgccshared-libraries
Albert
Albert

Reputation: 433

Can a dynamic library depend on a static library in C and vice-versa?

I am trying to understand static libraries and shared objects in C. I am trying to understand whether one type of library can depend on other type.

Consider a scenario:

libA.so has a function foo_A_dyn():

libA.so ---> foo_A_dyn()

foo_A_dyn() uses a function foo_B_static() which is defined in libB.a which is a static library.

libB.a ---> foo_B_static()

I have built my libraries in the following way:

gcc -c foo_B.c -o foo_B.o
ar -cvq libB.a foo_B.o

gcc -fPIC -c foo_A.c -o foo_A.o
gcc -shared libA.so foo_A.o -I.

gcc main.c -lA -lB -L. -I. -o EXE

Note: main.c makes call to foo_A_dyn() and does NOT call foo_B_static() directly.

And now when I am trying to build my executable EXE, I am getting the error "undefined reference to foo_B_static".

I think the error seems genuine but I am not able to decode the rationale behind this and put it to words.

Can someone please help?

Upvotes: 3

Views: 1450

Answers (2)

Albert
Albert

Reputation: 433

Here's what the linker is doing. When we link our executable ('EXE' above) it has some symbols (functions and other things) that are unresolved. It will look down the list of libraries that follow in sequential order, trying to resolve unresolved symbols. Along the way, it finds that some of the symbols are provided by libB.so, so it notes that they are now resolved by this library. While going through libB.so it finds some symbols which are unresolved and it tries to resolve them by looking up the library that follows.

When we are ordering the libraries like:

gcc main.c -lA -lB -L. -I. -o EXE

Linker is not able to lookup for the definition of symbols used in libB into libA. Reason could be that backward reference is not available.

I have also figured out that:

  1. shared object can depend on a static archive,

  2. a static archive can depend on a shared object, and

  3. one static archive can depend on another static archive

Please let me know if I have erred somewhere.

Upvotes: 0

KamilCuk
KamilCuk

Reputation: 141930

From gcc link options:

-llibrary
-l library
...
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file foo.o but before bar.o. If bar.o refers to functions in ‘z’, those functions may not be loaded.

Try:

gcc main.c -lB -lA -L. -I. -o EXE

Upvotes: 3

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