g++ linking order dependency when linking c code to c++ code

You can use --start-group archives --end-group and write the 2 dependent libraries instead of archives:

gcc main.o -L. -Wl,--start-group -lobj_A -lobj_b -Wl,--end-group

The order you specify object files and libraries is VERY important in GCC - if you haven't been bitten by this before you have lead a charmed life. The linker searches symbols in the order that they appear, so if you have a source file that contains a call to a library function, you need to put it before the library, or the linker won't know that it has to resolve it. Complex use of libraries can mean that you have to specify the library more than once, which is a royal pain to get right.

A static library is a collection of object files grouped into an archive. When linking against it, the linker only chooses the objects it needs to resolve any currently undefined symbols. Since the objects are linked in order given on the command line, objects from the library will only be included if the library comes after all the objects that depend on it.

So the link order is very important; if you're going to use static libraries, then you need to be careful to keep track of dependencies, and don't introduce cyclic dependencies between libraries.

The library order pass to gcc/g++ does actually matter. If A depends on B, A must be listed first. The reason is that it optimizes out symbols that aren't referenced, so if it sees library B first, and no one has referenced it at that point then it won't link in anything from it at all.