Resolving circular dependencies by linking the same library twice?

Question

We have a code base broken up into static libraries. Unfortunately, the libraries have circular dependencies; e.g., libfoo.a depends on libbar.a a

Answer 1

All I can offer is a lack of counter-example. I've actually never seen the first form before (even though it's clearly better) and always seen this solved with the second form, and haven't observed problems as a result.

Even so I would still suggest changing to the first form because it clearly shows the relationship between the libraries rather than relying on the linker behaving in a particular way.

That said, I would suggest at least considering if there's a possibility of refactoring the code to pull out the common pieces into additional libraries.

Answer 2

Since it is a legacy application, I bet the structure of the libraries is inherited from some arrangement which probably does not matter any more, such as being used to build another product which you no longer do.

Even if still structural reasons remain for the inherited library structure, almost certainly, it would still be acceptable to build one more library from the legacy arrangement. Just put all the modules from the 20 libraries into a new library, liballofthem.a. Then every single application is simply g++ -o myApp -lallofthem ...

Answer 3

The problem with

g++ -o myApp -lfoo -lbar -lfoo

is that there is no guarantee, that two passes over libfoo and one pass over libbar are enough.

The approach with Wl,--start-group ... -Wl,--end-group is better, because more robust.

Consider the following scenario (all symbols are in different object-files):

• myApp needs symbol fooA defined in libfoo.
• Symbol fooA needs symbol barB defined in libbar.
• Symbol barB needs symbol fooC defined in libfoo. This is the circular dependency, which can be handled by -lfoo -lbar -lfoo.
• Symbol fooC needs symbol barD defined in libbar.

To be able to build in the case above, we would need to pass -lfoo -lbar -lfoo -lbar to the linker. Why?

1. The linker sees libfoo for the first time and uses definitions of symbol fooA but not fooC, because so far it doesn't see a necessity to include also fooC into the binary. The linker however starts to look for definition of barB, because its definition is needed for fooA to function.
2. The linker sees -libbar, includes the definition of barB (but not barD) and starts to look for definition of fooC.
3. The definition of fooC is found in libfoo, when it processed for the second time. Now it becomes evident, that also the definition of barD is needed - but too late there is no libbar on the command line anymore!

The example above can be extended to an arbitrary dependency depth (but this happens seldom in real life).

Thus using

g++ -o myApp -Wl,--start-group -lfoo -lbar -Wl,--end-group

is a more robust approach, because linker passes as often over the library group as needed - only when a pass didn't change the symbol table will the linker move on to the the next library on the command line.

There is however a small performance penalty to pay: in the first example -lbar were scanned once more compared with the manual command line -lfoo -lbar -lfoo. Not sure whether it is worth mentioning/thinking about through.