Question

ld supports a few options for specifying “upward dependencies on OS X. What is an upward dependency here?

-upward-lx  This is the same as the -lx but specifies that the dylib is an upward dependency.
-upward_framework name[,suffix]
             This is the same as the -framework name[,suffix] but also specifies that the framework is an upward dependency.

Answer 1

An upward dependency is something that goes against normal dependency order. Suppose the code in library “A” calls functions implemented in library “B”; library “A” depends on “B”. Now suppose that library “B” ALSO contains code that depends on something in library “A”; this is an upward dependency (that is generally a very broken thing to have).

I think if you have a situation like this then you should solve the problem in code by making indirect callbacks through a mediator instead of relying on the linker.

An example of a callback/mediator scheme is as follows…

Imagine that code in library “A” still depends on library “B”, but library “B” has a mechanism for registering callback functions to perform specific actions. A 3rd module acting as the mediator could arrange for a function in library “A” to be invoked via callback from library “B” instead of making library “B” call library “A” directly. The linker now only has to resolve a dependency from “A” to “B”, and at runtime a controlled relationship from “B” to “A” is forged instead of requiring an upward-dependency chain in the link phase.