避免静态库中的符号引用

Question

I develop a static library that is distributed to other developers. I want to use CocoaLumberjack (DDLog) class if it is available in the final binary. In the static library I define the class interface and check [DDLog class] to see if it exists. But in the host App, if CocoaLumberjack isn't present, the linker complains because DDLog does not exist.

I know I can defer symbol checking to runtime in the App configuration, but is there a way to prevent the static library compilation from referencing the DDLog class in the compiled objects?

Answer 1

I don't think it is possible to have undefined symbols at final App link time, even if they are weak. From the docs for Mac OS X ld:

When creating a output file with the static link editor when -twolevel_namespace is in effect (now the default) all undefined references must be satisfied at static link time. The flags to allow undefined references, -Usymbol_name, -undefined warning and -undefined sup_press can't be used. When the environment variable MACOSX_DEPLOYMENT_TARGET is set to 10.3 then -undefined dynamic_lookup can also be used.

So if there are any undefined references at link time (including any API you use that is not in the current Base SDK version) it will produce an error. The only way around this is to use the -undefined dynamic_lookup linker option. Unfortunately this defers all symbol lookup to runtime, you can't specify just the symbols you want to skip when using two level namespace.

For me, I don't want to burden the end developer with that. So I switched to using objc_msgSend and NSClassFromString instead, avoiding all use of the symbol. It's a shame it has to be done in that way and this seems like something Apple could improve.