Re: Category Unknown ?

[Andreas Buff]

    Hi Andreas,
      
      thanks again for trying to help. I really appreciate it!
      
      Find feedback inlined.
      
      Best,
      Andreas

Am 15.10.21 um 17:56 schrieb Andreas Fink:

> i have done precompiled libraries for debian10 and 11 and they work fine
> for me.
>     
    Just to avoid misunderstandings: I do _not_ have a general issue
    using pre-compiled static libs. Its only about Objective-C
    Categories that cause issues.
    
    The behaviour is expected when linking a static lib containing
    Categories without the `-ObjC` linker flag.
    
    From the
      Apple Docs:
    "
    # Objective-C
    
    The dynamic nature of Objective-C complicates things slightly.
    Because the code that implements a method is not determined until
    the method is actually called, Objective-C does not define linker
    symbols for methods. Linker symbols are only defined for classes.
    
    For example, if main.m includes the code [[FooClass alloc]
    initWithBar:nil]; then main.o will contain an undefined symbol for
    FooClass, but no linker symbols for the -initWithBar: method will be
    in main.o.
    
    Since categories are a collection of methods, using a category's
    method does not generate an undefined symbol. This means the linker
    does not know to load an object file defining the category, if the
    class itself is already defined. This causes the same "selector not
    recognized" runtime exception you would see for any unimplemented
    method.
    
    # The -ObjC Linker Flag
    
    Passing the -ObjC option to the linker causes it to load all members
    of static libraries that implement any Objective-C class or
    category. This will pickup any category method implementations. But
    it can make the resulting executable larger, and may pickup
    unnecessary objects. For this reason it is not on by default
    "
    The problem seems to be that the `-ObjC` flag is used by the
    compiler (who doesn't know and thus ignores it) instead of the
    linker using the GNUstep make system.
    
    I have created a simple project to reproduce, showing the issue,
    taking your advice into account. 
    
    ## HowTo Reproduce
    
    ```
    # clone
    git clone
http://pep-security.lu/gitlab/andreas/category-linking-problem-sample.git
    
    # build
    cd category-linking-problem-sample
    sh build.sh
    # note: compiler output "clang: warning: argument unused during
    compilation: '-ObjC' [-Wunused-command-line-argument]"
    
    # run
    ./obj/LinkFailSample
    ```
    
    Expected:
    
    Console output is
    ```
    myDebugPrint
    myDebugPrintCategory
    ```
    
    Actual:
    Console output is
    ```
    myDebugPrint
    ./obj/LinkFailSample: Uncaught exception NSInvalidArgumentException,
    reason: MyClass(instance) does not recognize myDebugPrintCategory
    ```
    
    Additional Info:
    
    `nm` shows the Category _is_ contained in the lib:
    
    ```
    nm static-lib/obj/libmyLib.a | grep myDebugPrint
    0000000000000000 t _i_MyClass__myDebugPrint
    0000000000000000 V .objc_selector_myDebugPrint_v160:8
    0000000000000000 V .objc_sel_name_myDebugPrint
    0000000000000000 V .objc_str_myDebugPrint
    0000000000000000 t _i_MyClass_Extension_myDebugPrintCategory #HERE
    0000000000000000 V .objc_selector_myDebugPrintCategory_v160:8
    0000000000000000 V .objc_sel_name_myDebugPrintCategory
    0000000000000000 V .objc_str_myDebugPrintCategory
    ```
    
    `nm` shows it's _not_ in the excecutable:
    
    ```
    nm obj/LinkFailSample | grep myDebugPrint
    0000000000400b40 t _i_MyClass__myDebugPrint
    0000000000403220 d .objc_selector_myDebugPrintCategory_v160:8
    0000000000403210 d .objc_selector_myDebugPrint_v160:8
    0000000000400bf0 r .objc_sel_name_myDebugPrint
    0000000000400c10 r .objc_sel_name_myDebugPrintCategory
    0000000000403330 d .objc_str_myDebugPrint
    ```

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