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Technical Q&A QA1490
Building Objective-C static libraries with categories

Q: Why do I get a runtime exception of "selector not recognized" when linking against an Objective-C static library that contains categories?

A: The "selector not recognized" runtime exception occurs due to an issue between the implementation of standard UNIX static libraries, the linker and the dynamic nature of Objective-C. Objective-C does not define linker symbols for each function (or method, in Objective-C) - instead, linker symbols are only generated for each class. If you extend a pre-existing class with categories, the linker does not know to associate the object code of the core class implementation and the category implementation. This prevents objects created in the resulting application from responding to a selector that is defined in the category.

To resolve this issue, the static library should pass the -ObjC option to the linker. This flag causes the linker to load every object file in the library that defines an Objective-C class or category. While this option will typically result in a larger executable (due to additional object code loaded into the application), it will allow the successful creation of effective Objective-C static libraries that contain categories on existing classes.

Document Revision History

2006-10-03Changed "and" to "an" in order to make the content correct.
2006-09-25Describes how to properly build Objective-C static libraries that contain categories on existing classes.

Posted: 2006-10-03

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