What is objc-classrefs for?

In the context of macOS and iOS development, particularly when dealing with Objective-C, __objc_classrefs is a section within the Mach-O executable file. It's crucial for the Objective-C runtime and dynamic linking.

Specifically, __objc_classrefs (or more formally, the __DATA,__objc_classrefs section) holds pointers to Objective-C class structures that are referenced by the code in a binary but not directly defined within that binary. Think of it as a list of "external" Objective-C classes the current binary depends on.

Here's a breakdown:

1. Mach-O Executable Format: macOS and iOS executables (and dynamic libraries/frameworks) are typically in the Mach-O format. This format divides the file into segments and sections. Sections store specific types of data, like code, constants, and Objective-C metadata.

2. Objective-C Runtime: The Objective-C runtime is the engine that powers Objective-C's dynamic features like message sending, dynamic class loading, and introspection. It relies heavily on metadata stored in the Mach-O file.

3. Dynamic Linking: Objective-C applications often link against frameworks and libraries at runtime (dynamic linking). This means the application needs to find the classes and methods defined in those external libraries when the application is launched or when a particular library is loaded.

4. Role of __objc_classrefs: When your Objective-C code uses a class (for instance, NSString or a custom class defined in another framework), the compiler generates code that needs to access the class's metadata at runtime. The __objc_classrefs section contains pointers to these external class structures. The dynamic linker uses this section to resolve those pointers, ensuring your application can correctly call methods on those classes.

For Example, consider a scenario where your application uses a custom class named MyCustomView that's defined in a separate framework called MyFramework.

1. Your application binary will contain code that references MyCustomView.

2. The __objc_classrefs section in your application's Mach-O file will contain a pointer that is intended to point to the MyCustomView class structure.

3. At runtime, the dynamic linker will load MyFramework into memory.

4. The dynamic linker will then update the pointer in the __objc_classrefs section to point to the actual memory address of the MyCustomView class structure within the loaded MyFramework.

Without __objc_classrefs (and similar sections like __objc_selrefs for selectors and __objc_superrefs for superclasses), dynamic linking of Objective-C code wouldn't be possible. The Objective-C runtime would be unable to find the classes and methods required for your application to function correctly.

You can inspect the contents of these sections using tools like otool (a command-line tool for examining object files) on macOS. For example:

otool -v -s __DATA __objc_classrefs /path/to/your/binary

This command will display the contents of the __objc_classrefs section, showing the addresses of the referenced classes.

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