Welcome back to another Friday Q&A. This week I'd like to take Joshua Pennington's idea and elaborate on a particular facet last week's topic of the Objective-C runtime, namely messaging. How does messaging work, and what exactly does it do? Read on!

Definitions
Before we get started on the mechanisms, we need to define our terms. A lot of people are kind of unclear on exactly what a "method" is versus a "message", for example, but this is critically important for understanding how the messaging system works at the low level.

• Method: an actual piece of code associated with a class, and which is given a particular name. Example: - (int)meaning { return 42; }
• Message: a name and a set of parameters sent to an object. Example: sending "meaning" and no parameters to object 0x12345678.
• Selector: a particular way of representing the name of a message or method, represented as the type SEL. Selectors are essentially just opaque strings that are managed so that simple pointer equality can be used to compare them, to allow for extra speed. (The implementation may be different, but that's essentially how they look on the outside.) Example: @selector(meaning).
• Message send: the process of taking a message and finding and executing the appropriate method.

Methods
The next thing that we need to discuss is what exactly a method is at the machine level. From the definition, it's a piece of code given a name and associated with a particular class, but what does it actually end up creating in your application binary?

Methods end up being generated as straight C functions, with a couple of extra parameters. You probably know that self is passed as an implicit parameter, which ends up being an explicit parameter. The lesser-known implicit parameter _cmd (which holds the selector of the message being sent) is a second such implicit parameter. Writing a method like this:

    - (int)foo:(NSString *)str { ...

    int SomeClass_method_foo_(SomeClass *self, SEL _cmd, NSString *str) { ...

What, then, happens when we write some code like this?

    int result = [obj foo:@"hello"];

    int result = ((int (*)(id, SEL, NSString *))objc_msgSend)(obj, @selector(foo:), @"hello");

What that ridiculous piece of code after the equals sign does is take the objc_msgSend function, defined as part of the Objective-C runtime, and cast it to a different type. Specifically, it casts it from a function that returns id and takes id, SEL, and variable arguments after that to a function that matches the prototype of the method being invoked.

To put it another way, the compiler generates code that calls objc_msgSend but with parameter and return value conventions matched to the method in question.

Messaging
A message send in code turns into a call to objc_msgSend, so what does that do? The high-level answer should be fairly apparent. Since that's the only function call present, it must look up the appropriate method implementation and then call it. Calling is easy: it just needs to jump to the appropriate address. But how does it look it up?

The Objective-C header runtime.h includes this as part of the (now opaque, legacy) objc_class structure members:

    struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;

    struct objc_method_list {
        struct objc_method_list *obsolete                        OBJC2_UNAVAILABLE;
    
        int method_count                                         OBJC2_UNAVAILABLE;
    #ifdef __LP64__
        int space                                                OBJC2_UNAVAILABLE;
    #endif
        /* variable length structure */
        struct objc_method method_list[1]                        OBJC2_UNAVAILABLE;
    }                                                            OBJC2_UNAVAILABLE;

    struct objc_method {
        SEL method_name                                          OBJC2_UNAVAILABLE;
        char *method_types                                       OBJC2_UNAVAILABLE;
        IMP method_imp                                           OBJC2_UNAVAILABLE;
    }                                                            OBJC2_UNAVAILABLE;

So even though we're not supposed to touch these structs (don't worry, all the functionality for manipulating them is provided through functions in elsewhere in the header), we can still see what the runtime considers a method to be. It's a name (in the form of a selector), a string containing argument/return types (look up the @encode directive for more information about this one), and an IMP, which is just a function pointer:

    typedef id 			(*IMP)(id, SEL, ...);

One more detail needs to be considered here. The above procedure would work but it would be extremely slow. objc_msgSend only takes about a dozen CPU cycles to execute on the x86 architecture, which makes it clear that it's not going through this lengthy procedure every single time you call it. The clue to this is another objc_class member:

    struct objc_cache *cache                                 OBJC2_UNAVAILABLE;

    struct objc_cache {
        unsigned int mask /* total = mask + 1 */                 OBJC2_UNAVAILABLE;
        unsigned int occupied                                    OBJC2_UNAVAILABLE;
        Method buckets[1]                                        OBJC2_UNAVAILABLE;
    }

(There is actually one more detail beyond this which ends up being extremely important: what happens when no method can be found for a given selector. But that one is so important that it deserves its own post, so look for it next week.)

Conclusion
That wraps up this week's edition. Come back next week for more. Have a question? Think Objective-C's messaging system should be done differently? Post below.

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