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From: Evan Bowman <evan.a.bowman@gmail.com>
Date: Wed, 3 Mar 2021 12:05:49 -0500
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Subject: Question about selectors in the Objective-C ABI
To: gcc-help@gcc.gnu.org
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Hello,

Yesterday, I was playing around with objective-c and GCC, and I have a
question about module loading and selectors.

Given this minimal sample code:
```
#import "Object.h"
#include <stdio.h>

@interface HelloWorld:Object {
}
+(void) hello: (const char*) param;
+(void) world: (const char*) param;
@end

@implementation HelloWorld
+(void) hello: (const char*) param {
    printf("hello %s\n", param);
}

+(void) world: (const char*) param {
    printf("world %s\n", param);
}
@end

int main()
{
    [HelloWorld hello:"some text"];
}
```

I am able to get the code to compile and link with my own implementations
of the objective-c runtime ABI functions, but I'm confused about how to
determine the string name of a selector attached to a method definition.
For example, I have this C code, which I run upon sending the hello:
message to class HelloWorld, in the above example:

void debug_walk_class(struct objc_class_gsv1* class, int depth)
{
    printf("name: %s, info: %zu, inst_size: %zu, abi: %zu\n",
           class->name,
           class->info,
           class->instance_size,
           class->abi_version);

    if (class->methods) {
        struct objc_method_list_gcc* methods = class->methods;

        for (int i = 0; i < methods->count; ++i) {
            struct objc_method_gcc* method = &methods->methods[i];
            printf("method: %s %zu\n",
                   method->types ? method->types : "no types",
                   method->selector->index);
        }
    }

    puts("");

    if (class->isa && depth < 2) {
        debug_walk_class((struct objc_class_gsv1*)class->isa, depth + 1);
    }
}

id objc_msg_lookup(id receiver, SEL selector)
{
    debug_walk_class((struct objc_class_gsv1*)receiver, 0);
}

The output, when walking the class tree, looks like this:
name: (null), info: 0, inst_size: 94483748880412, abi: 1

name: HelloWorld, info: 1, inst_size: 8, abi: 0

name: HelloWorld, info: 2, inst_size: 104, abi: 0
method: v24@0:8r*16 5692614131986886519
method: v24@0:8r*16 8574917940748248424

So, I see the two methods in the metaclass, but when I try to print the
selector->name string rather than selector->index, the code segfaults, so I
guess the selector in this case holds an index rather than a string? Does
anyone know how to determine the string name of the selector attached to an
objc_method_gcc struct? If not, what does the index represent? The selector
passed to objc_msg_lookup, on the other hand, does seem to be a string, I
can print it just fine.