Never mind! I answered my own question. It seems that for short names, gcc
is doing a small size optimization, by storing string literals in-place
within the selectors, rather than as a pointer to the string.
On Wed, Mar 3, 2021 at 12:05 PM Evan Bowman <evan.a.bowman@xxxxxxxxx> wrote:
> 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.
>
