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.
