Unused code in call_category_loads()
While hacking around for the Mach-O-project I discovered a unused code block in the source code of Objective-C, version 4.646. Especially in file objc4-646/runtime/objc-loadmethod.mm, function call_category_loads(). This function is called on every App start on every Mac using OS X 10.10 Yosemite.
It’s caused by the variable loadable_categories_used. Take a closer look to call_category_loads.c:
// Original: http://www.opensource.apple.com/source/objc4/objc4-646/runtime/objc-loadmethod.mm
static BOOL call_category_loads(void){
// ...
int used = loadable_categories_used;
// ...
loadable_categories_used = 0;
// ...
new_categories_added = (loadable_categories_used > 0);
for (i = 0; i < loadable_categories_used; i++) {
if (used == allocated) {
allocated = allocated*2 + 16;
cats = (struct loadable_category *)
_realloc_internal(cats, allocated * sizeof(struct loadable_category));
}
cats[used++] = loadable_categories[i];
}
if (loadable_categories) _free_internal(loadable_categories);
if (used) {
loadable_categories = cats;
// ...
} else {
// ...
loadable_categories = nil;
// ...
}
// ...
return new_categories_added;
}
Between
loadable_categories_used = 0;
and
new_categories_added = (loadable_categories_used > 0);
for (i = 0; i < loadable_categories_used; i++) {
the variable loadable_categories_used is not used. So this function returns always false. The code in the for-loop will never be reached.
The same case occurs with variable loadable_categories. Even if the loop is running the line
cats[used++] = loadable_categories[i];
cause an error because loadable_categories is nil at this point.
To be sure I compiled my own version of this code block. It seems that this is legit.
Thanks to Christian Kremser for reviewing. I reported this bug (19846374) to Apple.
Today we made the Matrix a little bit better.
Update, 2015-03-19: It turns out I was wrong. Apple replied me on 2015-02-26:
That code is intended to handle the case where a category +load method calls dlopen() or NSBundle to load another library that itself has more category +load methods. In that case loadable_categories and loadable_categories_used will become populated inside the calls to
(*load_method)(cls, SEL_load).We are now closing this bug report.