Moving the kernel to modern C [LWN.net]

Moving the kernel to modern C

Posted Feb 24, 2022 21:11 UTC (Thu) by abatters (✭ supporter ✭, #6932)
In reply to: Moving the kernel to modern C by iabervon
Parent article: Moving the kernel to modern C

It would break code that does this:

list_for_each_entry(iterator, &foo_list, list) {
    	if (do_something_with(iterator)) {
    		break;
    	}
}
if (list_entry_is_head(iterator, &foo_list, list)) {
	// iteration finished
} else {
	do_something_else_with(iterator);
}

All this "compare to head" nonsense is why I prefer regular NULL-terminated linked lists to the kernel's circular linked lists. Insert/delete may take more instructions but iteration is much easier.

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Moving the kernel to modern C

Posted Feb 24, 2022 21:55 UTC (Thu) by nybble41 (subscriber, #55106) [Link]

Yes, and you also have macros like for_each_list_entry_continue() which depend on the value being left in the iterator. All of these would also break if the macro was changed to declare the iterator inside the `for` statement, C99-style.

One way to work around the problem in your example would be to move the condition inside the loop, like this:

list_for_each_entry(iterator, &foo_list, list) {
    // ...
    if (do_something_with(iterator)) {
        do_something_else_with(iterator);
        break;
    }
    // ...
    if (&iterator->list == &foo_list) {
        // this is the last entry; iteration finished
    }
}

The compiler should be smart enough to avoid checking the end condition twice in each iteration. Of course this becomes much less convenient if there is more than one break statement.