Eric Wong <e@xxxxxxxxx> writes:
> While we cannot rely on a `__typeof__' operator being portable
> to use with `offsetof'; we can calculate the pointer offset
> using an existing pointer and the address of a member using
> pointer arithmetic.
> +/*
> + * like offsetof(), but takes a pointer to type instead of the type
It actually takes "a pointer to a variable of the type". I had to
read the above twice to guess what is going on.
> + * @ptr is subject to multiple evaluation since we can't rely on TYPEOF()
> + */
> +#define OFFSETOF_VAR(ptr, member) \
> + ((uintptr_t)&(ptr)->member - (uintptr_t)(ptr))
> +
This unfortunately has funny interactions with using uninitialized
variables. "make CC=clang config.o" gives something like this:
config.c:1944:50: error: variable 'entry' is uninitialized when used here
[-Werror,-Wuninitialized]
hashmap_for_each_entry(&cs->config_hash, &iter, entry,
^~~~~
./hashmap.h:453:20: note: expanded from macro 'hashmap_for_each_entry'
OFFSETOF_VAR(var, member)); \
^~~
./git-compat-util.h:1346:16: note: expanded from macro 'OFFSETOF_VAR'
((uintptr_t)&(ptr)->member - (uintptr_t)(ptr))
^~~
./hashmap.h:445:61: note: expanded from macro 'hashmap_iter_first_entry_offset'
container_of_or_null_offset(hashmap_iter_first(map, iter), offset)
^~~~~~
config.c:1939:34: note: initialize the variable 'entry' to silence this warning
struct config_set_element *entry;
^
= NULL
Personally, I feel the workaround suggested by the compiler is just
as bogus as the warning itself X-<. Casting NULL as if it were a
pointer to an object of type typeof(*ptr), treating it as if it is a
valid address, and doing raw address arith is just as bogus as doing
the same raw address arith on an uninitialized ptr, isn't it?
Anyway...
