Re: [Libguestfs] [libnbd PATCH v3 05/29] vector: (mostly) factor out DEFINE_VECTOR_EMPTY

On Mon, Feb 20, 2023 at 06:03:13PM +0100, Laszlo Ersek wrote: > On 2/15/23 21:27, Eric Blake wrote: >> On Wed, Feb 15, 2023 at 03:11:34PM +0100, Laszlo Ersek wrote: >>> The "name##_iter" function is used 11 times in libnbd; in all those cases, >>> "name" is "string_vector", and the "f" callback is "free": >>> >>> string_vector_iter (..., (void *) free); >>> >>> Casting "free" to (void*) is ugly. (Well-defined by POSIX, but still.) >> >> Tangentially related: casting function pointers in general may get >> harder as more compilers move towards C23 and its newer rules (see for >> example >> https://lists.gnu.org/archive/html/bug-gnulib/2023-02/msg00055.html or >> this gcc 13 bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108694 >> which highlights some of the warnings that newer compilers will start >> warning us about). While this patch focuses on avoiding casts between >> fn(*)(type*) and void*, I don't know off-hand if C23 will permit >> fn(*)(void*) as a compatible function pointer with fn(*)(type). > > My understanding is that, per C99 at least, ret_type(*)(void*) is > compatible with ret_type(*)(type*) precisely if void* is compatible with > type* (6.7.5.3p15). > > Whether void* is compatible with type* depends on... ugh, that's hard to > deduce from the standard. 6.7.5.1p2 says, "For two pointer types to be > compatible, both shall be identically qualified and both shall > be pointers to compatible types". I don't think "void" (as a type in > itself) is compatible with any type! > > Now, there is one particular statement on void* -- 6.2.5p27 says, "A > pointer to void shall have the same representation and alignment > requirements as a pointer to a character type." > > (I think the statements about *converting* void* to type* and vice versa > do not apply here; AFAICT "compatibility" is about reinterpreting the > bit patterns, not converting values.) > > In Annex I (Common warnings, "informative"), the following is listed: > "An implicit narrowing conversion is encountered, such as the assignment > of [...] a pointer to void to a pointer to any type other than a > character type". > > All in all I don't think ret_type(*)(type*) is compatible with > ret_type(*)(void*) in the general case, and that's why in this patch I > didn't want to go more general than I absolutely needed to. Thanks for at least trying to find something definitive in the standard. Now you know why I skipped researching this particular issue - it's not straightforward to figure out when function pointers with differing parameter types are compatible.

>> >> Thinking higher-level now, your new macro is something where we have >> to do a two-step declaration of macro types where we want the new >> function. Would it make more sense to change the signature of the >> DEFINE_VECTOR_TYPE() macro to take a third argument containing the >> function name to call on cleanup paths, with the ability to easily >> write/reuse a no-op function for vectors that don't need to call >> free(), where we can then unconditionally declare name##_empty() that >> will work with all vector types? That is, should we consider instead >> doing something like: >> >> DEFINE_VECTOR_TYPE (string_vector, char *, free); >> >> DEFINE_VECTOR_TYPE (int_vector, int, noop); > > My counter-arguments: > > - this requires updates to all existent DEFINE_VECTOR_TYPE macro > invocations, > > - with "noop" passed to _reset, _reset and _empty become effectively the > same, so we get (at least partially) duplicate APIs, > > - this would be a step towards combinatorial explosion > > - if "noop" does nothing, then why call it on each element of the vector > anyway? It's not only the function call that becomes superfluous in the > loop bodym with the function being "noop", but the loop *itself* becomes > superfluous. So then we might want to compare the function pointer > against "noop" outside of the loop... and that way we get a bunch of > complications :) > > I chose this approach because it is purely additive and precisely as > generic/specific as it needs to be. We already have 11 use cases, so I > don't think it's *too* specific. We may still want some division of: DEFINE_VECTOR_TYPE (int_vector, int); DEFINE_POINTER_VECTOR_TYPE (string_vector, char *, free); where under the hood, DEFINE_POINTER_VECTOR_TYPE(type, base, fn) invokes both DEFINE_VECTOR_TYPE(type, base) and DEFINE_VECTOR_EMPTY(type, fn), or whatever we name the second function.

From your other mail in this subthread: >>> +#define DEFINE_VECTOR_EMPTY(name) \ >> >> I'm going to be that guy ... > > Yes, someone has to be! I knew it was virtually impossible for me to get > the name right at the first try :) > >> >> Can we call it ADD_VECTOR_EMPTY_METHOD or similar/better? > > Eric, any comments? ADD_VECTOR_EMPTY_METHOD() instead of DEFINE_VECTOR_EMPTY() works for me.

Whether we hard-code 'free()' as the lone function that will be utilized in the generated TYPE_vector_empty(), or if the macro takes fn as a parameter, is up to you.

Comparing: DEFINE_VECTOR_TYPE(string_vector, char *); ADD_VECTOR_EMPTY_METHOD(string_vector); vs. DEFINE_VECTOR_TYPE(string_vector, char *); ADD_VECTOR_EMPTY_METHOD(string_vector, free); does either one make it more obvious that we are doing a two-step definition (first of the type, then of the added cleanup method)?

And if so, does my idea of a single wrapper function that calls both

intermediate macros make sense

so that the 11 pointer vector types in libnbd are still one-liner macro calls, without penalizing the scalar vector types in nbdkit?