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

On 2/20/23 21:38, Eric Blake wrote: > 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. Let's be honest: it's staggeringly difficult to collect whatever "compatible type" means, in the standard. > >>> >>> 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.

This is doable, but I hope it's not expected that DEFINE_POINTER_VECTOR_TYPE() *enforce* that the element type be a pointer :)

> ADD_VECTOR_EMPTY_METHOD() instead of DEFINE_VECTOR_EMPTY() works for > me. OK, ADD_VECTOR_EMPTY_METHOD() can work with the above.