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

On 2/22/23 10:40, Laszlo Ersek wrote: > On 2/22/23 09:17, Richard W.M. Jones wrote: >> On Tue, Feb 21, 2023 at 05:23:52PM +0100, Laszlo Ersek wrote: > >>> This is doable, but I hope it's not expected that >>> DEFINE_POINTER_VECTOR_TYPE() *enforce* that the element type be a pointer :) >> >> >> You might ignore this for a first draft, but it is apparently possible >> to statically detect this (at least, if using GCC/clang): >> >> https://stackoverflow.com/questions/19255148/check-if-a-macro-argument-is... > > Right, we already use at least __builtin_types_compatible_p in > TYPE_IS_ARRAY(); that's what I wouldn't want more of, at least via this > series. OK, summarizing the TODOs for this particular patch: 1. keep _iter 2. rename DEFINE_VECTOR_EMPTY to ADD_VECTOR_EMPTY_METHOD 3. introduce DEFINE_POINTER_VECTOR_TYPE that expands to DEFINE_VECTOR_TYPE + ADD_VECTOR_EMPTY_METHOD 4. DEFINE_POINTER_VECTOR_TYPE (and ADD_VECTOR_EMPTY_METHOD) should not take "free" as a parameter; ADD_VECTOR_EMPTY_METHOD should hard-code it 5. in "common/utils/string-vector.h", don't ADD_VECTOR_EMPTY_METHOD; instead, replace DEFINE_VECTOR_TYPE with DEFINE_POINTER_VECTOR_TYPE. 6. in ADD_VECTOR_EMPTY_METHOD, consider checking that the element type is a pointer type.

--*-- Re: 6, I find that the stackoverflow solution above is too complicated. I mentioned our existent TYPE_IS_ARRAY macro: #define TYPE_IS_ARRAY(a) \ (!__builtin_types_compatible_p (typeof (a), typeof (&(a)[0]))) This is perfectly usable for our purposes, as !TYPE_IS_ARRAY(). The reason is that when this macro is applied to anything that's *neither* a pointer *nor* an array, we get a build error at once: #define TYPE_IS_ARRAY(a) \ (!__builtin_types_compatible_p (typeof (a), typeof (&(a)[0]))) int main(void) { int x[5]; int *y; int z; TYPE_IS_ARRAY (x); TYPE_IS_ARRAY (y); TYPE_IS_ARRAY (z); return 0; } ---> isptr.c: In function ‘main’: isptr.c:2:59: error: subscripted value is neither array nor pointer nor vector 2 | (!__builtin_types_compatible_p (typeof (a), typeof (&(a)[0]))) | ^ isptr.c:12:3: note: in expansion of macro ‘TYPE_IS_ARRAY’ 12 | TYPE_IS_ARRAY (z); | ^~~~~~~~~~~~~ (the "nor vector" part of the error message can most likely be ignored, I believe it refers to the C++ standard library vector class) Thus, with types that are neither pointers nor arrays nicely caught at compilation time, !TYPE_IS_ARRAY stands for "pointer".

I'll experiment with this a bit, but if it becomes too complex, I'll likely drop step 6.