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.
Thanks
Laszlo