Nick,
Thanks for your suggestions! They gave me some additional keywords to search for
more ideas.
After trying your code I found that I could actually use _Generic directly in
the C sources, no need for autoconf, e.g.:
time_t _unused_t;
#define TIME_T_FMT _Generic((_unused_t), long long int: "%lld", long int: "%ld", int: "%d")
Or even a generic int type format specifier handler:
#define _FTM(x) _Generic((x), long long int: "%lld", long int: "%ld", int: "%d", \
unsigned long long int: "%llu", unsigned long int: "%lu", unsigned int: "%u")
And for the case when we can't rely on C11, I just found there's
__builtin_types_compatible_p available since at least gcc 3.1.1 (2002) and cland 1.0:
#define TIME_T_FMT (__builtin_types_compatible_p(time_t, long long int) ? "%lld" : \
__builtin_types_compatible_p(time_t, long int) ? "%ld" : "%d")
On 23/10/20 15:03, Nick Bowler wrote:
> On 2020-10-23, Nick Bowler <nbowler@xxxxxxxxxx> wrote:
>> On 23/10/2020, Paul Eggert <eggert@xxxxxxxxxxx> wrote:
>>> On 10/22/20 6:09 PM, Russell Shaw wrote:
>>>> else if(sizeof(time_t) == sizeof(long int)) {
>>>
>>> This is not the right kind of test. You want to test whether time_t and
>>> int
>>> are
>>> the same types, not whether they're the same size. To do that, you should
>>> use
>>> code like this:
>>>
>>> extern time_t foo;
>>> extern long int foo;
>>>
>>> Of course this means you'll need to compile N programs rather than
>>> one, but that's life in the big Autoconf city.
>>
>> To improve configure performance when N is more than one or two,
>> you can use C11 _Generic and AC_COMPUTE_INT to pretty easily and
>> quickly determine which type (out of a finite list of candidates)
>> time_t or any other type is compatible with.
>>
>> But you'd need a fallback (probably by compiling one program
>> like the one shown abovce for each type) to handle the case
>> where _Generic is not supported by the implementation.
>>
>> Example (totally untested):
>>
>> AC_COMPUTE_INT([timetype],
>> [_Generic((time_t)0, long long: 3, default: 0)
>> + _Generic((time_t)0, long: 2, default: 0)
>> + _Generic((time_t)0, int: 1, default: 0)],
>
> On review, it is obvious this list of types could be more succinctly
> written with a single _Generic as they are all for sure different:
>
> _Generic((time_t)0, long long: 3, long: 2, int: 1, default: 0)
>
> But care must be taken if any of the generic cases are themselves
> typedefs, (for example, if we wanted to determine whether POSIX
> ssize_t is compatible with a list of types that includes ptrdiff_t),
> as it is an error to include compatible types among the list of
> cases:
>
> /* error if ptrdiff_t happens to be compatible with long long */
> _Generic((ssize_t)0, long long: 2, ptrdiff_t: 1, default: 0)
>
> Nesting avoids this problem better than adding as I did originally:
>
> _Generic((ssize_t)0, long long: 2, default:
> _Generic((ssize_t)0, ptrdiff_t: 1, default: 0))
>
> as only one "branch" can be matched.
>
>> [#include <time.h>],
>> [... slow fallback computation goes here])
>>
>> AS_CASE([$timetype],
>> [3], [... action when time_t is compatible with long long],
>> [2], [... action when time_t is compatible with long],
>> [1], [... action when time_t is compatible with int],
>> [... action when time_t's compatibility is undetermined])
>
> Cheers,
> Nick
>
