Re: determine base type of a typedef

[Nick Bowler]

On 23/10/2020, Paul Eggert <eggert@cs.ucla.edu> 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)],
    [#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