[PATCH v2] tst: Provide test for difftime

[Paul Eggert]

On 1/27/21 4:42 AM, Lukasz Majewski wrote:
> This change adds new test to assess difftime's functionality by
> adding some arbitrary offsets to current time_t value (read via
> time).

That would make tests irreproducible. Why is that helpful for difftime? 
I suggest using only reproducible tests; this should make the code 
simpler anyway.

> +static void
> +test_difftime_helper (time_t start, time_t t1, time_t t0, double exp_val)

The signature should be simpler:

static void
test_difftime_helper (long long t1, long long t0, double exp_val)

And test_difftime_helper can test whether its arguments fit into time_t 
with code like this:

    time_t u1;
    if (__builtin_add_overflow (t1, 0, &u1))
      return;

That way, you can omit the 'sizeof (time_t) > 4' test, which is a bit 
dubious anyway as it wouldn't work on hypothetical hosts where char is 
wider than 8 bits. You can do this sort of thing in all the time_t 
tests, to omit all uses of sizeof (time_t).