Re: [PATCH v3 3/4] tests: Avoid sprintf() with "%.6f"

[Markus Armbruster <armbru@redhat.com>]

Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp> writes:

> A string that represents a double can be long if it is an exponentially
> large number.

I'd like us to spell out that this is cleanup, not a bug fix.  I suggest
to start with the warning.  Something like this:

    tests: Clean up double comparisons to avoid compiler warning

    To enable -Wformat-overflow=2, we need to clean up a couple of false
    positives:

    <insert warnings here>

Now explain why they are false:

    These buffers cannot actually overflow because the doubles are
    between 0 and 31.0/3 inclusive.

Then justify the solution:

    However, formatting doubles just to compare them is silly.  Compare
    them directly instead.  To avoid potential rounding trouble, change
    the numbers tested to be representable exactly in double.

> Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> ---
>  tests/unit/test-qobject-input-visitor.c  | 8 ++------
>  tests/unit/test-qobject-output-visitor.c | 7 ++-----
>  2 files changed, 4 insertions(+), 11 deletions(-)
>
> diff --git a/tests/unit/test-qobject-input-visitor.c b/tests/unit/test-qobject-input-visitor.c
> index 84bdcdf702e0..beee11db4e47 100644
> --- a/tests/unit/test-qobject-input-visitor.c
> +++ b/tests/unit/test-qobject-input-visitor.c
> @@ -500,7 +500,7 @@ static void test_visitor_in_list_struct(TestInputVisitorData *data,
>      g_string_append_printf(json, "'number': [");
>      sep = "";
>      for (i = 0; i < 32; i++) {
> -        g_string_append_printf(json, "%s%f", sep, (double)i / 3);
> +        g_string_append_printf(json, "%s%f", sep, (double)i / FLT_RADIX);
>          sep = ", ";
>      }
>      g_string_append_printf(json, "], ");
> @@ -583,11 +583,7 @@ static void test_visitor_in_list_struct(TestInputVisitorData *data,
>  
>      i = 0;
>      for (num_list = arrs->number; num_list; num_list = num_list->next) {
> -        char expected[32], actual[32];
> -
> -        sprintf(expected, "%.6f", (double)i / 3);
> -        sprintf(actual, "%.6f", num_list->value);
> -        g_assert_cmpstr(expected, ==, actual);
> +        g_assert_cmpfloat(num_list->value, ==, (double)i / FLT_RADIX);
>          i++;
>      }
>  
> diff --git a/tests/unit/test-qobject-output-visitor.c b/tests/unit/test-qobject-output-visitor.c
> index 407ab9ed505a..3c47b28f6638 100644
> --- a/tests/unit/test-qobject-output-visitor.c
> +++ b/tests/unit/test-qobject-output-visitor.c
> @@ -538,7 +538,7 @@ static void test_visitor_out_list_struct(TestOutputVisitorData *data,
>      }
>  
>      for (i = 31; i >= 0; i--) {
> -        QAPI_LIST_PREPEND(arrs->number, (double)i / 3);
> +        QAPI_LIST_PREPEND(arrs->number, (double)i / FLT_RADIX);
>      }
>  
>      for (i = 31; i >= 0; i--) {
> @@ -571,12 +571,9 @@ static void test_visitor_out_list_struct(TestOutputVisitorData *data,
>      i = 0;
>      QLIST_FOREACH_ENTRY(qlist, e) {
>          QNum *qvalue = qobject_to(QNum, qlist_entry_obj(e));
> -        char expected[32], actual[32];
>  
>          g_assert(qvalue);
> -        sprintf(expected, "%.6f", (double)i / 3);
> -        sprintf(actual, "%.6f", qnum_get_double(qvalue));
> -        g_assert_cmpstr(actual, ==, expected);
> +        g_assert_cmpfloat(qnum_get_double(qvalue), ==, (double)i / FLT_RADIX);
>          i++;
>      }

With an improved commit message
Reviewed-by: Markus Armbruster <armbru@redhat.com>