Re: Configure warning/error in m4/frexp.m4

[Sam James]

Bruno Haible <bruno@clisp.org> writes:

> Hi Sam,

Hi Bruno,

>
>> The configure test in gl_FUNC_FREXP_WORKS within m4/frexp.m4 triggers
>> a Clang warning/error when Clang is passed
>> -Werror=incompatible-pointer-types.
>
> Gnulib does not support CC or CPPFLAGS or CFLAGS values with -Werror at
> configure time. Neither with gcc nor with clang.

Yes, I understand that. But Clang is likely to make this change to its
default and -Werror=... was just a way of emulating that. gnulib _does_
have to cater to the default strictness of compilers.

>
> The reason is that [1]
>   "Many GCC warning options usually don’t point to mistakes in the code;
>    these warnings enforce a certain programming style. It is a project
>    management decision whether you want your code to follow any of these
>    styles. Note that some of these programming styles are conflicting.
>    You cannot have them all; you have to choose among them."
>
> For example, there is a warning option that attempts to enforce
> explicit casts (no implicit conversions), and there is a warning option
> that attempts to enforce no explicit casts. When you use such a warning
> option together with -Werror, you are attempting to enforce a certain
> programming style on the configure tests. Obviously the configure test
> snippets cannot obey different, conflicting programming styles.

Yep, completely on board with that & I get it. The only reason I
reported this is because there's a reasonable chance this will be Clang's
new behaviour by default (see below).

>
>> /tmp/foo.c: In function ‘main’:
>> /tmp/foo.c:59:21: warning: passing argument 2 of ‘memcmp’ discards 
>> ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
>>    59 |     if (memcmp (&y, &x, sizeof x))
>>       |                     ^~
>
> There is nothing wrong with this code (m4/frexp.m4:159, m4/frexpf.m4:83).
>
> We don't want to add a cast here, because — as Paul argues — casts can make
> code more difficult to maintain in the long run.

Yes, I agree it's pedantic, but it's still going to be an issue if (as
planned) Clang flips '-Wincompatible-pointer-types' to error out by
default unless they carve out an exception for qualifiers here.

I'll advocate that they do, of course. I just came across it while
checking for configure changes w/ recent GCC changes.

>
> Bruno
>
> [1] https://www.gnu.org/software/gnulib/manual/html_node/manywarnings.html