RFR: JDK-8354316 : clang/linux build fails with -Wunused-result warning at XToolkit.c:695:9

[Alexey Ivanov]

On Tue, 13 May 2025 18:30:01 GMT, Harshitha Onkar <honkar at openjdk.org> wrote:

>> The following line results in unused-result warning on linux/clang.
>> 
>> 
>> /java.desktop/unix/native/libawt_xawt/xawt/XToolkit.c:695:9: error: ignoring return value of function
>>  declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
>>  695 | write ( AWT_WRITEPIPE, &wakeUp_char, 1 ); 
>> 
>> 
>> There are two ways to handle it 
>> 
>> 1) Make changes to XToolkit.c such that the warning is no longer thrown. But throwing an error based on the result of `write ( AWT_WRITEPIPE, &wakeUp_char, 1 );` will result in unexpected behavioral changes and the best way to handle it is to have an empty if block with an appropriate comment.
>> 
>> 2) Add unused-result to disabled warning section for clang similar to gcc - https://github.com/openjdk/jdk/blob/d1543429ff29ca0d761b8473b3fb8621abcd226d/make/modules/java.desktop/lib/AwtLibraries.gmk#L281. The 1st approach was picked over the 2nd since the usual recommendation is not to add to disabled warning section unless there is no other option.
>> 
>> NOTE: the fix has been tested on linux/gcc , it does need to be tested on linux/clang.
>
> make/modules/java.desktop/lib/AwtLibraries.gmk line 281:
> 
>> 279:       DISABLED_WARNINGS_gcc_XlibWrapper.c := type-limits pointer-to-int-cast, \
>> 280:       DISABLED_WARNINGS_gcc_XRBackendNative.c := maybe-uninitialized, \
>> 281:       DISABLED_WARNINGS_gcc_XToolkit.c := unused-result, \
> 
> Probably a question for build team -
> One thing to be noted here is that:  With unused-result line removed and WITHOUT the if block changes in XToolkit.c, I expected the build to fail due to unused-result on linux/gcc but it did not.
> So is there any chance that this disabled warning is being added from else where for gcc ?

Maybe gcc doesn't detect this specific case.

There could've been other cases which triggered the warning in gcc.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25217#discussion_r2087418018