[cfe-users] warning/error issues

[David Blaikie]

On Fri, Oct 11, 2013 at 6:58 PM, Eric Levy <contact at ericlevy.name> wrote:

> I stand corrected on the -Werror/-Wall issue.
>
> With respect to unused parameters, I do have -Wall enabled, however, the
> documentation states I can override it on particular types of warnings
> using -Wno-error=foo (this part is definitely different from gcc), however,
> I have been unsuccessful in doing so.
>

I demonstrated that in my example:

$ clang++-tot unused.cpp -Wextra -Werror -Wno-error=unused-parameter
unused.cpp:2:17: warning: unused parameter 'i' [-Wunused-parameter]
  void func(int i) {
                ^
1 warning generated.

demoting the unused-parameter warning back from an error to a warning,
worked for me. You do need to use the exact warning name, not a warning
group (eg: -Wno-error=unused or -Wno-error=all won't work).


>
> Eric Levy
>
>
>
> On 10/11/2013 09:49 PM, David Blaikie wrote:
>
>>
>>
>>
>> On Fri, Oct 11, 2013 at 6:34 PM, Eric Levy <contact at ericlevy.name
>> <mailto:contact at ericlevy.name>**> wrote:
>>
>>     Hello,
>>
>>     I began to experiment with clang to build an existing C++ (Qt-based)
>>     project.  I wanted to make warnings into errors in order to ensure
>>     that the code is solid.  However, there are several issues:
>>
>>     1) Clang is said to be designed to have the same command-line
>>     interface as gcc, but the options for warnings and errors are
>>     different.  The gcc command -Wall is -Werror in clang, and so on.
>>
>>
>> Actually Clang and GCC both have -Wall and -Werror and they do (roughly)
>> the same thing in each. -Wall is the "common likely-good warnings" and
>> -Werror is "whatever warnings are on, make them errors". The specific
>> set of things in -Wall differs somewhat between Clang and GCC since
>> there's no specific definition, but roughly speaking those flags do the
>> same thing.
>>
>>
>>     2) The user documentation states that specific warnings can be made
>>     into errors or not errors by using the -Werror=foo and
>>     -Wno-error=foo options.  However, no list of accepted values for foo
>>     is given.
>>
>>
>> This is a flaw in Clang's documentation - we basically don't document
>> our flags, so far as I know. You guess them, or you look at the
>> implementation, or try -Weverything (turns on all warnings) and see what
>> warnings fire and what their flags are called (we print the flag name by
>> default).
>>
>>
>>     3) Finally, gcc/clang both warn on unused variables and parameters.
>>     This often undesired, especially in C++ where, due to polymorphism
>>     some implementations of a method may not need certain parameters.
>>
>>
>> I don't believe Clang warns on unused parameters by default, if at all.
>> If it does so on virtual functions that's probably a bug to be fixed.
>>
>>       When clang produces an error for unused parameters (-Werror
>>     enabled), it reports the reason is -Wunused-parameter.  However,
>>     introducing -Wno-error=unused-parameter has no effect, despite
>>     documentation to the contrary.  The gcc-style -Wno-unused-parameter
>>     is equally ineffective.
>>
>>
>> Could you show examples of this?
>>
>> -Wunused-parameter isn't under -Wall, so you must've passed it
>> explicitly I assume (or perhaps via -Wextra)?
>>
>> All these options seem to work quite well for me with Clang:
>>
>> $ clang++-tot unused.cpp -Wextra
>> unused.cpp:2:17: warning: unused parameter 'i' [-Wunused-parameter]
>>    void func(int i) {
>>                  ^
>> 1 warning generated.
>> $ clang++-tot unused.cpp -Wextra -Werror
>> unused.cpp:2:17: error: unused parameter 'i' [-Werror,-Wunused-parameter]
>>    void func(int i) {
>>                  ^
>> 1 error generated.
>> $ clang++-tot unused.cpp -Wextra -Werror -Wno-error=unused-parameter
>> unused.cpp:2:17: warning: unused parameter 'i' [-Wunused-parameter]
>>    void func(int i) {
>>                  ^
>> 1 warning generated.
>> $ clang++-tot unused.cpp -Wextra -Werror -Wno-unused-parameter
>> $
>>
>>     So too is replacing unused-parameter with simply unused, which, at
>>     least according to gcc documentation, subsumes unused variables and
>>     parameters alike.
>>
>>     Whether these observations represent choice or oversight, I would
>>     like to know how to make all warnings errors except for unused
>>     variables and parameters.
>>
>>     Thank you.
>>
>>     Sincerely,
>>     Eric Levy
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>>
>>
>>
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