How to force gcc to report a warning/error

[Trevor Woollacott]

>From: "ppmoore" <polomora@gmail.com>
>Sent: Tuesday, October 26, 2010 10:17 AM
>To: <gcc-help@gcc.gnu.org>
>Subject: How to force gcc to report a warning/error
>
>>
>> Hello,
>>
>> We had an interesting error that was undetected in gcc:
>> {
>>  unsigned long a;
>>  unsigned long x=3;
>>  unsigned long y=4;
>>  a =
>>      min(x,y);
>> }
>>
>> I've simplified the example. In the original code, the contents of the 
>> min()
>> statement were longer, so that it was on a separate line from the 
>> assignment
>> operation.
>>
>> Because of a bug, the assignment line was removed, and we had the 
>> following
>> code:
>> {
>>  unsigned long a;
>>  unsigned long x=3;
>>  unsigned long y=4;
>>      min(x,y);
>> }
>>
>> Should this be picked up as a warning/error?
>> Neither -Wall or -Wextra picked it up.
>>
>> Many thanks,
>> Paul
>> -- 
>> View this message in context: 
>> http://old.nabble.com/How-to-force-gcc-to-report-a-warning-error-tp30055230p30055230.html
>> Sent from the gcc - Help mailing list archive at Nabble.com.
>>

Hi Paul

The compiler doesn't know if you are purposely ignoring the result of min, 
therefore
it doesn't warn you. However, using -Wall in the above code would have 
warned you that
'a' was an unused variable.

If you want to make sure that a return value is used, then you can declare 
min
using __attribute((__warn_unused_result__)).
For example:
int min(int x, int y) __attribute((__warn_unused_result__));

And compiling would give you the following warning:
warning: ignoring return value of 'min', declared with attribute 
warn_unused_result

Regards,
Trevor