Policy for return by reference values in error cases

[Gedare Bloom]

On Tue, Mar 23, 2021 at 12:37 PM Joel Sherrill <joel at rtems.org> wrote:
>
>
>
> On Tue, Mar 23, 2021 at 12:58 PM Sebastian Huber <sebastian.huber at embedded-brains.de> wrote:
>>
>> On 23/03/2021 18:48, Joel Sherrill wrote:
>>
>> > My first thought is that I don't like covering up for applications
>> > that do the wrong thing.
+.5

>> This topic just came up recently in a discussion about defensive
>> programming. We also test for NULL pointers.
+1

>> >
>> > I'm overall rather ambiguous. It is possible that setting the value at
>> > the top of the function could lead to overridden before used issues
>> > with warnings and static analysis.
>>
>> You mean code like this:
>>
>> void (int *x, int y)
>>
>> {
>>
>>    *x = 0;
>>
>>   if (y) {
>>
>>     *x = 1;
>>
>> } else {
>>
>>   *x = 2;
>>
>> }
>>
>> ?
>
>
> Yep. That's a pretty clear case.
>
> Others should speak up but I just don't want the solution pattern
> to introduce warnings or static analysis reports. It easily could.
>

Generally, the error-producing path should allow the static analysis
to find that the value gets set. In fact, I'm a bit surprised the
static analysis doesn't complain about the original problem, that some
variable can be used uninitialized when for example
rtems_event_receive() returns before updating its pointer argument.
Probably, the points-to analysis is complicated to find this case, but
it seems like a good case to reduce and bring to some expert in static
analysis.

So I don't see a fundamental problem with a pattern that initializes
these out parameters at the top of the function to a default value.

>
>>
>> > I don't want to see every error case assign a value to an output
>> > parameter though.
>> Yes, I don't like this also.
>
>
> I have my own wish list for error paths eventually if we ever get bored. :)
>
>
> --joel
>
>>
>>
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