LGTM.com - false positive in "Potentially uninitialized local variable"

[olehermanse]

Description of the false positive

I think this kind of pattern is not so unusual:

int res;
bool was_attempted = false;

if (...)
{
    res = SomeOperation();
    was_attempted = true;
}

if (...)
{
    res = SomeOperation();
    was_attempted = true;
}

if (was_attempted && res != 0)
{
    printf("Some error: %d\n", res);
}

This causes the LGTM to give a false positive alert - for all cases were was_attempted is true, res is initialized.

URL to the alert on the project page on LGTM.com

https://lgtm.com/projects/g/cfengine/core/snapshot/1e3d1567db2ff8d470d3473ba68d27e933c1a7ae/files/libpromises/cf3parse_logic.h?sort=name&dir=ASC&mode=heatmap#x26838ee62fa30f43:1

[geoffw0]

Hi @olehermanse,

The query in question (cpp/uninitialized-local) has a precision of 'medium' (severity 'warning'), so a certain level of false positives are to be expected if you choose to use it. The reason for this precision is in fact exactly the pattern you've described, which occurs quite widely - but unfortunately appears to be difficult to fix [with accuracy and sufficient generality].

I'll poke a few people to try and [re-]start a discussion about this subject. I don't think we should expect a quick solution.

[jbj]

The cfengine project has opted in to imprecise queries by putting include: "*" in their lgtm.yml. I'm afraid that false positives are to be expected from this query, which is why it's not shown by default.