Clang Static Analyzer unexpected NULL pointer warning

Question

I have the following code:

int s[4096];
unsigned char o = 0;

int main(void) {
    int *n;
    return ((char *) (s + o)) == 0 ? *n : 0;
}

When I run the Clang Static Analyzer on that code, it warns me that I'm dereferencing n because (char *) (s + o) is a null pointer, which it's not (I can even print it and get an address that's definitely not zero).

What am I missing?

I'm noticing that removing the (char *) cast makes the warning disappear.

It warns you because the condition could technically evaluate to true, in which case you dereference a uninitialized pointer. — tkausl
– tkausl, Commented Oct 30, 2019 at 19:51
@tkausl, that makes sense, but I can't figure out in which conditions this could be true. For whatever value o has, this still won't be true. And this is the only codepath in the entire program, so neither s nor o can have any other values than the ones in the program. I'm also noticing that removing the (char *) cast makes the warning disappear. — rid
– rid, Commented Oct 30, 2019 at 19:52
@OznOg, but o is an unsigned char, so it can only have values between 0 and 255. — rid
– rid, Commented Oct 30, 2019 at 19:57

rid · Accepted Answer · 2019-10-30 20:42:17Z

1

I was testing with the Clang Static Analyzer version 8. Version 10 no longer reports the warning.

answered Oct 30, 2019 at 20:42

rid

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