clang static analyzer questions

Question

I'm confused about making a decision.

For example, when I wrote a code 'test.c' like this.

 int main(void){ int b = 2; int c = 0; int d = b/c; printf("d: %d\n", d); return 0; }

And then, I typed the command clang --analyze test.c then the statement warning: Division by zero [core.DivideZero]" appeared

After that, I typed the command clang test.c. Then no warning comes out. However, when I run this program, error Floating point exception(core dumped) comes out.

In this case, which is the right one? is it a true-positive or false positive? Can someone explain it to me?

The analyzer succesfully predicted that the code will fail at runtime. Looks like an obvious true positive to me? — HolyBlackCat
– HolyBlackCat, Commented Dec 20, 2022 at 9:54
You have the code in front of you where you do a division by zero. And you get an exception telling you something is broken. What makes you think it could be a false positive, then? The compiler just doesn't do all the static analyzis if you don't ask it to. — Gerhardh
– Gerhardh, Commented Dec 20, 2022 at 10:49
Executing clang test.c with the source code shown in the problem does not result in no warning. it results in a warning about printf not being declared, since <stdio.h> was not included. The source code in the question is not the code you compiled. When asking questions like this, always include a correct minimal reproducible example that includes code that is just enough to demonstrate the problem but that is complete, meaning a reader should be able to paste exactly the source code you show into a file and compile it to reproduce the problem, with no changes or additions. — Eric Postpischil
– Eric Postpischil, Commented Dec 20, 2022 at 11:42
Compilers aren't obliged to do static analysis. They might do it as a bonus if you've been nice. Both clang and gcc warns for b/0 so at least they check integer constant expressions. They don't have to do that either. — Lundin
– Lundin, Commented Dec 20, 2022 at 15:47

Eric Postpischil · Accepted Answer · 2022-12-20 11:41:19Z

And then, I typed the command clang --analyze test.c then the statement warning: Division by zero [core.DivideZero]" appeared

The Clang static analyzer correctly determined there is a division by zero in the program.

After that, I typed the command clang test.c. Then no warning comes out.

Clang compiled the program in conformance with the C standard. The C standard does not require a compiler to warn you that there is a division by zero in the program.

However, when I run this program, error Floating point exception(core dumped) comes out.

The program was executed in conformance with the C standard. For division, the C standard says “if the value of the second operand is zero, the behavior is undefined.” Since the behavior is undefined, aborting the program with an error message (even a misleading one about “Floating point exception”) is permitted by the C standard.

In this case, which is the right one? is it a true-positive or false positive?

All three behaviors are correct.

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