cppcheck 2.3: false positive: zerodivcond (introduced by generic reverse valueflow)

[Michał Kępień]

Hi there,

Thank you for your work on cppcheck!

For the following C program:

int main(int argc, char *argv[]) {
    int quotient, remainder;

    remainder = argc % 2;
    argc /= 2;
    quotient = argc;

    if (quotient != 0) {
        return quotient;
    }

    return remainder;
}

cppcheck 2.3 (with --enable=warning) reports:

Checking test.c ...
test.c:4:19: warning: Either the condition 'quotient!=0' is redundant or there is division by zero at line 4. [zerodivcond]
 remainder = argc % 2;
                  ^
test.c:8:15: note: Assuming that condition 'quotient!=0' is not redundant
 if (quotient != 0) {
              ^
test.c:6:13: note: Assignment to 'quotient=argc'
 quotient = argc;
            ^
test.c:5:2: note: Compound assignment '/=', assigned value is 0
 argc /= 2;
 ^
test.c:5:10: note: Assignment to 'argc/=2'
 argc /= 2;
         ^
test.c:4:19: note: Division by zero
 remainder = argc % 2;
                  ^

Of course the sample program itself is silly and does not serve any real purpose, it is just the simplest piece of code that I was able to trigger this false positive with.

Applying any of the following changes prevents the false positive from being triggered:

• removing the if (quotient != 0) { ... } block
• removing the argc /= 2; line
• replacing argc /= 2; quotient = argc; with quotient = argc / 2;

git bisect claims that commit bd7e915c208219d6a5e3efca40168d31f9c0248c, i.e. pull request #2878, is the culprit. I also carried out an additional bisect on that pull request's branch (pfultz2/valueflow-generic-reverse) and it pointed at commit 5c75d1f0c2b23f74694318a060b43e3724df2b73 ("Add support for reassignment").

Please let me know if any further information and/or experiments would help.

[Daniel Marjamäki]

Thanks! This is a great report we should have the info we need. I have created this ticket:
https://trac.cppcheck.net/ticket/10049