knownConditionTrueFalse false positive

[Cameron Watt]

Hi,

When analysing some code with cppcheck 2.2 I seem to have found a false positive. It looks like cppcheck does not recognise that the pointer is incremented and so a subsequent check is not the same as a previous one.

The following code shows that the increment actually occurs, the second conditional check of if (*ptr >= max_value) is not the same as the first one.

Any advice would be appreciated.

#include <stdio.h>
#include <stdbool.h>

bool inc_ptr_test(int max_value, int *ptr)
{
    if (*ptr >= max_value)
    {
        printf("Returning after initial *ptr >= max_value check\n");

        return false;
    }

    *ptr = *ptr + 1;

    if (*ptr >= max_value)
    {
        printf("Taken 'if' branch, *ptr==%d, max_value==%d\n", *ptr, max_value);

        return false;
    }
    else
    {
        printf("Taken 'else' branch, *ptr==%d, max_value==%d\n", *ptr, max_value);

        return true;
    }
}

int cnt = 0;
int *foo = &cnt;

int main(void)
{

    while(inc_ptr_test(10, &cnt));


    printf("main() : pre increment cnt==%d\n", cnt);

    *foo = *foo + 1;

    printf("main() : post increment cnt==%d\n", cnt);

    return 0;
}

[Cameron Watt]

I should have added the output of the test, this is shown below...

$ /home/cameron/Desktop/testcase/build/cppcheck_testcase
Taken 'else' branch, *ptr==1, max_value==10
Taken 'else' branch, *ptr==2, max_value==10
Taken 'else' branch, *ptr==3, max_value==10
Taken 'else' branch, *ptr==4, max_value==10
Taken 'else' branch, *ptr==5, max_value==10
Taken 'else' branch, *ptr==6, max_value==10
Taken 'else' branch, *ptr==7, max_value==10
Taken 'else' branch, *ptr==8, max_value==10
Taken 'else' branch, *ptr==9, max_value==10
Taken 'if' branch, *ptr==10, max_value==10
main() : pre increment cnt==10
main() : post increment cnt==11

[Daniel Marjamäki]

Thanks! I created ticket https://trac.cppcheck.net/ticket/10004