false negative: uninitialized variable usage not detected in case variable set in for loop

[Pieter D]

Hi,

I am not sure this is already tracked by a bug ticket.
GCC and cppcheck do not detect that entries_bad_format is used as unitialized variable in case of for loops not being executed.
Simplified code snippet:

#include <stdio.h>
#include <stdbool.h>
int main()
{
     bool entries_bad_format;
     unsigned int n_entries = 0; // In real example can be assigned by anything mostly > 0 but could be 0 (json_array_size)

    for (size_t i = 0; i < n_entries; i++)
    {
        entries_bad_format = true;
    }
    if(entries_bad_format)
        printf("hello");
     return 0;
}

gcc -Wall -Wextra test.c -o test
cppcheck test.c --enable=all

Checking test.c ...
test.c:9:23: style: Checking if unsigned expression 'i' is less than zero. [unsignedLessThanZero]
 for (size_t i = 0; i < n_entries; i++)
                      ^
test.c:7:28: note: Assignment 'n_entries=0', assigned value is 0
  unsigned int n_entries = 0;
                           ^
test.c:9:23: note: Unsigned less than zero
 for (size_t i = 0; i < n_entries; i++)

When modifying code to:

#include <stdio.h>
#include <stdbool.h>
int main()
{
     bool entries_bad_format;
    if(entries_bad_format)
        printf("hello");
     return 0;
}

GCC:
test.c: In function ‘main’:
test.c:9:4: warning: ‘entries_bad_format’ is used uninitialized in this function [-Wuninitialized]
9 | if(entries_bad_format)
| ^

cppcheck:
Checking test.c ...
test.c:9:5: error: Uninitialized variable: entries_bad_format [uninitvar]
if(entries_bad_format)
^

Thank you for making cppcheck better every day!

Kind regards,
Pieter

[Daniel Marjamäki]

hmm it works for me from cppcheck-2.4. which version do you have?

[Pieter D]

Sorry for the inconvenience.
Apparantly I was running it with 1.90, indeed on 2.4.1 this particular simplified case above is fixed. In our case the n_entries is filled in by size_t json_array_size(const json_t *array (jansson lib) and as cppcheck does not know that lib (no lib cfg yet) it fails to find it propably?