Is this a big bug in the implementation of tolower in ntoskrnl.exe?

Question

The following code works fine in user mode:

#include <stdio.h>
#include <ctype.h>

int main()
{
    //
    // 0x7f51 is the unicode code of Chinese character '网'
    //
    int n = tolower(0x7f51); // n will equal 0x7f51
}

However, if we are in kernel mode, n will equal 0x7f71 !!!

The simplest sample code:

#include <ntifs.h>

ULONG NTAPI DriverEntry(PDRIVER_OBJECT, PUNICODE_STRING)
{
    int n = tolower(0x7f51); // n will equal 0x7f71 !!!

    return 0;
}

Is this a big bug in the implementation of tolower in ntoskrnl.exe?

Answer 1

tolower(int c) is defined only for integers c, that are EOF or representable as an unsigned char. 0x7f51 is neither. Therefore, the behaviour of tolower(0x7f51) is undefined.