FIX: VC++ scanf functions don't return EOF(-1) on Windows 10 Enterprise 2015 LTSB

Symptoms

When you run an application that calls the _sntscanf_s function, the call should return an EOF(-1) value. However, when you run the application on Windows 10 Enterprise 2015 LTSB, the injected null character is ignored. This causes the numFields field to receive an erroneous value of 1 instead of the EOF (-1) value.

For example, this issue occurs in the following Visual C++ application:

#include "stdafx.h"
#include "string.h"
#include "stdlib.h"

int main()
{
    TCHAR szBuff[256];
    double temp = 0;
    wcscpy_s(szBuff, L"77777777");

    // this should force the _sntscanf_s to return an EOF (-1) value
    szBuff[0] = '\0';

    int numFields = _sntscanf_s(szBuff, _countof(szBuff), _T("%lf"), &temp);
    if (numFields == EOF)
    {
        printf("Correct Behavior!!!\r\n");
    }
    else
    {
        printf("BUG: _sntscanf_s failed to return EOF!!!\r\n");
    }
    return 0;
}

Cause

This issue occurs because of a known problem in the common_vsscanf function in the Ucrtbase.dll file that is included in Windows 10 Enterprise 2015 LTSB. This problem affects the whole scanf family of functions:

• _vsscanf_l

• vsscanf

• _sscanf_l

• sscanf

• _vsscanf_s_l

• vsscanf_s

• _sscanfs_l

• sscanf_s

• _snscanf_l

• _snscanf_s

• _snscanf_s_l

• _snscanf_s

• _vswscanf_l

• vswscanf

• _swscanf_l

• swscanf

• _vswscanf_s_l

• vswscanf_s

• _wsscanfs_l

• swscanf_s

• _snwscanf_l

• _snwscanf_s

• _snwscanf_s_l

• _snwscanf_s

Resolution

To resolve this issue, install servicing update May 8, 2018—KB4103716 (OS Build 10240.17861).

References

The Great C Runtime (CRT) Refactoring

Introducing the Universal CRT