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

Applies to: 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).