DOCUMENT:Q149966  07-FEB-2002  [visualc]
TITLE   :BUG: istream::operator>>(long&) Does Not Clear errno
PRODUCT :Microsoft C Compiler
PROD/VER::1.52,2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
OPER/SYS:
KEYWORDS:kbCRT kbVC kbVC152 kbVC200bug kbVC210bug kbVC220bug kbVC400bug kbVC410bug kbVC420bug kb

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - The C Run-Time (CRT), used with:
    - Microsoft Visual C++ for Windows, 16-bit edition, version 1.52 
    - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1 
    - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 
    - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 
    - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
    - Microsoft Visual C++.NET (2002) 
-------------------------------------------------------------------------------

SYMPTOMS
========

If an error occurs when extracting long values from the stream, the error cannot
be cleared and other streams also report errors.

CAUSE
=====

When the istream::operator>>(long& n) encounters an error, it sets
ios::failbit, but does not clear errno. This causes the stream to continue to
fail even after ios::clear is called. It also causes other streams to fail.

RESOLUTION
==========

To work around the bug, set errno to zero in addition to completing normal
stream error checking.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

MORE INFORMATION
================

Sample Code
-----------

   /*
   The following sample code demonstrates only the bug and workaround, not
   normal error checking.

   Compile options needed: none
   */ 

   #include    <iostream.h>
   #include    <strstrea.h>
   #include    <stdio.h>
   #include    <stdlib.h>
   #include    <string.h>
   #define     MAX_STRING_LEN      100
   //#define     CLEAR_ERRNO    // uncomment to work around bug

   void main()
   {
       char    dc1a, dc1b, dc2a, dc2b;
       long    input1a=0, input1b=0;
       long    input2a=0, input2b=0;
       char    szTest1[MAX_STRING_LEN];
       char    szTest2[MAX_STRING_LEN];

       strcpy(szTest1,"a2147483648z50"); // 
       strcpy(szTest2,"a500z50");

       istrstream    StrStream1(szTest1); //String stream 1
       istrstream    StrStream2(szTest2); //String stream 2

       //    The following command will fail since value input to
       //    input1a=2147483648 is
       //    bigger that maximum value long can hold.

       StrStream1 >> dc1a >> input1a >> dc1b >> input1b;
   #ifdef    CLEAR_ERRNO
       errno = 0;
   #endif
       StrStream2 >> dc2a >> input2a >> dc2b >> input2b;

       cout << "\n    dc1a: " << dc1a
            << "\n input1a: " << input1a
            << "\n    dc1b: " << dc1b
            << "\n input1b: " << input1b << endl;

       cout << "\n    dc2a: " << dc2a
            << "\n input2a: " << input2a
            << "\n    dc2b: " << dc2b
            << "\n input2b: " << input2b << endl;

       cout << "\n\nPress any key to continue..." << endl;
       dc1a = getchar();
   }

   /*
   The output is:
       dc1a: a
    input1a: 2147483647
       dc1b:
    input1b: 0

       dc2a: a
    input2a: 500
       dc2b:
    input2b: 0

   If CLEAR_ERRNO is set, the output is as expected:
       dc1a: a
    input1a: 2147483647
       dc1b:
    input1b: 0

       dc2a: a
    input2a: 500
       dc2b: z
    input2b: 50
   */ 

Additional query words: kbVC400bug

======================================================================
Keywords          : kbCRT kbVC kbVC152 kbVC200bug kbVC210bug kbVC220bug kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600bug 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : :1.52,2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Issue type        : kbbug

=============================================================================

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