Q149966: BUG: istream::operator>>(long&) Does Not Clear errno

Article: Q149966
Product(s): Microsoft C Compiler
Version(s): 1.52,2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbCRT kbVC kbVC152 kbVC200bug kbVC210bug kbVC220bug kbVC400bug kbVC410bug kbVC420bug kb
Last Modified: 07-FEB-2002

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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) 
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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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