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

Last reviewed: July 24, 1997
Article ID: Q149966

The information in this article applies to:

The C Run-time (CRT) included with: - Microsoft Visual C++ for Windows, version 1.52 - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0,
```
     4.1, 4.2, 5.0
```

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.

WORKAROUND

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. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

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
   */

Keywords : CRTIss kbprg vcbuglist400 vcbuglist500
Version : 1.52 2.0 2.1 2.2 4.0 4.1 4.2 5.0
Platform : NT WINDOWS
Issue type : kbbug

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Last reviewed: July 24, 1997
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