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

• 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, 4.2, 5.0, 6.0

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.

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
Version : winnt:2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Platform : winnt
Issue type : kbbug