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

View products that this article applies to.

This article was previously published under Q149966

SYMPTOMS

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

Back to the top

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.

Back to the top

RESOLUTION

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

Back to the top

STATUS

This bug was corrected in Microsoft Visual C++ .NET 2003.

Back to the top

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
*/

Back to the top