BUG: Filebuf overflow() and underflow() Call filebuf::sync()

• Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.0a, 4.1, 4.2, 5.0, 6.0

SYMPTOMS

The filebuf class allows you to use buffered file input/output (I/O), and provides the sync() function to flush the file buffers. However, the filebuf::overflow() and filebuf::underflow() functions do not support the use of virtual functions in the filebuf class. Classes derived from filebuf might incorrectly call filebuf::sync() when they should call the sync() function from the derived class.

CAUSE

The filebuf class calls filebuf::sync() instead of calling sync().

RESOLUTION

You can use one of the following workarounds to this problem:

• Do not define the virtual function sync() in your filebuf-derived class.
  
  -or-
  
  define your own versions of overflow() and underflow() in the derived class.



• Change the filebuf::overflow() and filebuf::underflow() functions to call sync() instead of filebuf::sync(). You can find the source code for filebuf class functions in the following location:

  
  ...\VC\Src\FileBuf.cpp 

  The source code for filebuf::overflow() follows:




      int filebuf::overflow(int c)
      {
          if (allocate()==EOF)
              return EOF;
          if (filebuf::sync()==EOF) // Change to sync() for the workaround.
              return EOF;

          if (!unbuffered())
              setp(base(),ebuf());
          if (c!=EOF)
              {
              if ((!unbuffered()) && (pptr() < epptr()))
                  sputc(c);
              else
                  {
                  if (_write(x_fd,&c,1)!=1)
                      return(EOF);
                  }
              }
          return(1);
     } 

STATUS

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

MORE INFORMATION

© Microsoft Corporation 1999, All Rights Reserved.
Contributions by Mark Hagen, Microsoft Corporation

Additional query words: inheritance inherit

Keywords : kbcode kbLangCPP kbVC kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600bug
Version : winnt:4.0,4.0a,4.1,4.2,5.0,6.0
Platform : winnt
Issue type : kbbug