Q70784: INFO: Data May Be Lost During fflush() If Write Is Unsuccessful

Article: Q70784
Product(s): Microsoft C Compiler
Version(s): MS- DOS:5.1,6.0,6.00a,6.00ax,7.0;WIN3X:1.0,1.5;WINNT:1.0,2.0,2.1,4.0,5.0;
Operating System(s): 
Keyword(s): kbCRT kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 26-JUL-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The C Run-Time (CRT), included with:
   - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax 
   - Microsoft C/C++ for MS-DOS, version 7.0 
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0 
-------------------------------------------------------------------------------

SUMMARY
=======

Because of the way the fflush() function clears the stream buffer, data may be
lost after a failed write. This may become apparent when setting up a critical
error handler for a file that uses stream I/O [for example, fopen(), fclose(),
fwrite(), and so on]. To work around this situation, turn buffering off [with
the setvbuf() function] or use low-level I/O routines instead [for example,
open(), close(), write(), and so on].

MORE INFORMATION
================

When using stream I/O, input and output is buffered. This can provide
significant performance benefits because data is read and written in larger
"chunks." The file buffer is 512 bytes by default but can be adjusted with the
setvbuf() function.

When fflush() is called, either explicitly or implicitly through fclose(), the
data still in the stream is written to the file by an implicit call to write().
Once that is done, the current location pointer is reset to the start of the
stream buffer and the buffer's character count is set to zero. Unfortunately,
this occurs whether or not the write() was successful.

In an environment where a critical error handler has been installed, the
application will usually try to rectify the error (that is, request that the
user close the disk drive door, turn on the printer, insert a formatted disk,
and so on), and then retry the operation. For example:

  IOStatus = FAIL;
  while(IOStatus == FAIL)
  {
     fflush(FilePtr);
     IOStatus = CriticalErrorChk();
  }

However, the second time fflush() is called, the stream has already been cleared
of data and the data is effectively "lost." Calling setvbuf() with the
mode-parameter of _IONBF eliminates this problem by unbuffering the stream, but
file I/O will be slower.

Note that this fflush() behavior is compatible with the ANSI standard. As
required by ANSI, fflush() does return EOF to indicate there was a problem with
flushing the file.

The Help for fflush() in Microsoft C/C++ versions 7.0 and Visual C++ gives the
following information:

  Note: If fflush returns EOF, data may have been lost because of a failed
  write. When setting up a critical error handler, it is safest to turn
  buffering off with the setvbuf function or to use low-level I/O routines such
  as _open, _close, and _write instead of the stream I/O functions.

Additional query words: _harderr _hardrtn _hardresume

======================================================================
Keywords          : kbCRT kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : MS- DOS:5.1,6.0,6.00a,6.00ax,7.0;WIN3X:1.0,1.5;WINNT:1.0,2.0,2.1,4.0,5.0;
Issue type        : kbinfo

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.