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

ID: Q70784

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
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;
Platform : MS-DOS NT WINDOWS
Issue type : kbinfo