ID Number: Q57889
5.10 6.00 6.00a 6.00ax 7.00 | 5.10 6.00 6.00a
MS-DOS | OS/2
Summary:
In Microsoft C versions 5.0, 5.1, 6.0, 6.0a, 6.0ax, and C/C++ version
7.0, if disk space is exceeded, file buffering fails when buffering is
done by the stream I/O functions. For example, if a file buffer for a
stream is set to 10K by setvbuf() but only 2K of disk space is
available, all data above 2K written to the buffer will be lost. This
problem can occur even when setvbuf() and fwrite() return successful
return codes.
More Information:
When there is less space on the disk than there is in the stream
buffer, all stream I/O functions will seem to work properly until the
stream buffer is filled [for example, fwrite() returns the number of
bytes written as if it were successful]. However, the status of these
functions is valid only for the data going to the buffer and is not
reflected in the file that is written to the disk.
The problem is due to the existence of both the C run-time buffers and
the MS-DOS buffers. Only when the MS-DOS buffers try to write to disk
does it become evident that the disk is full. Then, the next return
value from fwrite() will indicate failure.
The following is the series of events that leads to the loss of data
with buffered stream I/O functions:
1. The stream is opened with fopen().
2. Buffering is set on the stream, either 512 bytes default or the
number of bytes selected by the user with setvbuf().
3. There is less disk space than the size of the buffer set by step 2.
4. Bytes are written to the file [for example, fwrite()], with
successful return codes.
5. The buffer is filled and then the stream I/O function attempts to
write all the data to MS-DOS.
6. The bytes that can fit on the disk are written and all remaining
data that was in the buffer is lost.
7. Successive calls to write data to the file fail.
The following are possible workarounds:
1. Turn buffering off by setting the file buffer to NULL, using
setvbuf().
2. Use nonbuffered I/O functions, such as open(), read(), and write().
3. Check the result of closing the stream with fclose(), which flushes
all the buffers associated with the given stream prior to closing.
4. Set buffering to the same size as the records that are being
written. The third workaround will force fwrite() to return a
"failure" return code at a point where the program can easily
recover because the program knows exactly which records were
successfully written to disk and which ones were not.
Additional reference words: 5.00 5.10 6.00 6.00a 6.00ax 7.00