Q57889: File Buffering Can Hide Out-of-Disk-Space Condition

Article: Q57889
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc s_quickasm | mspl13_c
Last Modified: 22-JAN-1990

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 disk space is available,
all data written to the buffer above 2K will be lost. This problem can
occur even when setvbuf() and fwrite() return successful return codes.

In the case where 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 DOS buffers. Only when the 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 data to 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 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 one was not.

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