QUESTION 19
What does a+ mean?
2 marksAnswer
It opens a stream for reading and appending; writes go to the end. Do not assume a seek changes an append-mode write into an overwrite.
QUESTION 20
Explain append and update streams and safe transitions between reading and writing.
5 marksAnswer in brief
Update modes permit both input and output on a stream, but changing direction requires the appropriate operation.
Mode choice
r+ opens an existing file for reading and writing without initial truncation. w+ creates or truncates a readable/writable file. a+ permits reading and append writes. Choose the mode according to whether old contents must be preserved.
Direction rules
After output, use fflush or a file-positioning operation before input. After input, use a file-positioning operation before output, unless the input operation encountered end of file. These are stream rules, not merely style suggestions. An explicit successful fseek is a clear way to express a planned transition.
Record update
For a fixed-size binary file, identify the target record, seek to its start, write the replacement and check the result. A seek after reading avoids an invalid direction change. Arbitrary variable-length text replacement is harder because changing a field’s length can shift later contents.
Safer application design
For important files, validate all new data before modifying the stored version. A temporary-file-and-replace approach can reduce partial-update risks, but platform rename and durability behaviour need a deliberate design. The teaching examples use separate demonstration filenames and do not claim crash-safe database transactions.
Complexity
Appending a known record does not require searching old keys, but device costs are not specified by C. Finding a record sequentially remains O(n) record comparisons; processing or rewriting n records requires work proportional to that data.