Chapter 13: File I/O Buffering

Exercise 13-1

To build the solution, run make ex13_1.

The command options are:

1. -b block_size - sets the size of the input buffer
2. -s - enables the O_SYNC flag when opening the output file
3. -v - shows more detailed output
4. input file name
5. output file name

To generate a log file of multiple runs, do

./test_ex13_1.sh >>test_run_2020_02_20.log

Output file has the following fields:

1. Block Size
2. Synchronisation flag set?
3. Number of bytes read
4. Wall clock time in seconds
5. User time in seconds
6. System time in seconds

Part (a)

$ ./ex13_1 -b 512  /tmp/ex13_1.in /tmp/ex13_1.out 
512     -       67108864        0.28    0.07    0.21
$ ./ex13_1 -b 65536 /tmp/ex13_1.in /tmp/ex13_1.out 
65536   -       67108864        0.09    0.00    0.08

Note The fourth column (Wall clock time) appears not to match the output from time.

Increasing the internal buffer size from 512 to 65536 (64K) reduced the overall time (user + sys) from 0.21s to 0.08s (a 62% reduction).

Part (b)

$ ./ex13_1 -b 65536 -s /tmp/ex13_1.in /tmp/ex13_1.out 
65536   s       67108864        0.41    0.00    0.40
$ ./ex13_1 -b 512 -s /tmp/ex13_1.in /tmp/ex13_1.out 
512     s       67108864        27.10   1.51    25.59

Note The actual elapsed time for the 512 byte buffer case was several hours (not 27 seconds).

Adding O_SYNC significantly increasing the run time (but the effect is much less with larger internal buffers).

Part (c)

Not done yet.

Exercise 13-2

To compile the solution, run:

make ex13_2

To execute the solution, run:

./ex13_2 -b 4096 -f -d -n 409600 test.dat

Sample output is:

4096    -       f       d       409600  0.04    0.00    0.03

Exercise 13-3

Refer to Ex13-3.md

Exercise 13-4

To compile the solution, run:

make ex13_4

To execute the solution, run:

./ex13_4

The expected output is:

If I had more time,
I would have written you a shorter letter.

To run the solution with a file, run:

./ex13_4 >test.dat
cat test.dat

The expected output is:

I would have written you a shorter letter.
If I had more time,

Exercise 13-5

To compile the solution, run:

make my_tail

To execute the solution, run:

./my_tail -n 2000 results.csv | wc -l

The expected output is:

2000