System call programming and debugging Solution



Useful pointers

  • man strace

Laboratory: Buffered versus unbuffered I/O

As usual, keep a log in the file lab.txt of what you do in the lab so that you can reproduce the results later. This should not merely be a transcript of what you typed: it should be more like a true lab notebook, in which you briefly note down what you did and what happened.

For this laboratory, you will implement transliteration programs tr2b and tr2u that use buffered and unbuffered I/O respectively, and compare the resulting implementations and performance. Each implementation should be a main program that takes two operands from and to that are byte strings of the same length, and that copies standard input to standard output, transliterating every byte in from to the corresponding byte in to. Your implementations should report an error from and to are not the same length, or if from has duplicate bytes. To summarize, your implementations should like the standard utility tr does, except that they have no options, characters like [, – and \have no special meaning in the operands, operand errors must be diagnosed, and your implementations act on bytes rather than on (possibly multibyte) characters.

!. Write a C transliteration program tr2b.c that uses getchar and putchar to transliterate bytes as described above.

#. Write a C program tr2u.c that uses read and write to transliterate bytes, instead of using getchar andputchar. The nbyte arguments

to read and write should be 1, so that the program reads and writes single bytes at a time.

$. Use the strace command to compare the system calls issued by

your tr2b and tr2u commands (a) when copying one file to another, and (b) when copying a file to your terminal. Use a file that contains at least 5,000,000 bytes.

  • Use the time command to measure how much faster one program is, compared to the other, when copying the same amount of data.

Homework: Encrypted sort revisited

Rewrite the sfrob program you wrote previously so that it uses system calls rather than <stdio.h> to read standard input and write standard output. If standard input is a regular file, your program should initially allocate enough memory to hold all the data in that file all at once, rather than the usual algorithm of reallocating memory as you go. However, if the regular file grows while you are reading it, your program should still work, by allocating more memory after the initial file size has been read.

Your program should do one thing in addition to sfrob. If given the -f option, your program should ignore case while sorting, by using the standard toupper function to upper-case each byte after decrypting and before comparing the byte. You can assume that each input byte represents a separate character; that is, you need not worry about multi-byte encodings. As noted in its specification, toupper’s argument should be either EOF or a nonnegative value that is at

most UCHAR_MAX (as defined in <limits.h>); hence one cannot simply pass a char value to toupper, as char is in the range CHAR_MIN..CHAR_MAX.

Call the rewritten program sfrobu. Measure any differences in performance between sfrob and sfrobu using thetime command. Run your program on inputs of varying numbers of input lines, and estimate the number of comparisons as a function of the number of input lines.

Also, write a shell script sfrobs that uses standard tr and sort to sort encrypted files using a pipeline (that is, your script should not create any temporary files). Your shell script should also accept an -f option, with the same meaning as with sfrobu. Use the time command to compare the overall performance of sfrob, sfrobu, sfrobs, sfrobu -f, and sfrobs -f.


Submit a compressed tarball syscall.tgz containing the following files.

  • The files lab.txt, tr2b.c, and tr2u.c as described in the lab.

  • A single source file sfrobu.c as described in the homework.

  • A single shell script sfrobs as described in the homework.

  • A text file sfrob.txt containing the results of your sfrob performance comparison as described in the homework.

All files should be ASCII text files, with no carriage returns, and with no more than 200 columns per line. The C source file should contain no more than 132 columns per line. The shell commands

tar xf syscall.tgz

expand lab.txt sfrob.txt |

awk ‘/\r/ || 200 < length’

expand tr2b.c tr2u.c sfrobu.c sfrobs |

awk ‘/\r/ || 132 < length’

should output nothing.

  • 2005, 2007, 2009–2011, 2013–2017 Paul Eggert. See copying rules. $Id: assign5.html,v 1.34 2017/02/04 22J45J48 eggert Exp $