CMSC216 HW10: UNIX I/O and Directories

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                             HW10 QUESTIONS
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Write your answers to the questions below directly in this text file to
prepare for the associated quiz. Credit for the HW is earned by
completing the associated online quiz on Gradescope.


PROBLEM 1: `append_all.c'
=========================

  Examine the code in `append_all.c' which makes use of the `read()' and
  `write()' I/O system calls in an interesting pattern.


A
~

  Compile and run the program and experiment with entering data into it
  and pressing 'Enter'. Start with simple single letter inputs and then
  extend you inputs to longer strings.  Show your session and explain
  why read() always seems to read one more character than you type.


B
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  Describe the initialize size of the array `input' in `append_all.c'
  and how it changes over the run of the program.
  - What standard C function is used to initially allocate memory for
    `input'? How does it work and how much space does `input' initially
    occupy?
  - what C function is used to alter its size? How does it work?


C
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  Restart the `append_all.c' program and type the specific input below
  in at the prompts. Show the output produced and describe *exactly how
  many read() calls* result from entering this input. This should
  solidify your understanding of the main loop in the program.

  ,----
  | > 123456
  `----


D
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  In append_all.c, the read call is followed by a commented line:
  ,----
  |     int nread = read(STDIN_FILENO, input+cur_pos, max_read); // perform read()
  |  // int nread = read(STDIN_FILENO, input, max_read);         // this read() call would be an error: why?
  `----
  This commented line contains a common error for those new to `read()'
  system call or input accumulation in general.

  Comment the current read() call and uncomment the line marked as an
  error so that the code reads:
  ,----
  |  // int nread = read(STDIN_FILENO, input+cur_pos, max_read); // perform read()
  |  int nread = read(STDIN_FILENO, input, max_read);            // this read() call would be an error: why?
  `----

  Recompile the program and run it entering various inputs. Describe why
  this line is error and relate it to what the program now erroneously
  produces for output.


PROBLEM 2: `dirops.c'
=====================

A
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  Examine the source code of `dirops.c' closely. It makes use of a
  variety of system calls to produce a semi-interesting effect.  Compile
  and run it several times. Describe the overall intent of the program
  based on its output and the code you understand.


B
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  What set of system calls is used by the program to determine all the
  files in the current directory? Describe briefly how these calls work
  together.


C
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  Identify the system call that `dirops.c' uses to find the sizes of
  files. Describe briefly how this call works.


D
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  The following line sets up the read/write permissions that the copied
  file will have.
  ,----
  | mode_t perms = S_IRUSR | S_IWUSR;
  `----
  Modify this line so that the copied file is readable by the group in
  addition to the other permissions present.

  /Optional challenge:/ Set the permissions to be identical to the
  original file. `stat()' is one way to find out the permissions for the
  original file and the manual page for the system call shows the fields
  of the `struct stat', one of which shows the permissions/mode. Note:
  you may need to use `man 2 stat' to get the system call version of
  `stat' rather than the command line utility of the same name.


E
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  `dirops.c' contains a subtle bug in the following bit of code towards
  the end of the file.
  ,----
  |   while( (nbytes = read(infd, buf, BUFSIZE)) > 0){
  |     write(outfd, buf, BUFSIZE);
  |   }
  `----
  You should observe that every time program is run, it will identify a
  copied file as the largest and make another copy due to this bug. It
  may help to examine the ends of the copied files with the `tail
  file.txt.copy' command which will show the last 10 lines.

  Explain what is wrong with the loop and paste a fixed version below.