ICS 54
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Assignment #4
Due Thur 24 Feb 2005 8:00am in lecture
1) [10 marks] Say you are really low on disk space on openlab, and you have some
utility programs in C that you like to use often, but you don't have enough
disk space to keep the compiled executables around all the time. Each program
consists of just one .c file. However, you don't want to manually re-compile
each C program each time you want to use it.
Write a short shell script called "C-interp" which is intended to have
links point at it, and pretends to be a C language interpreter. That is,
if you have C file "foo.c", then you would make a link "ln -s C-interp foo".
Then, what C-interp does, when called as "foo", is compile "foo.c" and run
the resulting executable on the arguments given to "foo". (You can test it
on your solutions to the other questions in this assignment.)
Some caveats:
- be sure to delete any temporary files generated, inculding the executable
after it's been executed.
- To ensure you don't delete any files that exist before you start whose
names may conflict with the temporary filenames you choose, put *all*
temporary files (including the executable) in /tmp.
- the executable should be called with argv[0] equal to the name of the .c
file, without the '.c', eg "foo.c" gets called as "foo".
- to ensure that your executable name doesn't conflict with other users,
you should put the executable in a uniquely-named subdirectory of /tmp.
In fact, it would be best if all your temp files went into this directory,
as long as the entire directory is removed when the executable is finished.
- ensure that the temporary files are deleted even if the program is
interrupted. ie, use the "trap" command in the Bourne shell to trap
signals 0 (Exit), 1 (Hangup), 2 (Interrupt) ,3 (Quit), and 15 (Terminate).
See signal(5) for a list and more details about signals.
2) [20 marks] Write a filter in C that prints M lines out of every N. It can
be done using the shell and awk (see ~wayne/pub/ics54). It's more simple
and efficient in C. The program's name is "every". It is called like this:
$ every [-N,M] [<list-of-files>]
where N and M are both positive integers, M <= N. (Anything in square
brackets '[]' is optional, and doesn't need to appear on the command line.
This is standard for Unix manual pages.) The option argument, if present,
must come before any filenames. If no "-N,M" option is on the command line,
then "every" should look for an environment variable called EVERY and take
its options from there, in the same format as the command line. If "every"
can't find options either on the command line or in the environment variable
EVERY, then the default is "-1,1". That is, with no options, "every"
acts just like cat(1). For example, if we number lines starting at 0, then
$ every -10,2 foo.c
prints out the following lines of foo.c: 0,1, 10,11, 20,21, 30,31,
etc. If M is omitted, eg
$ every -10 foo.c
then it defaults to 1. (If either N or M is specified on the command line,
the environment variable EVERY should be ignored.) If multiple files are
given on the command line, each one should be handled INDEPENDENTLY, so
"-10,2" means lines 0,1,10,11, etc. of each file.
3) [30 or 40 marks, see below] Re-write lss in C. (This is the last time you'll
see "lss", I promise.) You may use the standard I/O library; you don't
need to restrict yourself to Unix system calls like read(2) and write(2),
although you will still need to use SOME Unix system calls, like stat(2).
You may not use any shells directly or indirectly. (For example, you
can't use system(3s).)
There are two versions of this program; an easier one, and a harder one.
The easier one is described first, and will get you at most 30 marks.
For 30 marks, it doesn't need to support any options, it doesn't need to take
any filenames, it doesn't need to print a "total" line, and it doesn't need
to do anything special with symbolic links or special files (like the ones
found in /dev). However, if a link points nowhere, an appropriate error
message should be output, using perror(3c). This is different than what
"ls -L" does; that's OK. It does not ignore files whose names begin with
'.'. It needs to work correctly for directories and regular files (S_IFDIR
and S_IFREG in stat(5), respectively). In other words, in a directory
that contains only other directories, regular files, and symbolic links,
the output should look just like
$ /usr/ucb/ls -Lla | grep -v '^total' | sort +3nr
except symbolic links that point nowhere should cause an error. (The -L
makes it easier for you: you only need to use stat(2), rather than *both*
lstat(2) and stat(2).)
In addition to the standard C I/O library, you should read the following
manual pages: stat(2), stat(5), opendir(3c), readdir(3c), closedir(3c),
dirent(4), qsort(3c), getpwuid(3c), perror(3c), time(2), ctime(3c). You MUST
use qsort(3c) to do the sort. You MUST check the return values of *all*
system calls and standard I/O functions for errors, and use perror(3c)
to print the error. The time should be printed by clipping characters 4
through 15 of the string returned by ctime(3c). (ie, you don't need to
do anything fancy for files older than 1 year, like ls(1) does.)
Extra 10 marks: (these are not bonus marks; this question is out of 40, but
you may choose to implement the subset above, and if you do it correctly,
it's worth 30 marks out of 40). Implement a more full version of lss.
It supports the options -a, -A, and -L in standard Unix format like "-a -L"
or "-La" (see getopt(3c)). It needs to handle symbolic links just like
"ls -l" does (ie, you'll need to use lstat(2) by default and print where
symlinks point, but if "-L" is specified and the file is a symlink then call
stat(2).) Finally, if the "file" is actually a directory, then it descends
the directory just like ls(1) does. No "total" line is necessary, though.
GENERAL COMMENTS: Style and modularity of questions 2 and 3 are important.
Try to keep the design simple and elegant. You will probably have to make
extensive use of the course texbook and the online manual.