PURPOSE

This assignment requires you to write another C$++$ program using the Unix system calls. It should give you experience in

using the system calls open $(2),$ close$(2),$ write$(2),$ stat $(2),$ chmod $(2),$ as well as working with command line arguments

$($using getopt $(3)$ is optional$).$ The numbers in parentheses here are section numbers so you can find the right page in the

manual. You should use perror $(3)$ to print the error message whenever an error occurs in one of the system calls.

PROGRAM

A typical Unix system will have many files that contain sensitive information. Permissions can keep these files secure$.$ Some

files can be publicly read, but can not be altered by a regular user $($eg$./$etc$/$passwd$).$ Other files can't be read at all by a

regular user $($eg$./$etc$/$shadow$).$ Your task is to write a C$++$ program that will allow you to add messages to a file that has

NO permissions granted for any user. Obviously, writing to a file that you don't have write permissions to is impossible, so

you will be temporarily granting yourself the necessary privileges, then immediately revoking them when done.

The program you develop will take a message as a command line argument and append that message to the end of a file

$($which is also specified as a command line argument$).$ It will also support a $-$ c command$-$line option that, when supplied,

will cause your program to clear the file before the message is appended.

The file should have all permissions denied for any user, both before and after the message is appended. For this to work,

the person running the program needs to be the owner of the file, so they can change the permissions as needed.

ALGORITHM

Your program should follow the following basic algorithm to accomplish the required task. Accomplish these using the

system calls named above.

$(1)$ Check to see whether the output file exists. If it doesn't, create it$.$ It will make things easier for you if any newly created

file is closed at the end of this step.

$(2)$ Check the output file's permissions. If any exist, print a useful error message and exit.

$(3)$ Change the permissions on the file to allow writing by the user.

$(4)$ Open the file for output. If the $-$c command line option is present, your program should truncate the file contents,

replacing whatever's there with the new message. Otherwise, the message should be appended to the end of the file's

contents.

$(5)$ Write the message passed in on the command line into the output file. Write an additional newline character so that

the output has a nicer format.

$(6)$ Remove the permissions and close the file. $($These two operations can be performed in either order, but the implemen$-$

tation will be slightly different.$)$

USEFUL HINTS

Don't use the command line arguments directly. Their position in the argument list may change depending on com$-$

mand line options $($like your $"-$c $").$ Create meaningful char $*$ variables and fill these with the appropriate entries from

the argument list, once you've determined their proper positions. I would recommend using getopt $(3)$ to handle the

command line options$/$arguments$,$ but its use is not required.

This program will not be reading in any kind of input except for the command line arguments. If you're trying to use

cin or read for something, you're probably doing something wrong.

ERROR CHECKING

If the message log file cannot be opened, an appropriate error message should be printed to standard error and the

program should exit immediately. If the file has any permissions at all, the file should be rejected as insecure, and the

program should exit.

Check for error values after every system call that can fail $($that$\text{'}$s most of them$),$ and print out what happened when an

error did occur. It is more work to set up$,$ but when something goes wrong, you'll actually have an idea what caused

the problem.