CS 243 Homework #3

In your home directory,
 create a subdirectory named: homework3

Part 1:
In your homework3 directory,
 create part1.c, compile, and run part1
  which should do the following:
   writes these 3 letters: abc
    followed by newline
     to part1.out
      in the current directory.
       Create the file if it does not already exist.
        Over write the file if it already exists.
   Hint: see open on p. 98, write on p. 96.
   Testing:
    put more than 10 chars in part1.out
     then run your program
      make sure there are only 4 chars after your program runs.

Part 2:
In your homework3 directory,
 create part2.c, compile, and run part2
  which should do the following:
   writes these 3 letters: abc
    followed by newline
     to part2.out
      in the current directory.
       Create the file if it does not already exist.
        Set the permissions so that everyone can read, write, and
         execute if umask is 0.
          Append to the file if it already exists.
           Hint: set umask to 0 before you test your program.
            You program should not call chmod.

Part 3:
Initially part3.c should be a copy of part2.c.
 but change all references from part2 to part3.
 Then, if your program is unable to create or write to the
  destination file,
   use perror (see p 126)
    to show the error message.
     Hint: turn off write perm to the file part3.out
      before you run your program.
       Another way to test: turn off write perm
        to this dir so that you can't create any files in it.
     Make sure that your program shows an error msg
      if you can't create the file
       and if you can't write to an existing file.
   Hint 2:
      Check for errors twice:
         1. immediately after open (error here should be fatal)
         2. immediately after write
      Your code to check for errors should look like this:

           errno=0;
           /* make some system call */
           if (errno) {
              perror("progname or function");
              exit(errno);   /* if the error is fatal */
           }

       It is wrong to only check errno after the write,
        because,
         if the open fails,
          the file descriptor returned by open will not be valid
           so the write will get an error
            which will set errno
             but this error will be misleading
              and won't show you the open error.