File attributes
-----------------
name
contents
owner
group
protection (mode)
date created, modified, accessed

to change protection of files
-----------------------------
chmod new-protection list-of-file-names

e.g. I want new protection to be
     rw-r-----
     110100000
     6  4  0

     chmod 640 filenames

unix functions
--------------
   int open(char * fname, int flags {, protectionbits} )
   the int is called a file descriptor (fd)

   int close(int fd)

return value
   negative, there was an error
   positive or zero means all OK
   >0 usually some useful information

to create anew output file
  int fd = open("outfile.txt", O_WRONLY | O_CREAT | O_TRUNC, 0640);
OR int fd = open("outfile.txt",
                 O_WRONLY | O_CREAT | O_TRUNC,
                 S_IWUSR | S_IRUSR | S_IRGRP);
  int rf = open("infile.txt", O_RDONLY);

using stat
----------

  struct stat sb;
  int r = stat("filename", & sb);
  if (r < 0)
  { perror("stat:");
    exit(1); }