Macros and how they work (and other things about inodes)

Reading: man 7 inode 
https://man7.org/linux/man-pages/man7/inode.7.html

We were shown macros in Assignment h1

Where m represents some st_mode for a file:

 S_ISREG(m) 	regular file?
 S_ISDIR(m)        directory?
 S_ISLNK(m)    symbolic link?

each macro returns either a 1 or 0, depending on what type of file it is.

But how do these work?

the st_mode is generally represented as a 7 digit octal number (base 8).
	- The leftmost three digits define the file type
	- The rightmost four define the file permissions

Note that in your program, leading zeroes aren't printed by default.
So an st_mode value of 100644 will be represented here as 0100644

Let's look at file types:
The following mask values (also octal numbers) are defined for the file type

			S_IFMT     0170000   bit mask for the file type bit field

			S_IFSOCK   0140000   socket
			S_IFLNK    0120000   symbolic link
			S_IFREG    0100000   regular file
			S_IFBLK    0060000   block device
			S_IFDIR    0040000   directory
			S_IFCHR    0020000   character device
			S_IFIFO    0010000   FIFO

To determine the filetype, we need to AND our st_mode value with the bit mask, and check the value

AND - Both bits need to be a 1 to get a 1

So if our st_mode value was 0100644, it would go something like this

	st_mode : 0100644
	S_IFMT  : 0170000

	Result: : 0100000

	We can probably see right away without needing to convert these octal characters to binary 
	that the result will be 0100000, which is a regular file.

Now suppose we get an st_mode value of 40755 (or 0040755)

	st_mode : 0040755
	S_IFMT  : 0170000

	It's not as obvious here, so to see it better we can convert this to binary, which is easy to do
	as each octal character represents three binary bits.

	st_mode : 0040755  000 000 100 000 111 101 101
	S_IFMT  : 0170000  000 001 111 000 000 000 000
    Answer  :          000 000 100 000 000 000 000  -> 0040000

    We get a value of 0040000, whis cooresponds to a directory

Then, for the macros above, it checks that value produced with the value defined for the type of 
file specified to see if these are equal. If it returns 1, then the answer is yes, otherwise it's no.

######    FILE PERMISSIONS   ####
Finding file permissions work much the same way

File permissions have a setting for the file owner, the group owner, and all other users


The following mask values are defined for the file mode component
of the st_mode field:

	Fourth digit:
		           S_ISUID     04000   set-user-ID bit (see execve(2))
		           S_ISGID     02000   set-group-ID bit (see below)
		           S_ISVTX     01000   sticky bit (see below)

Last three Digits:

	3rd - Owner section:
		           S_IRWXU     00700   owner has read, write, and execute permission

		           S_IRUSR     00400   owner has read permission
		           S_IWUSR     00200   owner has write permission
		           S_IXUSR     00100   owner has execute permission

	2nd - Group Section:
		           S_IRWXG     00070   group has read, write, and execute permission

		           S_IRGRP     00040   group has read permission
		           S_IWGRP     00020   group has write permission
		           S_IXGRP     00010   group has execute permission

	1st - Others section:
		           S_IRWXO     00007   others (not in group) have read, write, and execute permission

		           S_IROTH     00004   others have read permission
		           S_IWOTH     00002   others have write permission
		           S_IXOTH     00001   others have execute permission

These are represented here as 5-digit octal numbers, the last 4 digits determine the file-ownership

We can determine permissions in our st_mode by the following
	- If digit is a 7, we know we have read-write-execute
	- All other digits, we look at the table and determine what two numbers out of 4, 2, or 1 add up 
	  to our desired digit

	The second digit in 0040755 (or the group digit) is 5, so we need to determine which two numbers 
	out of 4, 2, or 1 add to 5, which is 4 and 1. this indicates that the group has read or execute/