Notes (07-08).sync-conflict-20250716-154051-MBE4QS2

File system stuff!

execute doesn’t just mean “run-able”. It also means being able to cd into (if it’s a directory) and allow use of pathnames going through the directory

To change perms u use chmod and specify: u=rw means user reads and writes. g=r means group can read

• But there are several notations chmod(const char *path, mode_t mode) also is a thing

To change owning user or group you do any of these:

i-node

File system has an array of i-nodes i-node number = array index Each file/dir is identified by an i-node, not by filename i-node also stores file/directories metadata:

• type
• perms
• owning user and owning group
• size
• timestamps
• where the data is on disk
• others
• but not filename! stat gives the i-node info!

A directory simply stores mappings from filenames to i-node numbers

• Different data on different systems. Regardless though opendir, readdir, and closdir lets you access “portably” in Albert’s words important

Hard Link

• ln command creates another filename to have the same i-node number as an existing file. This creates a “hard link”
• System call version of ln is link (idk the difference though)
• . and .. are implemented hard links!
  • Otherwise hard-linking directories is disallowed (besides the two exceptions above)

Unlinking = Deleting a file

• Keeps removing references (these are stored in the i-node)
• If there’s no more, then it actually frees the file from disk space (as well as the i-node)
  • If some processes still have it open, we wait for closing
• Soft links (Symlinks)
  • Forwards you to another pathname.
  • Most systems follows “symlink forwarding” todo
  • symlink and ln create symlinks
• If the path is relative, it’s relative to the directory that linkname lives intodo I don’t think he can english
• symlinks break on name change. Hard links don’t..
  • symlinks will still work if the file of a certain name is deleted and a new one is created with the same name. hard links would still point the file you deleted

File Attributes

• These are the system calls to get file attributes (a.k.a. “statuses”)
• int stat(const char *path, struct stat statBuf)
• int lstat(const char *path, struct stat statBuf)
• Both of which return 0 if success and -1 if error (also sets errno but idk what that istodo)
• for stat, if you pass a path to a symlink, then it gives info on the file it points to. However, for lstat it gives info on the symlink itself

The field st_mode has file type and permission flags, e.g., it represents “drwxr-xr-x”. Meaning and useful macros in ‘man inode’

• Try to understand thistodo but from what I get of it:
  • st_mode is some field mentioned in man 3 stat (stat being a struct)

Bit stuff

Base then stuff: 26 is twenty six: $2\times {\text{ten}}^{\text{one}}+6\times {\text{ten}}^{\text{zero}}$ The base is ten. Wow. intuitive Hexadecimal means the base is 16 instead. $1A=1\times 16^1+A\times 16^0$

Bitwise operations

Bitwise & and, | or, ~ not, and ^ xor

Example: a = 10001001 and b = 00000011

a & b = 00000001 a | b = 10001011 ~ a = 01110110 a ^ b = 10001010

Check/set/clear/flip

Here’s how you check/set/clear/flip bit 5 of b where b = 00000011 Let there be an m = 00100000. Note that m = 00100000 since that’s the 5th bit! Bit 5 is on; the others are off.

1. Check if bit 5 is set with if (b & m)
2. Then we do the algorithm:
   1. b = b | m (set bit 5 to be on)
   2. b = b & ~m (set bit 5 to 0)
   3. I don’t get the point of setting 1 and removing 1…todo
   4. b = b ^ m (flip. )
3. You can also do b |= m or b ^= n etc. etc.

File Perms (bitwise layout)