---
title: "15-file-systems"
aliases: 
tags: 
- cosc204
- lecture
---

# what is a file (LINUX)
an interface of OS to users

os manages files just as files - it doesn't care what type of file it is - they are all bytes arranges logically in sequence.

they are arrangges logically in sequence

files need to be interpreted. THis is done by applications. e.g., powerpoint, ms paint, etc.


- the OS provides a logical unit of storage for the user
	- the user refers to files
	- the operating system maps files onto regions of the storage
	- files are really an artifact of the dialogue between the user and the OS

## how to define it
- a collection of related data
- e.g., the set of lines in a program

structure:
- a byte stream or a sequence of bytes, text file is a stream of ascii characgers

what can the user do
- create, write, read, reposition, delete, truncate (through system calls)

## attributes
- name
- owner
	- Uid and Gid
- file type
- location of data
- size of data
- permission
- "housekeeping" information
	- time and created modified etc
- and more

## where is the data stored
- in an index node (inode) pointed by an entry of a directory. (you can think of a directory as just a set of <name, inode> entries. )

internal fragmentation
- dont know exactly how many bytes to allocate to a file
- extra space is lost

external fragmentaion TODO

these issues have been solved using block allocaiton instead of continuous allocation

## system calls for file systems
- open close, read, write, ioctl, etc

`ssize_t write(int fd, const, void *buf, size_t count)`
	- fd - file descriptor
	- write up to count butes from the buffer pointed buf to the file referred to by the file descriptor
	- returns -1 if error. and errno is set
	- should return number of bytes written if ok

# file access pattersns
# directory structure
# file system implementation
# disk allocation methods