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

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