File-System Interface

Electrical and Computer Engineering Stephen Kim ([email protected])

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Chapter 11: File-System Interface n File Concept n Access Methods n Directory Structure n File System Mounting n File Sharing n Protection

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1 File Concept (1) n Contiguous logical address space n Usually nonvolatile such as a hard-disk n Most RTOS’s do not use a hard-disk because of

u its mechanical components that cannot sustain in harsh environments like impact and vibration

u its power consumer (mostly battery-powered RTOS)

u exceptions are some printers, digital recorder, mp3 players, which required huge storage n Recently, non-mechanical storage becomes very popular

u Simulating hard-disk as an interface

u Use FlashRAM technology n In future, file-system in embedded/real-time system will be more popular ECE/IUPUI RTOS & APPS 3

File Concept (2) n File types:

u Data – numeric, character, alphanumeric, binary

u Program n Generally, a file is sequence of bits, bytes, lines, or records

u The meaning is defined by its creator or user

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2 File Structure n None - sequence of words, bytes n Simple record structure u Lines u Fixed length u Variable length n Complex Structures u Formatted document u Relocatable load file n Can simulate last two with first method by inserting appropriate control characters. n Who decides: u Operating system u Program ECE/IUPUI RTOS & APPS 5

File Attributes n Name – only information kept in human-readable form.

u Unix differentiates upper- and lowercase

u DOS and its predecessors do not distinguish them n Identifier – unique tag internal to the file system n Type – needed for systems that support different types. n Location – pointer to file location on device. n Size – current file size. n Protection – controls who can do reading, writing, executing, and so on. n Time , date , and user identification – data for protection, security, and usage monitoring. n Information about files are kept in the directory structure, which is maintained on the disk. ECE/IUPUI RTOS & APPS 6

3 File Operations (1)

n OS provides system calls for each operation

u Create – space allocation and a new entry in the directory

u Write – For a given file name, search the directory. OS keeps a write-pointer to the location where the next write is to take place

u Read – Search the directory for a given name. OS maintains a read-pointer to the location where the next read is to take place

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File Operations (2)

u Reposition within file – the current-file-position is set to a given value

u Delete – release all file space

u Truncate – erase the contents of a file, but keep the file attribute.

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4 File Open and Close

n Open(Fi) - search the directory structure on disk for entry Fi, and move the content of entry to an open-file table. So, the system does not need to search directory for subsequent read or write n Close (Fi) – remove the entry of Fi from the open-file table and finish the file access. n Implicit open/close

u open a file when the first reference is made

u close a file when the job opened the file terminates n Explicit open/close

u Most systems require the programmer open and close a file explicitly.

u Open system calls returns a pointer to the entry in the open-file table. The pointer, or file descriptor, is used all subsequent IO operation

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Open/Close in Multi-process Systems n Multiple users can open a file simultaneously n OS uses two-level of internal tables,

u per-process table – keep info for all files a process has open (current file pointer, access right, accounting info), and pointing to the system-wide table entry

u system-wide table – contains process-independent info (file location, access date, file size)

u Typically, the open-file table has a counter indicating the number of processes opening the file

t increment when a process opens the file

t decrement when a process closes the file

t when the counter is zero, the file is not in-use anymore. remove the file entry from the open-file table.

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5 File Types n A part of filename indicates the file type

u For example, In DOS, .com, .exe, .bat indicates an executable file n In some systems, the use of file extension is optional and just hint. n In Apple, each file has a type made by the creator of the file n Unix does not support any file type, even though file extension can be used as a hint n What is the advantage for supporting file types by OS? n What is the disadvantage?

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File Types – Name, Extension

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6 Access Methods, Sequential Access n most common access method n based on a tape model of a file, but work well on random-access devices such as hard-disk n read-operation reads the next position of a file n write-operation appends to the end of file and advances to the end of the newly written material n A file may move the CP

u to the beginning of the file

u to n records forward or backward

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Sequential-access File

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7 Access Methods, Direct Access n based on a disk model

u block, sector, cylinder n A file is made up of fixed length logical records n A record can be accessed without particular order n Relative block number – index relative to the beginning of the file

u the first relative block is the index 0 n Given a logical record length L and a request for record N

u IO request must be made starting at location N´ L n Simulation of sequential access using direct access reset Þ cp=0; read next Þ read cp; cp++; write next Þ write cp; cp++;

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Access Methods, Index Access n a file containing the record pointers, to access the file record directly, with little IO

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8 Directory Structure (1)

n A disk can be divided into one or more partitioned n MS-DOS/Windows – each partition is treated as a separate storage. ie. only dividing n In Unix, several partitions can be organized into one logical structure n Each partition contains a device directory, or volume table of contents (VTOC) u to keep information about file within it t name, location, size, type, and so forth

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Directory Structure (2) n A collection of nodes containing information about all files.