Files

System Calls for File System Accessing files –Open, read, write, lseek, close Creating files –Create, mknod

System Calls for File System Manipulation & Navigation –Chdir, chroot, chown, chmod, stat, fstat Special file and operations –Pipes, dup Extending and changing the file system –Mount, umount, link and unlink

Open Opens a file located on the file system by returning an integer – file descriptor (fd) Other system calls then use the fd to carry out operations on the opened file –Read, write, lseek, etc. Fd = open(char, flags, mode)

Open Input to the open system call is –Pathname – char string –Open flags – integer value –Mode – integer value Kernel converts the char file name into an inode

Open Allocates an entry in file table File table entry has a pointer to the inode and field that indicates the byte offset Byte offset is typically set to Zero, the beginning of the file, but can be set to the end of the file.

Open Algorithm

Data Structures After Open

Data Structure After 2 Processes

Notes about Open System Call Each open system call –Returns a single user file descriptor –Points to a unique file table index –Inode table entry can have multiple links

Reading and Writing Once a file is opened –We can then operate on it via the file descriptor (i.e. fd) Open system call syntax –open(fd, buffer, count) fd – file descriptor buffer – place to store read data count – number of bytes to read

Read and U Area Sets I/O parameters namely Mode (Read or Write) Count field Target address for user data buffer Byte offset in file where I/O should begin Address Flag (User or Kernel)

Read Example

Writing Similar to read Except –Allocate new data blocks –Allocate new indirect blocks Updates inode for file. –What does datum does it update in inode? –Disk Block, indirect block, file size, modification time, etc,

Write System Call Syntax of write system call write(fd, buffer, modes); –fd is file descriptor –buffer is memory where data to write is found –Count is number of bytes to write

Writing Notes Data is written in at most one block size Write system call may have to read a block before it writes Inode for file is locked until write() is completed Allocation of new blocks Modifies inode

Pipes Allows the exchange of data between processes Data exchange is in a FIFO manner Can be used to synchronize process executions

Pipes Two classes of pipes Named pipes –Use open Unnamed pipes –Use pipe system call Use traditional