Files

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Files
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System Calls for File System

• Accessing files
• Open, read, write, lseek, close
• Creating files
• Create, mknod

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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

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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)

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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

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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.

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Open Algorithm
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Data Structures After Open
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Data Structure After 2 Processes
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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

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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

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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)

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Read Example
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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,

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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

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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

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Pipes

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

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Pipes

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