Advanced I/O

1
Advanced I/O

• functions involved in atypical I/O
• non-blocking functions
• record locking
• streams
• I/O multiplexing
• select() function
• poll() function
• readv(), writev() functions
• memory mapped I/O

2
Nonblocking I/O

• term for I/O using system calls that arent
  slow
• a slow call can theoretically block
  indefinitely
• reads from files where data may be delayed or
  never arrive
• write to files that cant accept data immediately
• conditional open
• reads and writes to files with mandatory locks
• other IOCTL and IPC functions
• calls to open can be designated non-blocking
• fcntl() functions can alter an open FD to make it
  non-blocking
• non-blocking calls return immediately if ops
  cannot be performed, with a would have blocked
  error

3
Record Locking

• used by one process to prevent other processes
  from changing a portion of file in use by the
  first process
• some UNICES only support locking entire file
• locking can be advisory or mandatory
• advisory locking depends on cooperative functions
• mandatory locking is enforced by the kernel

4
fcntl() record locking

• uses flock structure to describe lock
  requirements
• locks can start and extend beyond the current
  EOF, but not beyond the current SOF
• locks can be shared read or exclusive write
• multiple read locks are allowed
• only one write lock is allowed
• a read locked file segment cannot be write
  locked, and vice versa
• if two processes are each waiting on the others
  lock, you have deadlock, which is typically
  overridden by the system

5
Inheritance and Release of Locks

• locks are associated with a process and a file
• if a process exits, all locks are released
• if a file is closed, all locks are released
• multiple descriptors for a file to NOT prevent
  lock release, locks are associated with the file
  and not the descriptor
• locks are not inherited by children
• locks are typically inherited across an exec, but
  are not required to be so by POSIX

6
Advisory vs. Mandatory Locking

• advisory locking is essentially voluntary
• requires cooperating functions
• mandatory locking is essentially involuntary
• enforced by the kernel
• limited to open(), read() and write() functions
• mandatory locking can result in unexpected
  consequences
• file may still be modifiable under certain
  circumstances
• can be used by to maliciously control files

7
Streams

• SYSV methodology for interfacing comm drivers
  with the kernel
• streams are full duplex paths between processes
  and device drivers
• streams are normally loaded into the kernel at
  compile time
• data flows upstream to the process,
  downstream to the driver
• streams uses a messaging paradigm for passing
  information to and from processes and devices

8
I/O Multiplexing

• multiplexing is a methodology for handling
  unpredictable I/O on multiple file descriptors
• using nonblocking I/O results in polling
• using blocking I/O may cause available I/O on
  another descriptor to be ignored
• multiplexing uses a function that returns when
  any of a given list of FDs is available for I/O

9
select() Function

• common function for multiplexing
• takes
• highest FD1
• descriptor lists read, write, exception
• timeout
• returns
• -1 error OR
• 0 no FDs ready OR
• number of descriptors ready
• select() clears all bits on descriptors that are
  not ready

10
working with select()

• four macros are available to work with FD sets
  used with select
• FD_ZERO clears all bits
• FD_SET turns on bit for particular FD
• FD_CLR turns off bit for particular FD
• FD_ISSET tests for bit on in particular FD
• descriptors are ready if
• in read set and read wont block
• in write set and write wont block
• in except set and exception is pending
• whether or not an FD is blocking is irrelevant to
  the operation of select()

11
poll() Function

• variant form of select()
• uses array of nfds structures, in place of FD
  sets
• more fine grained control of events that
  trigger return

12
Asynchronous I/O

• asynchronous I/O is a method whereby I/O is
  requested (queued), and the called process
  returns immediately, allowing the calling process
  to continue
• on some systems AIO involves a kernel generated
  signal when queued I/O requests change status
• on other systems AIO is handled by the process
  calling a function to determine status

13
readv() and writev() Functions

• functions for writing to/from multiple buffers
  from a single function call
• scatter read
• gather write
• uses array of iovec structures for buffers
• improves efficiency over multiple read/writes
• cleaner then combining smaller buffers into
  single large buffer

14
readn() and writen() Functions

• repeating read() and write() functions
• used for reading and writing fixed amounts of
  data to and from end devices which may cause a
  return before all data is transferred
• terminal devices, network devices, SVR4 streams
  typically
• functions re-read or re-write until all data
  is transferred

15
Memory Mapped I/O

• methodology for manipulating files transparently,
  via the manipulation of a memory buffer
• when data is written to the buffer, the file (or
  a copy) is written, when read from the buffer,
  the file is read
• mapped regions can be shared across processes
• mapped regions are unmapped when the process
  terminates
• closing an FD does not unmap the region