Interprocess Communication Mechanisms

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Interprocess Communication Mechanisms
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IPC

• Signals
• Pipes
• System V IPC

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Signals

• Signal asynchronous events to one or more
  processes
• Oldest IPC method used by UNIX
• Intel Box
• 1)SIGHUP 2) SIGINT 3) SIGQUIT 4)
  SIGILL
• 5) SIGTRAP 6) SIGIOT 7) SIGBUS
  8) SIGFPE 9) SIGKILL 10) SIGUSR1 11)
  SIGSEGV 12) SIGUSR2 13) SIGPIPE 14)
  SIGALR 15)SIGTERM 17) SIGCHLD 18) SIGCONT
  19) SIGSTOP 20) SIGTSTP 21) SIGTTIN 22)
  SIGTTOU 23) SIGURG 24) SIGXCPU 25)
  SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28)
  SIGWINCH 29) SIGIO 30) SIGPWR

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Signal

• Processes can ignore most of the signals except
  SIGSTOP, SIGKILL
• A process can choose to block, handle or allow
  kernel to handle it
• For example, SIGFPE(floating point exception)
  core dump and the exit

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Linux Signals

• task_struct
• Pending signals signal (32 bits)
• A mask of blocked signal blocked
• sigaction array address of handling routine or a
  flag to ignore the signal (modified by system
  calls)
• Normal process can only send signals to processes
  with the same uid and gid or to the processes in
  the same process group

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Signal Handling

• Signals are not presented to the process
  immediately they are generated., they must wait
  until the process is running again.
• Every time a process exits from a system call its
  signal and blocked fields are checked and, if
  there are any unblocked signals, they can now be
  delivered

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Signal Handling

• Processes can elect to wait for signals if they
  wish, they are suspended in state Interruptible
  until a signal is presented
• The Linux signal processing code looks at the
  sigaction structure for each of the current
  unblocked signal

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Pipe

• ls pr lpr
• Linux using two file data structure which point
  at the same temporary VFS inode (points to a
  physical page within memory)
• use standard read/write library function

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Pipe
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Named Pipe

• FIFO
• created by mkfifo command

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System V IPC Mechanism

• Message queues, semaphores and shared memory
• UNIX System V in 1983

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Message Queues

• Allow one or more processes to write messages,
  which will be read by one or more reading
  processes
• msgque --gt msqid_ds
• Linux kernel wait queue

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Message Queues
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Semaphores

• A semaphore is a location in memory whose value
  can be tested and set (atomic) by more than one
  processes
• Can be used to implement critical regions
• To prevent deadlock sem_undo data structure

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Semaphores
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Shared Memory

• Allow one or more processes to communicate via
  memory that appears in all of their virtual
  address space
• shmid_ds data structure

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Shared Memory