Share information between processes

1. 透過硬碟上的文件溝通
   • 超慢
2. 透過 kernel buffer
   • 滿快的，但這樣要一直在 user mode 和 kernel mode 來回切換，因為kernel buffer 在 kernel space
3. 透過 shared memory region
   • shared memory region 在 user space

Mechanisms

1. Signals

2. Communication

   1. Data transfer
      • Byte Stream
        • Pipes
        • FIFOs(Named Pipes)
        • stream sockets
      • Message Passing
        • SystemV MsgQ
        • POSIX MsgQ
        • datagram sockets
   2. Shared Memory
      • SystemV S.M
      • POSIX S.M
      • Memory Mapping
        • anonymous memory mapping
        • memory mapped file

3. Synchronization

Pipes

• Related processes
  • parent-child
  • sibling
• Executing on same machine
• 用法
  • cmd1 | cmd2
    • cmd1 不是輸出到 stdout，而是由 kernel 維護的 buffer，也就是 pipe
    • cmd 不是從 stdin 獲取輸入，而是從 pipe 獲取
  • cmd1 | cmd2 | … | cmdn

Named Pipes / FIFOs

• Related / Unrelated processes

• Executing on same machine

• creat a FIFO

  • commands
    • mkfifo
    • mknod

嘗試寫入或讀取 FIFO 時，會被 redirect 到 pipe

Signal Handling

Signal

• Used by OS to notify running process some event has occured without the process needing to pull for that event
• process 收到 signal 後會先停止執行並執行 signal handler

1. A process did something
   • SIGSEGV(11), SIGFPE(8), SIGILL(4), SIGPIPE(13)…
2. A process wants to tell another process something
   • SIGCHILD(17)
     • child process terminated
3. User sends sig to foreground processes
   • Ctrl + C SIGINT(2)
   • Ctrl + \ SIGQUIT(3)
   • Ctrl + Z SIGTSTP(20)

disposition

決定 process 遇到 signal 時該怎麼處理

• Term
  • teminate process
• Ign
  • ignore
• Core
  • terminate the process and dump core
• Stop
  • stop the process
• Cont
  • continue the process if it is stopped

Signal can’t not be caught

• SIGKILL(9)
• SIGSTOP(19)

Commands

• trap

  可以 handle signal

kill

kill - L 可以看到 standard signal 和 real-time signal

standard signal 開頭是 SIG，realt-time signal 是 SIGRT