CS230 System Programming

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CS230 System Programming

• Project 3 IPC
• (Interprocess Communication)

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1. Specification(1/2)

• Develop who command using shared memory
• List IDs of shell processes the same shell
• Unlink shared memory semaphore when exit the
  shell
• You need to think about structure to save
  process pid

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1. Specification(2/2)

• Develop built-in command msg using shared
  memory signal
• msg pid Message
• Use single shared memory to send and receive
  message
• Send a signal to receiving shell to consume the
  message
• Issue semaphore to deal a race condition
• Do not use busy-waiting policy

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2. Interprocess Communication

• IPC communication between processes
• Shared memory
• Message queuing
• Semaphore
• cf. Socket

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3. Shared Memory(1/3)

• Memory space that some processes use together

Process I
Process II
Shared memory
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3. Shared Memory(2/3)

• shmget
• Make shared memory segment
• Int shmget(key_t key, int size, int shmflg)
• shmat
• Get address of shared memory segment
• Int shmat(int shmid, char shmaddr, int shmflg)

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3. Shared Memory(3/3)

• shmdt
• Unload shared memory segment
• Be careful with unload because there can be other
  process who is dealing with segment
• int shmdt(char shmaddr)

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4. Semaphore (1/4)

• Certain condition
• Two processes want to write something on same
  position of shared memory at same time-gt
  collision!
• Use semaphore to give the permission to only one
  process

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4. Semaphore(2/4)

• P() get semaphore to contact critical point
• V() release semaphore after using critical point

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4. Semaphore(3/4)

• semget
• Create semaphore
• int semget(key_t key, int nsems, int semflg)
• semop
• Get or release semaphrore
• int semop(int semid, struct sembuf sops,
  unsigned nsops)

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4. Semaphore(4/4)

• semctl
• Control semaphore
• Int semctl(int semid, int semnum, int cmd, union
  semun arg)

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5. ipcs, ipcrm

• Deal IPC control
• shared memory
• Semaphore
• Message queue
• ipcs show using IPC control
• Ex ipcs m
• ipcrm remove IPC control
• Ex ipcrm M shm_key