3191932525 Distributed Software Programming Lecture 10 Week 14

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31919/32525 Distributed Software
ProgrammingLecture 10 (Week 14)

• Posix Threads

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Outline

• Disadvantages of Using fork()
• Advantages of Using Threads
• Basic Thread Functions

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Problems with fork()

• fork is expensive
• Memory is copied to child
• All fds are duplicated in the child.
• Need IPC to pass information between the parent
  and child after the fork.

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Features of Threads

• Threads share
• the same global variables
• process instructions
• most data
• open files
• signal handlers and signal dispositions
• current working directory
• user and group IDs

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pthread_create Function

• Initial or main thread is created by exec.
• Additional threads are created by
• int pthread_create(pthread_t tid, const
  pthread_attr_t attr, void (func)(void ), void
  arg)
• Thead has attributes priority, stack size etc
• Thread specifies a function to execute
• arg is the single argument to the function
• On success, pthread_create returns 0 and a thread
  is created. Otherwise a positive value is
  returned.

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Start Function of a Thread

• If multiple arguments are needed, the arguments
  are packaged into a data structure.
• The function takes only a single argument.
• The argument is a generic pointer pointing to the
  data structure.
• The function returns a generic pointer.

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pthread_join Function

• One can wait for a given thread to terminate by
  calling
• int pthread_join(pthread_t tid, void status)
• tid specifies the thread to be waited for
• If the status pointer is not NULL, the return
  value from the thread function is stored in the
  location pointed by status
• pthread_join returns 0 on success, and a positive
  value on failure.

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Joinable Thread and Detached Thread

• Joinable threads ID and status are retained when
  the thread terminates until another thread calls
  pthread_join.
• Detached threads resources are released when the
  thread terminates.
• By default, a thread is joinable.
• To change from joinable to detached thread, use
• int pthread_detach (pthread_t tid)

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Other Basic Functions

• One way for a thread to terminate is to call
• void pthread_exit (void status)
• status must not point to an object that is local
  to the calling thread.
• Another two ways for a thread to terminate
• The thread function returns
• The main() function calls exit().
• A thread fetches its own ID by calling
• pthread_t pthread_self (void)

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Readings

• Stevens, UNIX Network Programming, Volumn 1 (2nd
  Edition)
• Chapter 23