Advanced%20UNIX%20programming

1
Advanced UNIX programming

• Fall 2002
• Instructor Ashok Srinivasan
• Lecture 29

Acknowledgements The syllabus and power point
presentations are modified versions of those by
T. Baker and X. Yuan
2
Announcements

• Reading assignment
• Chapter 23
• Project Outline due Monday
• Midterm 2
• Bonus points test
• 20 Nov 2002
• HW 4 will be announced Friday

3
Week 11 Topics

• Threads
• Basic thread functions
• Some more thread functions
• Thread-specific data
• Mutual exclusion
• Conditional variables

4
Threads

• A thread is an independent sequence of execution
  of program code inside a process
• A thread is often called a lightweight process
• But it is NOT a process
• It is something smaller than a process
• The process context includes
• Memory, instruction, program counter, stack
  pointer, registers, open file descriptor, etc.
• A thread has less
• Program counter, a stack, and a set of
  registers, etc.

5
Threads introduction (continued)

• Threads share
• Process instructions, most data, open files,
  signal handlers and signal dispositions, current
  working directory, user and group ID
• Threads do not share
• Thread ID, set of registers (including pc
  program counter -- and sp stack pointer),
  stack, errno, signal mask, priority
• POSIX standard pthreads
• Specifies the API and the way other POSIX
  interfaces deal with threads

6
Threads vs Processes

• Advantages of threads
• It is less expensive to create a thread
• Inter-thread communication is much simpler and
  cheaper than inter-process communication
• Thread programming can be dangerous
• It is easy to make mistakes
• Some system functions may break with threads

7
Basic thread functions

• Thread creation
• int pthread_create(pthread_t thread, const
  pthread_attr_t attr, void (start_routine)(void
  ), void arg)
• Thread attributes (stack size, scheduling policy)
  can be modified
• Use NULL for default
• Returns 0 if successful, and an error number if
  it fails
• EAGAIN System lacks resources
• EINVAL Invalid attribute
• EPERM Caller does not have appropriate
  permission
• Pthread functions return error values
• They do not set errno
• New thread executes the function given by
  start_routine
• arg is the argument to this function
• The thread ID is stored in thread

8
Basic thread functions (continued)

• Wait for a thread to terminate
• int pthread_join(pthread_t thread, void
  value_ptr)
• Note that the thread ID, and not a pointer to it,
  is given as the first argument
• The return value of the function executed by the
  thread is stored in value_ptr
• example1.c and example2.c
• Link the thread program with lpthread