Note: Multithreaded Programming

####Creating POSIX Threads

pthread_create

• returns a positive value indicating failure

####Structure pointer approach

P46

####Passing arguments to threads

1. copy all arguments to the thread’s stack: isn’t supported in either POSIX or Win-32
2. pass a pointer to local storage: should make sure this storage doesn’t go out of scope until the thread is finished with it
3. pass a pointer to static or global storage: works only if one thread at a time is using the storage
4. pass a pointer to dynamically allocated storage: works only if we can free the storage exactly when the thread is finished with it

####Threads terminate

• 2 ways:
  1. return from its first procedure(a value of type void*)
  2. call pthread_exit, supplying a void* argument
• difference:
  1. pthread_exit: terminates just the calling thread
  2. exit: terminates the entire process, including other threads in the process

####Deadlock conditions

1. a thread can hold an item while waiting for another
2. a thread cannot be forced toyield the items it is holding
3. each container has a finite capacity
4. a circular wait

####Semaphore

• binary semaphores
• counting semaphres

####Conditional variables

• P67: why not if --writers == 0
• P70: Now when the waiting threads wake up, they will find the guard false….

####async-signal

• fork
• _exit
• open
• close
• read
• write
• sem_post

####Question

• POSIX cancellation

####Bear in mind

1. it’s unsafe to make more than 1 call to pthread_join on any particular thread
2. for each call to pthread_create there should be exactly one call to either pthread_join or pthread_detach.
3. pthread_cleanup_push must match exactly with pthread_cleanup_pop

####Notes during db assignment

• It’s a common(if standard) approach to mask the signal you want to handle in the main thread, then create a handler thread, specifically unblocking the signal and waiting for the signal, to handle that signal.
• If you did not pthread_detach the thread, then you’ll have to pthread_join the thread, or the resources allocated by the thread will not be collected.
• Signal is not that hard at all, just need to figure out the current state.
• If you cherish your life, stay away from pthread_detach!!!