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GIOS Lecture Notes - Part 2 Lesson 3 - PThreads Case Study

PThreads References

• An Introduction to Programming with Threads
• PThreads Programming Resource
• PThread Coding Examples

PThread Creation

• pthread_attr_t
  • specified in pthread_create
  • defines featuress of nrew thread
    • stack size
    • joinable
    • priority
    • inheritance
    • scheduling policy
    • system/process scope
  • has default behavior with NULL in pthread_create
• detaching pthreads
  • default: joinable thread
  • parent thread should not exit until children threads have joined back
  • if parent exits early you get zombies
  • in pthreads you can detach children such that they cannot be joined, and can continue on after parent is killed
  • int pthread_detach() - takes thread as argument
  • for detached threads, must be killed off using void pthread_exit() example: There is a typo @4:58 with the code example. The pthread_create() function should be passed the following parameters: pthread_create(&tid, &attr, foo, NULL)

Compiling Pthreads

• need #include <pthread.h> for library
• need to link at compile time. which flag depends on platform.
  • gcc -o main main.c -lpthread
  • gcc -o main main.c -pthread
• check return values of common functions! always good, but extra needed here

Example 1

• Create threads with default attributes (first NULL in pthread create)
• Pass no arguments to threads (second NULL in pthread create)
• Default configuration of pthread is joinable, so can pass to pthread_join() without issue

Example 2

• Threads each execute a function that takes an argument, *pArg, so we pass &i as the argument to each pthread_create call

Example 3

• passing global variables to pthread_create arg is dangerous. May not pass what you expect, as the variable may be updated globally before thread can execute whatever they use it for
• This is called a data race, or race condition. This is when a thread tries to read a value while another thread modifies it
• Fix:
  • create static copy of argument that isn’t modified so it will always be passed and consumed correctly by appropriate thread

PThread Mutexes

• Mutexes provide the mechanism to solve mutual exclusion problems among concurrent threads
• Mutex safety tips
  • each piece shared data should always be accessed through a single mutex
  • mutex scope must be visible to all
  • globally order locks/unlocks
    • for all threads, lock mutexes in order
  • always unlock a mutex
    • always unlock the correct mutex

PThread Condition Variables

• Condition variable safety tips
  • do not forget to notify waiting threads
    • predicate change => signal/broadcast correct condition variable
  • when in doubt which to use, use broadcast instead of signal
    • safer, but will cause performance loss
  • You do not need a mutex to signal/broadcast
    • may be better to move signal/broadcast out of mutex block sometimes. See example from P2L2

Produce and Consumer Example in PThreads

• All source code provided in reference links at top of notes