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Questions and Answers
What is one main feature of POSIX threads compared to traditional Unix processes?
What is one main feature of POSIX threads compared to traditional Unix processes?
Which of the following is NOT a synchronization mechanism provided by Pthreads?
Which of the following is NOT a synchronization mechanism provided by Pthreads?
When compiling code that uses Pthreads on a modern Linux system, which option must be included?
When compiling code that uses Pthreads on a modern Linux system, which option must be included?
Which of the following is true about the Pthreads API?
Which of the following is true about the Pthreads API?
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Which combination of options might be required for Pthreads on systems other than Linux?
Which combination of options might be required for Pthreads on systems other than Linux?
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What is one way a POSIX thread can terminate?
What is one way a POSIX thread can terminate?
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What happens when you pass NULL as an argument to pthread_create()?
What happens when you pass NULL as an argument to pthread_create()?
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Which of the following is NOT an attribute of a thread defined by pthread_attr_t?
Which of the following is NOT an attribute of a thread defined by pthread_attr_t?
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What does joining a thread refer to in POSIX threads?
What does joining a thread refer to in POSIX threads?
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Which of the following methods cannot be used to terminate a POSIX thread?
Which of the following methods cannot be used to terminate a POSIX thread?
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What happens when a thread waits on a condition variable?
What happens when a thread waits on a condition variable?
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How are condition variables typically created in Pthreads?
How are condition variables typically created in Pthreads?
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What is a key characteristic of the Linux implementation of Pthreads regarding condition variables?
What is a key characteristic of the Linux implementation of Pthreads regarding condition variables?
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What must be considered when using pthread_cond_wait?
What must be considered when using pthread_cond_wait?
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What does it mean when threads can spuriously wake while waiting on a condition variable?
What does it mean when threads can spuriously wake while waiting on a condition variable?
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What is the return value of pthread_mutex_trylock when the mutex is already locked?
What is the return value of pthread_mutex_trylock when the mutex is already locked?
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Which of the following statements about destroying mutexes is correct?
Which of the following statements about destroying mutexes is correct?
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What happens when you attempt to destroy a locked mutex?
What happens when you attempt to destroy a locked mutex?
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What must a thread hold to wait on a condition variable?
What must a thread hold to wait on a condition variable?
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How can mutexes be initialized to allow recursive locking?
How can mutexes be initialized to allow recursive locking?
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What is a direct consequence of attempting to use a non-recursive mutex in the following code example?
What is a direct consequence of attempting to use a non-recursive mutex in the following code example?
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Which function is used to unlock a mutex?
Which function is used to unlock a mutex?
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What is the purpose of the pthread_mutex_trylock function?
What is the purpose of the pthread_mutex_trylock function?
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What is the purpose of the function pthread_cond_signal?
What is the purpose of the function pthread_cond_signal?
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What happens when pthread_cond_signal is called while no threads are waiting?
What happens when pthread_cond_signal is called while no threads are waiting?
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What is required before calling pthread_cond_signal?
What is required before calling pthread_cond_signal?
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What is the primary purpose of the pthread_create() function?
What is the primary purpose of the pthread_create() function?
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What could potentially happen if condition variables are destroyed while threads are waiting?
What could potentially happen if condition variables are destroyed while threads are waiting?
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Which function is used to destroy a condition variable?
Which function is used to destroy a condition variable?
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Which of the following is a requirement for the start function given to pthread_create()?
Which of the following is a requirement for the start function given to pthread_create()?
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What does pthread_create() do after creating a new execution context?
What does pthread_create() do after creating a new execution context?
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In the provided example, what does the function signal_done() do?
In the provided example, what does the function signal_done() do?
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What can be inferred about thread attributes when using pthread_create()?
What can be inferred about thread attributes when using pthread_create()?
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What must be initialized to use condition variables in pthreads?
What must be initialized to use condition variables in pthreads?
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How are Pthread objects typically declared?
How are Pthread objects typically declared?
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What is incorrect regarding the use of condition variables in pthreads?
What is incorrect regarding the use of condition variables in pthreads?
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When defining a thread function, what is the expected return type?
When defining a thread function, what is the expected return type?
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Which of the following statements is true regarding thread functions and their parameters?
Which of the following statements is true regarding thread functions and their parameters?
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What happens to the original thread after pthread_create() is called?
What happens to the original thread after pthread_create() is called?
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Study Notes
POSIX Threads and Synchronization
- POSIX threads (pthreads) add threading to Unix
- Early Unix used a process model for concurrency, while POSIX threads share more resources like processes
- Every POSIX thread begins with a function
POSIX Synchronization
- Pthreads offer various synchronization mechanisms. A rich set of options is available
- Mutexes
- Semaphores
- Condition variables
- Thread joining
- Memory barriers (not discussed in detail)
Compilation with Pthreads
- Pthreads may require specific compiler/linker options during compilation and linking
- On modern Linux, use the
-pthreadoption - Other systems may require alternative options:
- Different compiler or linker options (e.g.,
-pthreads) - Preprocessor defines (e.g.,
-DPTHREAD,-D_REENTRANT) - Linking with a library (e.g.,
-lpthread)
- Different compiler or linker options (e.g.,
- Consult documentation for specific requirements
Thread Creation
- Threads are created using the
pthread_create()function - Syntax:
int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_function) (void *), void *arg) - The created thread:
- Starts execution at the specified start function.
- Has the provided data passed as an argument
Pthread Object Declarations
- Threads (and other Pthread objects) are declared as values, but are often used as pointers.
- Example:
pthread_t thread; pthread_create(&thread, NULL, thread_function, NULL); - This eliminates the need for dynamic allocation
Thread Functions
- Thread start functions have the following signature:
void *(*start_function) (void* ); - This is a function that:
- Accepts a single
void *argument - Returns a
void *value
- Accepts a single
- Example:
void *thread_main(void *arg) { return NULL; }
Thread Semantics
-
pthread_create()creates a new execution context (including a stack). - It establishes a concurrent flow based on the provided stack and context.
- It then invokes the given function and passes the provided argument.
- The separation of function and arguments enables one function to handle diverse tasks.
- The new thread operates independently of the original thread.
Thread Attributes
-
pthread_create()accepts a thread attribute object (pthread_attr_t) - Using
NULLimplies using default attributes - Attributes include:
- Processor affinity
- Desired scheduler, including configurations
- Detach state of the new thread
- Stack location and size
- Attributes will not be used in the current semester
Thread Termination
- POSIX threads can terminate in various ways:
- Application exit
- Returning from the thread's start function
- Cancellation by another thread using
pthread_cancel()
Thread Joining
- Joining a thread is a synchronous operation.
-
pthread_join()blocks the caller until the target thread exits. - It retrieves the thread's exit status.
- Syntax:
int pthread_join(pthread_t thread, void **retval);
- Syntax:
Examples
-
counter.c: Mutexes to protect critical sections -
deadlock.c: Deadlock scenarios -
odds_evens.c: Condition variables -
printer.c: Thread scheduling and joining
POSIX Mutexes
-
POSIX mutexes are of type
pthread_mutex_t. -
Features include:
- Optional recursive lock detection
- A try lock operation that returns immediately whether or not the mutex can be locked
-
It's an error to unlock a POSIX mutex on a thread different from the one that locked it
Mutex Initialization
-
POSIX mutexes have static and dynamic initializers:
- Static:
pthread_mutex_t fastmutex = PTHREAD_MUTEX_INITIALIZER; - Dynamic:
int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutexattr_t *mutexattr);
- Static:
-
Older POSIX versions only allow static initializers for compile-time use.
-
The dynamic initializer accepts attributes to configure the mutex;
NULLimplies default behavior
Mutex Operations
-
The functions
pthread_mutex_lock,pthread_mutex_trylock, andpthread_mutex_unlockoperate as expected -
pthread_mutex_trylock()immediately returns whether a lock could be successfully obtained -
Returns
EBUSYif already locked -
Returns
0if unlocked, locking it and returning
Destroying Mutexes
- Destroy mutexes when no longer needed
- On Linux, destroying a mutex is a no-op (has no effect).
- Destroying a locked mutex is an error
- Destroying a mutex being waited upon is an error
Default Mutex Behaviors
- Default mutexes may not support recursive locking
- Example code demonstrating potential deadlock on Linux
Condition Variables
-
Condition variables work with mutexes.
-
A thread needs a mutex to wait on a condition variable
-
Waiting on a condition variable atomically unlocks the mutex and puts the thread to sleep until signaled
-
A thread can signal one or all threads sleeping on a condition variable.
Creating a Condition Variable
- Condition variables are created like mutexes:
-
pthread_cond_t cond = PTHREAD_COND_INITIALIZER; -
int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *cond_attr);
-
- Linux Pthread implementation does not support condition variable attributes
Waiting on Condition Variables
- A thread can wait using
pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex); - Associating mutexes with conditional variables protects the conditional state
- Spurious wakeups can occur in threads
Signaling Conditional Variables
-
Condition variables can signal:
- One waiting thread
- All waiting threads
-
pthread_cond_signal()signals a single waiting thread. -
pthread_cond_broadcast()signals all waiting threads.
Destroying Condition Variables
- When complete, destroy condition variables.
- On Linux, destroying a condition variable has no side effects.
- Destroying a condition variable with waiting threads is an error.
POSIX Semaphores
- POSIX semaphores can work between threads or processes
- Provide counting semaphore semantics
- Replace the System V semaphores
-
sem_init(),sem_wait(),sem_trywait(),sem_post()are the main POSIX functions
POSIX Semaphore Creation
-
sem_init()creates a semaphore withpshared(allowing inter-process use), initial value. - No static initialization for POSIX semaphores
-
psharedbeing true implies semaphore can be used between processes - It needs to be in shared memory to work properly
POSIX Semaphore Manipulation
-
sem_wait(): Decrements semaphore; blocks if it is zero. -
sem_trywait(): Attempts to decrement semaphore without blocking. -
sem_post(): Increments the semaphore. - The wait operation mirrors Dijkstra's P() operation, while
postmaps to V()
Summary
- POSIX threads provide a thread abstraction on Unix.
- POSIX provides various synchronization primitives (mutexes, semaphores, condition variables, thread joining)
-
CS:APPdetails semaphores
References
- (List of references, if needed)
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Description
This quiz covers the fundamentals of POSIX threads, including their creation, synchronization mechanisms, and compilation requirements. Explore mutexes, semaphores, and the intricacies of linking with pthreads in different systems. Master the essentials to effectively implement threading in Unix environments.
