Alex's Notes

Week 4 - PThreads

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Week 4 - PThreads

Additional reading

Background

Portable operating system interface (POSIX)

Portable operating system interface (POSIX)

Portable Operating System Interface (POSIX) is a set of standardised APIs and interfaces that define how software should interact with an operating system. POSIX ensures compatibility and portability of applications across different UNIX-like operating systems by standardising system calls, command-line interfaces, and utility functions. It is widely used to maintain cross-platform compatibility in software development.

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POSIX threads (PThreads)

POSIX threads (PThreads)

This is the POSIX interface for threads, mutex, and conditional variables.

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Thread interface

The core data structure in the PThreads library is pthread_t which represents a thread. This can be created through:

int pthread_create(
	pthread_t *thread,
	const pthread_attr_t *attr,
	void * (*start_routine)(void *),
	void *arg
)

Void is C's any type

The first argument thread will be filled with the thread once it is created. (We will come back to pthread_attr_t later - if you pass NULL you will get default behaviour.) The third argument start_routine is a function to call this arguments arg. It will return the status code of the operation.

This can be joined to the main thread using:

int pthread_join(
	pthread_t thread,
	void **status
)

where thread is the thread you want to join and status will be the return object. It will return the status code of the operation.

Pthread attributes

The pthread_attr_t type controls the type of thread created with the properties:

  • Stack size,
  • Scheduling policy,
  • Priority,
  • System/process thread,
  • inheritance (if it should inherit attributes from the parent thread), and
  • joinable. The object can be created and destroyed through the following interface.
int pthread_attr_init(pthread_attr_t *attr)
int pthread_attr_destroy(pthread_attr_t *attr)

You can then set/get attribute values through the following function.

pthread_attr_{set/get}{attribute}

For example pthread_attr_setjoinable.

Detachable threads

Threads that are created from a parent need to be joined by that parent in order for them to be cleaned up by the OS. Though if the parent thread exits before this happens it can create zombie threads that can not be cleaned up.

If you need to create a child thread that will outlive its parent then you can make it detachable. This means it is no longer needs to be joined to exit - it can instead use the following function.

void pthread_exit()

Threads can be made detachable using the joinable property or by calling detach from within the executing function.

pthread_attr_setjoinable(attr, PTHREAD_CREATE_DETACHED)
int pthread_detach()

Example

#include <stdio.h>
#include <pthread.h>
 
void *foo (void *arg) {		/* thread main */
	printf("Foobar!\n");
	pthread_exit(NULL);
}
 
int main (void) {
 
	int i;
	pthread_t tid;
	
	pthread_attr_t attr;
	pthread_attr_init(&attr); // Required!!!
	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
	pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
	pthread_create(&tid, &attr, foo, NULL);
 
	return 0;
}

When using Pthreads:

  • Include the pthread library #include <pthread.h>
  • Compile code using the pthreads flag: either -lpthread or -pthread.
  • Check the return value of pthread function calls in case of errors.

Mutex

The PThreads library allows you to create and use mutexes.

pthread_mutex_t aMutex;
int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);

For example to implement safe lock and unlock using this would be:

list<int> my_list;
pthread_mutex_t list_mutex;
 
void safe_insert(int to_add){
	pthread_lock(list_mutex);
	my_list.insert(to_add);
	pthread_unlock(list_mutex);
}

You must initialise and cleanup mutexes.

int pthread_mutex_init(
	pthread_mutex_t *mutex,
	pthread_mutexattr_t *attr,
);
int pthread_mutex_destroy(pthread_mutex_t *mutex);

Mutex have 3 attributes:

  • Mutex Type: can be changed to allow for deadlock detection.
  • Mutex Protocol: Determines the thread priority when holding the mutex.
  • Process-Shared Attribute: Determines if the mutex applies just to this process or can be shared.
  • Robustness Attribute: Determines if the mutex should be recoverable from a crash or not.

Another nice feature is the trylock which allows a thread to check if a mutex is free without getting blocked by it.

int pthread_mutex_trylock(pthread *mutex);

This will return 0 if it is free or EBUSY if not.

Lastly a couple reminders about working with mutexes:

  • Shared data must be accessed through a single mutex.
  • A mutex must be visible to all threads. i.e. declare them as global variables.
  • Globally order locks to prevent deadlock.
  • Always unlock the correct mutex.

Conditional variables

PThreads supports the API we saw in the last lecture.

pthread_cond_t aCond;
int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);
int pthread_cond_signal(pthread_cond_t *cond);
int pthread_cond_broadcast(pthread_cond_t *cond);

Conditional variables need to be created and destroyed.

int pthread_cond_init(
	pthread_cond_t *cond,
	pthread_condattr_t *attr,
);
int pthread_cond_destroy(pthread_cond_t *cond);

The attributes you can provide are:

  • Process-Shared (PTHREAD_PROCESS_SHARED, PTHREAD_PROCESS_PRIVATE): Determines whether the condition variable is shared between processes or only within a single process.
  • Clock (CLOCK_REALTIME, CLOCK_MONOTONIC): Specifies which clock to use for timed wait operations on the condition variable.

Some notes on conditional variables:

  • Make sure any predicate change is followed by the required signal/broadcast.
  • If you do not know whether to use signal or broadcast, use broadcast (though check this later as you will lose performance).
  • To avoid spurious wakeups think about if you need to hold the mutex when doing your signal or broadcast.

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