Thread

Basic Concepts
When to Use
Available APIs

Basic Concepts

Threads are the minimum running units that compete for system resources. They can use or wait to use CPUs and use system resources such as memory. They run independently from one another.

Threads in each process of the OpenHarmony kernel run and are scheduled independently. The scheduling of threads in a process is not affected by threads in other processes.

Threads in the OpenHarmony kernel use the preemptive scheduling mechanism, either round-robin (RR) scheduling or First In First Out (FIFO) scheduling.

Threads in the OpenHarmony kernel are assigned 32 priorities, ranging from 0 (highest) to 31 (lowest).

A high-priority thread in a process can preempt the resources of a low-priority thread in this process. The low-priority thread can be scheduled only after the high-priority thread is blocked or terminated.

A thread may have the following states:

Init: The thread is being created.
Ready: The thread is in the ready list and waits for being scheduled by the CPU.
Running: The thread is running.
Blocked: The thread is blocked and suspended. The Blocked states include pending (blocked due to lock, event, or semaphore issues), suspended (active pending), delay (blocked due to delays), and pendtime (blocked by waiting timeout of locks, events, or semaphores).
Exit: The thread stops running and waits for the parent thread to reclaim its control block resources.

Figure 1 State transition of a thread

Description of the thread state transition:

Init→Ready:

When a thread is created, the thread enters the Init state to start initialization after obtaining the control block. After the thread is initialized, the thread is inserted into the scheduling queue and therefore enters the Ready state.
Ready→Running:

When a thread switchover is triggered, the thread with the highest priority in the ready list is executed and enters the Running state. This thread will be deleted from the ready list.
Running→Blocked:

When a running thread is blocked (for example, is pended, delayed, or reading semaphores), the thread is deleted from the ready list, and its state changes from Running to Blocked. Then, a thread switchover is triggered to run the thread with the highest priority in the ready list.
Blocked→Ready:

After the blocked thread is restored (the thread is restored, the delay times out, the semaphore reading times out, or the semaphore is read), the thread is added to the ready list and changes from the Blocked state to the Ready state.
Ready→Blocked:

A thread may also be blocked (suspended) in the Ready state. The blocked thread will change from the Ready state to the Blocked state and is deleted from the ready list. In this case, the thread will not be scheduled until it is restored.
Running→Ready:

After a thread with a higher priority is created or restored, threads will be scheduled. The thread with the highest priority in the ready list will change to the Running state. The originally running thread will change from the Running state to the Ready state and be added to the ready list.
Running→Exit:

When a running thread is terminated, its state changes from Running to Exit. The thread without the PTHREAD_CREATE_DETACHED attribute will present the Exit state after being terminated.

When to Use

After a thread is created, it can be scheduled, suspended, restored, and delayed in user space. In addition, you can set and obtain the scheduling priority and scheduling policy of the thread.

Available APIs

The following table describes the APIs provided by the thread management module of the OpenHarmony kernel.

Table 1 APIs provided by the thread management module

Header File	Function	Description	Remarks
pthread.h	pthread_attr_destroy	Destroys a thread attribute object.	N/A
pthread.h	pthread_attr_getinheritsched	Obtains inherit scheduler attributes of a thread attribute object.	N/A
pthread.h	pthread_attr_getschedparam	Obtains scheduling parameter attributes of a thread attribute object.	N/A
pthread.h	pthread_attr_getschedpolicy	Obtains scheduling policy attributes of a thread attribute object.	OpenHarmony supports the SCHED_FIFO and SCHED_RR scheduling policies.
pthread.h	pthread_attr_getstacksize	Obtains the stack size of a thread attribute object.	N/A
pthread.h	pthread_attr_init	Initializes a thread attribute object.	N/A
pthread.h	pthread_attr_setdetachstate	Sets the detach state for a thread attribute object.	N/A
pthread.h	pthread_attr_setinheritsched	Sets inherit scheduler attributes for a thread attribute object.	N/A
pthread.h	pthread_attr_setschedparam	Sets scheduling parameter attributes for a thread attribute object.	A larger value represents a higher priority of the thread in the system. Note: The inheritsched field of the pthread_attr_t attribute must be set to PTHREAD_EXPLICIT_SCHED. Otherwise, the configured thread scheduling priority does not take effect. The default value is PTHREAD_INHERIT_SCHED.
pthread.h	pthread_attr_setschedpolicy	Sets scheduling policy attributes for a thread attribute object.	OpenHarmony supports the SCHED_FIFO and SCHED_RR scheduling policies.
pthread.h	pthread_attr_setstacksize	Sets the stack size for a thread attribute object.	N/A
pthread.h	pthread_getattr_np	Obtains the attributes of a created thread.	N/A
pthread.h	pthread_cancel	Sends a cancellation request to a thread.	N/A
pthread.h	pthread_testcancel	Requests delivery of any pending cancellation request.	N/A
pthread.h	pthread_setcanceltype	Sets the cancelability type for the calling thread.	N/A
pthread.h	pthread_setcancelstate	Sets the cancelability state for the calling thread.	N/A
pthread.h	pthread_create	Creates a thread.	N/A
pthread.h	pthread_detach	Detaches a thread.	N/A
pthread.h	pthread_equal	Compares whether two thread IDs are equal.	N/A
pthread.h	pthread_exit	Terminates the calling thread.	N/A
pthread.h	pthread_getschedparam	Obtains the scheduling policy and parameters of a thread.	OpenHarmony supports the SCHED_FIFO and SCHED_RR scheduling policies.
pthread.h	pthread_join	Waits for a thread to terminate.	N/A
pthread.h	pthread_self	Obtains the ID of the calling thread.	N/A
pthread.h	pthread_setschedprio	Sets a static scheduling priority for a thread.	N/A
pthread.h	pthread_kill	Sends a signal to a thread.	N/A
pthread.h	pthread_once	Enables the initialization function to be called only once.	N/A
pthread.h	pthread_atfork	Registers a fork handler to be called before and after fork().	N/A
pthread.h	pthread_cleanup_pop	Removes the routine at the top of the clean-up handler stack.	N/A
pthread.h	pthread_cleanup_push	Pushes the routine to the top of the clean-up handler stack.	N/A
pthread.h	pthread_barrier_destroy	Destroys a barrier (an advanced real-time thread).	N/A
pthread.h	pthread_barrier_init	Initializes a barrier (an advanced real-time thread).	N/A
pthread.h	pthread_barrier_wait	Synchronizes participating threads at a barrier.	N/A
pthread.h	pthread_barrierattr_destroy	Destroys a barrier attribute object.	N/A
pthread.h	pthread_barrierattr_init	Initializes a barrier attribute object.	N/A
pthread.h	pthread_mutex_destroy	Destroys a mutex.	N/A
pthread.h	pthread_mutex_init	Initializes a mutex.	N/A
pthread.h	pthread_mutex_lock	Locks a mutex.	N/A
pthread.h	pthread_mutex_trylock	Attempts to lock a mutex.	N/A
pthread.h	pthread_mutex_unlock	Unlocks a mutex.	N/A
pthread.h	pthread_mutexattr_destroy	Destroys a mutex attribute object.	N/A
pthread.h	pthread_mutexattr_gettype	Obtains the mutex type attribute.	N/A
pthread.h	pthread_mutexattr_init	Initializes a mutex attribute object.	N/A
pthread.h	pthread_mutexattr_settype	Sets the mutex type attribute.	N/A
pthread.h	pthread_mutex_timedlock	Blocks the calling thread to lock a mutex.	N/A
pthread.h	pthread_rwlock_destroy	Destroys a read-write lock.	N/A
pthread.h	pthread_rwlock_init	Initializes a read-write lock.	N/A
pthread.h	pthread_rwlock_rdlock	Applies a read lock to a read-write lock.	N/A
pthread.h	pthread_rwlock_timedrdlock	Blocks the calling thread to lock a read-write lock for reading.	N/A
pthread.h	pthread_rwlock_timedwrlock	Blocks the calling thread to lock a read-write lock for writing.	N/A
pthread.h	pthread_rwlock_tryrdlock	Attempts to apply a read lock to a read-write lock.	N/A
pthread.h	pthread_rwlock_trywrlock	Attempts to apply a write lock to a read-write lock.	N/A
pthread.h	pthread_rwlock_unlock	Unlocks a read-write lock.	N/A
pthread.h	pthread_rwlock_wrlock	Applies a write lock to a read-write lock.	N/A
pthread.h	pthread_rwlockattr_destroy	Destroys a read-write lock attribute object.	N/A
pthread.h	pthread_rwlockattr_init	Initializes a read-write lock attribute object.	N/A
pthread.h	pthread_cond_broadcast	Unblocks all threads that are currently blocked on the condition variable cond.	N/A
pthread.h	pthread_cond_destroy	Destroys a condition variable.	N/A
pthread.h	pthread_cond_init	Initializes a condition variable.	N/A
pthread.h	pthread_cond_signal	Unblocks a thread.	N/A
pthread.h	pthread_cond_timedwait	Blocks the calling thread to wait for the condition set by pthread_con_signal() for a period of time specified by ts.	N/A
pthread.h	pthread_cond_wait	Blocks the calling thread to wait for the condition set by pthread_con_signal().	N/A
semaphore.h	sem_destroy	Destroys a specified anonymous semaphore that is no longer used.	N/A
semaphore.h	sem_getvalue	Obtains the count value of a specified semaphore.	N/A
semaphore.h	sem_init	Creates and initializes an anonymous semaphore.	N/A
semaphore.h	sem_post	Increments the semaphore count by 1.	N/A
semaphore.h	sem_timedwait	Obtains the semaphore, with a timeout period specified.	N/A
semaphore.h	sem_trywait	Attempts to obtain the semaphore.	N/A
semaphore.h	sem_wait	Obtains the semaphore.	N/A

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