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The pthread_mutexattr_setprotocol() and pthread_mutexattr_getprotocol() functions, respectively, set and get the protocol attribute of a mutex
attribute object pointed to by attr, which was previously created by the pthread_mutexattr_init() function.
The protocol attribute defines the protocol to be followed in utilizing mutexes. The value of protocol may be one of PTHREAD_PRIO_NONE, PTHREAD_PRIO_INHERIT,
or PTHREAD_PRIO_PROTECT, which are defined by the header <pthread.h>.
When a thread owns a mutex with the PTHREAD_PRIO_NONE protocol attribute, its priority and scheduling are not affected by its mutex ownership.
When a thread is blocking higher priority threads because of owning one or more mutexes with the PTHREAD_PRIO_INHERIT protocol attribute, it executes at the higher of its priority or the priority of the highest priority thread waiting on any of the mutexes
owned by this thread and initialized with this protocol.
When a thread owns one or more mutexes initialized with the PTHREAD_PRIO_PROTECT protocol, it executes at the higher of its priority or the highest of the priority ceilings of all the mutexes owned by this thread and initialized with this attribute, regardless
of whether other threads are blocked on any of these mutexes.
While a thread is holding a mutex that has been initialized with the PRIO_INHERIT or PRIO_PROTECT protocol attributes, it will not be subject to being moved to the tail of the scheduling queue at its priority in
the event that its original priority is changed, such as by a call to sched_setparam(). Likewise, when a thread unlocks a mutex that has been initialized with the PRIO_INHERIT or PRIO_PROTECT protocol
attributes, it will not be subject to being moved to the tail of the scheduling queue at its priority in the event that its original priority is changed.
If a thread simultaneously owns several mutexes initialized with different protocols, it will execute at the highest of the priorities that it would have obtained by each of these protocols.
When a thread makes a call to pthread_mutex_lock(), if the symbol _POSIX_THREAD_PRIO_INHERIT is defined and the mutex was initialized with the protocol attribute having the value PTHREAD_PRIO_INHERIT,
when the calling thread is blocked because the mutex is owned by another thread, that owner thread will inherit the priority level of the calling thread as long as it continues to own the mutex. The implementation updates its execution priority to the maximum of its assigned priority and all its inherited
priorities. Furthermore, if this owner thread becomes blocked on another mutex, the same priority inheritance effect will be propagated to the other owner thread, in a recursive manner.
If the symbol _POSIX_THREAD_PRIO_INHERIT is defined, when a mutex initialized with the protocol attribute having the value PTHREAD_PRIO_INHERIT dies, the behavior depends on the robustness attribute of the mutex. See pthread_mutexattr_getrobust_np(3THR).
A thread that uses mutexes initialized with the PTHREAD_PRIO_INHERIT or PTHREAD_PRIO_PROTECT protocol attribute values should have its contentionscope attribute equal to PTHREAD_SCOPE_SYSTEM (see pthread_attr_getscope(3THR)) and its scheduling policy equal to SCHED_FIFO or SCHED_RR (see pthread_attr_getschedparam(3THR) and pthread_getschedparam(3THR)).
If a thread with contentionscope attribute equal to PTHREAD_SCOPE_PROCESS and/or its scheduling policy equal to SCHED_OTHER uses a mutex initialized with the PTHREAD_PRIO_INHERIT or PTHREAD_PRIO_PROTECT protocol attribute value, the effect on the thread's scheduling and priority is unspecified.
The _POSIX_THREAD_PRIO_INHERIT and _POSIX_THREAD_PRIO_PROTECT options are designed to provide features to solve priority inversion due to mutexes. A priority inheritance or priority ceiling mutex is designed to minimize the dispatch latency of a high priority
thread when a low priority thread is holding a mutex required by the high priority thread. This is a specific need for the realtime application domain.
Threads created by realtime applications need to be such that their priorities can influence their access to system resources (CPU resources, at least), in competition with all threads running on the system.
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