Tribblix: manual page: pthread_mutexattr

PTHREAD_MUTEXATTR_GETPROTOCOL(3C) Standard C Library Functions

NAME

pthread_mutexattr_getprotocol, pthread_mutexattr_setprotocol - get or
set protocol attribute of mutex attribute object

SYNOPSIS

cc -mt [ flag... ] file... -lpthread [ library... ]
#include <pthread.h>

int pthread_mutexattr_getprotocol(
const pthread_mutexattr_t *restrict attr,
int *restrict protocol);

int pthread_mutexattr_setprotocol(pthread_mutexattr_t *attr,
int protocol);

DESCRIPTION

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.

If a thread makes a call to pthread_mutex_lock() for a mutex that was
initialized with the protocol attribute PTHREAD_PRIO_INHERIT, and if
the calling thread becomes blocked because the mutex is owned by
another thread, then the owner thread inherits the priority level of
the calling thread for 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.

A thread that uses mutexes initialized with the PTHREAD_PRIO_INHERIT
or PTHREAD_PRIO_PROTECT protocol attribute values should have its
scheduling policy equal to SCHED_FIFO or SCHED_RR (see
pthread_attr_getschedparam(3C) and pthread_getschedparam(3C)).

If a thread with 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.

RETURN VALUES

Upon successful completion, the pthread_mutexattr_getprotocol() and
pthread_mutexattr_setprotocol() functions return 0. Otherwise, an
error number is returned to indicate the error.

ERRORS

The pthread_mutexattr_getprotocol() and
pthread_mutexattr_setprotocol() functions will fail if:

EINVAL
The value specified by attr is NULL.

ENOSYS
Neither of the options _POSIX_THREAD_PRIO_PROTECT and
_POSIX_THREAD_PRIO_INHERIT is defined and the system does
not support the function.

ENOTSUP
The value specified by protocol is an unsupported value.

The pthread_mutexattr_getprotocol() and
pthread_mutexattr_setprotocol() functions may fail if:

EINVAL
The value specified by attr or protocol is invalid.

EPERM
The caller does not have the privilege to perform the
operation.