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All Rights Reserved. .\" .TH PTHREAD_MUTEXATTR_GETPROTOCOL 3C "Feb 5, 2008" .SH NAME pthread_mutexattr_getprotocol, pthread_mutexattr_setprotocol \- get or set protocol attribute of mutex attribute object .SH SYNOPSIS .LP .nf cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ] #include \fBint\fR \fBpthread_mutexattr_getprotocol\fR( \fBconst pthread_mutexattr_t *restrict\fR \fIattr\fR, \fBint *restrict\fR \fIprotocol\fR); .fi .LP .nf \fBint\fR \fBpthread_mutexattr_setprotocol\fR(\fBpthread_mutexattr_t *\fR\fIattr\fR, \fBint\fR \fIprotocol\fR); .fi .SH DESCRIPTION .sp .LP The \fBpthread_mutexattr_setprotocol()\fR and \fBpthread_mutexattr_getprotocol()\fR functions, respectively, set and get the protocol attribute of a mutex attribute object pointed to by \fIattr\fR, which was previously created by the \fBpthread_mutexattr_init()\fR function. .sp .LP The \fIprotocol\fR attribute defines the protocol to be followed in utilizing mutexes. The value of \fIprotocol\fR may be one of \fBPTHREAD_PRIO_NONE\fR, \fBPTHREAD_PRIO_INHERIT\fR, or \fBPTHREAD_PRIO_PROTECT\fR, which are defined by the header <\fBpthread.h\fR>. .sp .LP When a thread owns a mutex with the \fBPTHREAD_PRIO_NONE\fR protocol attribute, its priority and scheduling are not affected by its mutex ownership. .sp .LP When a thread is blocking higher priority threads because of owning one or more mutexes with the \fBPTHREAD_PRIO_INHERIT\fR 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. .sp .LP When a thread owns one or more mutexes initialized with the \fBPTHREAD_PRIO_PROTECT\fR 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. .sp .LP While a thread is holding a mutex that has been initialized with the \fBPRIO_INHERIT\fR or \fBPRIO_PROTECT\fR 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 \fBsched_setparam()\fR. Likewise, when a thread unlocks a mutex that has been initialized with the \fBPRIO_INHERIT\fR or \fBPRIO_PROTECT\fR 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. .sp .LP 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. .sp .LP If a thread makes a call to \fBpthread_mutex_lock()\fR for a mutex that was initialized with the protocol attribute \fBPTHREAD_PRIO_INHERIT\fR, 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. .sp .LP A thread that uses mutexes initialized with the \fBPTHREAD_PRIO_INHERIT\fR or \fBPTHREAD_PRIO_PROTECT\fR \fIprotocol\fR attribute values should have its scheduling policy equal to \fBSCHED_FIFO or SCHED_RR\fR (see \fBpthread_attr_getschedparam\fR(3C) and \fBpthread_getschedparam\fR(3C)). .sp .LP If a thread with scheduling policy equal to \fBSCHED_OTHER\fR uses a mutex initialized with the \fBPTHREAD_PRIO_INHERIT\fR or \fBPTHREAD_PRIO_PROTECT\fR \fIprotocol\fR attribute value, the effect on the thread's scheduling and priority is unspecified. .sp .LP The \fB_POSIX_THREAD_PRIO_INHERIT\fR and \fB_POSIX_THREAD_PRIO_PROTECT\fR 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. .sp .LP Threads created by realtime applications need to be such that their priorities can influence their access to system resources (\fBCPU\fR resources, at least), in competition with all threads running on the system. .SH RETURN VALUES .sp .LP Upon successful completion, the \fBpthread_mutexattr_getprotocol()\fR and \fBpthread_mutexattr_setprotocol()\fR functions return \fB0\fR. Otherwise, an error number is returned to indicate the error. .SH ERRORS .sp .LP The \fBpthread_mutexattr_getprotocol()\fR and \fBpthread_mutexattr_setprotocol()\fR functions will fail if: .sp .ne 2 .na \fB\fBEINVAL\fR\fR .ad .RS 11n The value specified by \fIattr\fR is \fINULL\fR. .RE .sp .ne 2 .na \fB\fBENOSYS\fR\fR .ad .RS 11n Neither of the options \fB_POSIX_THREAD_PRIO_PROTECT\fR and \fB_POSIX_THREAD_PRIO_INHERIT\fR is defined and the system does not support the function. .RE .sp .ne 2 .na \fB\fBENOTSUP\fR\fR .ad .RS 11n The value specified by \fIprotocol\fR is an unsupported value. .RE .sp .LP The \fBpthread_mutexattr_getprotocol()\fR and \fBpthread_mutexattr_setprotocol()\fR functions may fail if: .sp .ne 2 .na \fB\fBEINVAL\fR\fR .ad .RS 10n The value specified by \fIattr\fR or \fIprotocol\fR is invalid. .RE .sp .ne 2 .na \fB\fBEPERM\fR\fR .ad .RS 10n The caller does not have the privilege to perform the operation. .RE .SH ATTRIBUTES .sp .LP See \fBattributes\fR(5) for descriptions of the following attributes: .sp .sp .TS box; c | c l | l . ATTRIBUTE TYPE ATTRIBUTE VALUE _ Interface Stability Committed _ MT-Level MT-Safe _ Standard See \fBstandards\fR(5). .TE .SH SEE ALSO .sp .LP \fBpthread_attr_getschedparam\fR(3C), \fBpthread_mutex_init\fR(3C), \fBpthread_mutexattr_init\fR(3C), \fBsched_setparam\fR(3C), \fBsched_setscheduler\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5)