11. Mutex Manager — RTEMS POSIX API Guide 5.4-rc2 (21st August 2026) documentation

# 11. Mutex Manager

## 11.1. Introduction

The mutex manager implements the functionality required of the mutex manager as defined by POSIX 1003.1b-1996. This standard requires that a compliant operating system provide the facilties to ensure that threads can operate with mutual exclusion from one another and defines the API that must be provided.

The services provided by the mutex manager are:

- pthread_mutexattr_init - Initialize a Mutex Attribute Set

- pthread_mutexattr_destroy - Destroy a Mutex Attribute Set

- pthread_mutexattr_setprotocol - Set the Blocking Protocol

- pthread_mutexattr_getprotocol - Get the Blocking Protocol

- pthread_mutexattr_setprioceiling - Set the Priority Ceiling

- pthread_mutexattr_getprioceiling - Get the Priority Ceiling

- pthread_mutexattr_setpshared - Set the Visibility

- pthread_mutexattr_getpshared - Get the Visibility

- pthread_mutex_init - Initialize a Mutex

- pthread_mutex_destroy - Destroy a Mutex

- pthread_mutex_lock - Lock a Mutex

- pthread_mutex_trylock - Poll to Lock a Mutex

- pthread_mutex_timedlock - Lock a Mutex with Timeout

- pthread_mutex_unlock - Unlock a Mutex

- pthread_mutex_setprioceiling - Dynamically Set the Priority Ceiling

- pthread_mutex_getprioceiling - Dynamically Get the Priority Ceiling

## 11.2. Background

### 11.2.1. Mutex Attributes

Mutex attributes are utilized only at mutex creation time. A mutex attribute structure may be initialized and passed as an argument to the `mutex_init` routine. Note that the priority ceiling of a mutex may be set at run-time.

blocking protcol | is the XXX
priority ceiling | is the XXX
pshared | is the XXX

### 11.2.2. PTHREAD_MUTEX_INITIALIZER

This is a special value that a variable of type `pthread_mutex_t` may be statically initialized to as shown below:

    pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;

This indicates that `my_mutex` will be automatically initialized by an implicit call to `pthread_mutex_init` the first time the mutex is used.

Note that the mutex will be initialized with default attributes.

## 11.3. Operations

There is currently no text in this section.

## 11.4. Services

This section details the mutex manager’s services. A subsection is dedicated to each of this manager’s services and describes the calling sequence, related constants, usage, and status codes.

### 11.4.1. pthread_mutexattr_init - Initialize a Mutex Attribute Set

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutexattr_init(
        pthread_mutexattr_t *attr
    );

STATUS CODES:

EINVAL

  The attribute pointer argument is invalid.

DESCRIPTION:

The `pthread_mutexattr_init` routine initializes the mutex attributes object specified by `attr` with the default value for all of the individual attributes.

NOTES:

XXX insert list of default attributes here.

### 11.4.2. pthread_mutexattr_destroy - Destroy a Mutex Attribute Set

CALLING SEQUENCE:

    #include <pthread.h>
        int pthread_mutexattr_destroy(
        pthread_mutexattr_t *attr
    );

STATUS CODES:

`EINVAL` | The attribute pointer argument is invalid.
`EINVAL` | The attribute set is not initialized.

DESCRIPTION:

The `pthread_mutex_attr_destroy` routine is used to destroy a mutex attributes object. The behavior of using an attributes object after it is destroyed is implementation dependent.

NOTES:

NONE

### 11.4.3. pthread_mutexattr_setprotocol - Set the Blocking Protocol

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutexattr_setprotocol(
        pthread_mutexattr_t *attr,
        int                  protocol
    );

STATUS CODES:

`EINVAL` | The attribute pointer argument is invalid.
`EINVAL` | The attribute set is not initialized.
`EINVAL` | The protocol argument is invalid.

DESCRIPTION:

The `pthread_mutexattr_setprotocol` routine is used to set value of the `protocol` attribute. This attribute controls the order in which threads waiting on this mutex will receive it.

The `protocol` can be one of the following:

`PTHREAD_PRIO_NONE` | in which case blocking order is FIFO.
`PTHREAD_PRIO_INHERIT` | in which case blocking order is priority with the priority inheritance protocol in effect.
`PTHREAD_PRIO_PROTECT` | in which case blocking order is priority with the priority ceiling protocol in effect.

NOTES:

There is currently no way to get simple priority blocking ordering with POSIX mutexes even though this could easily by supported by RTEMS.

### 11.4.4. pthread_mutexattr_getprotocol - Get the Blocking Protocol

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutexattr_getprotocol(
        pthread_mutexattr_t *attr,
        int                 *protocol
    );

STATUS CODES:

`EINVAL` | The attribute pointer argument is invalid.
`EINVAL` | The attribute set is not initialized.
`EINVAL` | The protocol pointer argument is invalid.

DESCRIPTION:

The `pthread_mutexattr_getprotocol` routine is used to obtain the value of the `protocol` attribute. This attribute controls the order in which threads waiting on this mutex will receive it.

NOTES:

NONE

### 11.4.5. pthread_mutexattr_setprioceiling - Set the Priority Ceiling

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutexattr_setprioceiling(
        pthread_mutexattr_t *attr,
        int                  prioceiling
    );

STATUS CODES:

`EINVAL` | The attribute pointer argument is invalid.
`EINVAL` | The attribute set is not initialized.
`EINVAL` | The prioceiling argument is invalid.

DESCRIPTION:

The `pthread_mutexattr_setprioceiling` routine is used to set value of the `prioceiling` attribute. This attribute specifies the priority that is the ceiling for threads obtaining this mutex. Any task obtaining this mutex may not be of greater priority that the ceiling. If it is of lower priority, then its priority will be elevated to `prioceiling`.

NOTES:

NONE

### 11.4.6. pthread_mutexattr_getprioceiling - Get the Priority Ceiling

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutexattr_getprioceiling(
        const pthread_mutexattr_t *attr,
        int                       *prioceiling
    );

STATUS CODES:

`EINVAL` | The attribute pointer argument is invalid.
`EINVAL` | The attribute set is not initialized.
`EINVAL` | The prioceiling pointer argument is invalid.

DESCRIPTION:

The `pthread_mutexattr_getprioceiling` routine is used to obtain the value of the `prioceiling` attribute. This attribute specifies the priority ceiling for this mutex.

NOTES:

NONE

### 11.4.7. pthread_mutexattr_setpshared - Set the Visibility

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutexattr_setpshared(
        pthread_mutexattr_t *attr,
        int                  pshared
    );

STATUS CODES:

`EINVAL` | The attribute pointer argument is invalid.
`EINVAL` | The attribute set is not initialized.
`EINVAL` | The pshared argument is invalid.

DESCRIPTION:

NOTES:

### 11.4.8. pthread_mutexattr_getpshared - Get the Visibility

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutexattr_getpshared(
        const pthread_mutexattr_t *attr,
        int                       *pshared
    );

STATUS CODES:

`EINVAL` | The attribute pointer argument is invalid.
`EINVAL` | The attribute set is not initialized.
`EINVAL` | The pshared pointer argument is invalid.

DESCRIPTION:

NOTES:

### 11.4.9. pthread_mutex_init - Initialize a Mutex

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutex_init(
        pthread_mutex_t           *mutex,
        const pthread_mutexattr_t *attr
    );

STATUS CODES:

`EINVAL` | The attribute set is not initialized.
`EINVAL` | The specified protocol is invalid.
`EAGAIN` | The system lacked the necessary resources to initialize another mutex.
`ENOMEM` | Insufficient memory exists to initialize the mutex.
`EBUSY` | Attempted to reinialize the object reference by mutex, a previously initialized, but not yet destroyed.

DESCRIPTION:

NOTES:

### 11.4.10. pthread_mutex_destroy - Destroy a Mutex

CALLING SEQUENCE:

    #include <pthread.h>
        int pthread_mutex_destroy(
        pthread_mutex_t *mutex
    );

STATUS CODES:

`EINVAL` | The specified mutex is invalid.
`EBUSY` | Attempted to destroy the object reference by mutex, while it is locked or referenced by another thread.

DESCRIPTION:

NOTES:

### 11.4.11. pthread_mutex_lock - Lock a Mutex

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutex_lock(
        pthread_mutex_t *mutex
    );

STATUS CODES:

`EINVAL` | The specified mutex is invalid.
`EINVAL` | The mutex has the protocol attribute of `PTHREAD_PRIO_PROTECT` and the priority of the calling thread is higher than the current priority ceiling.
`EDEADLK` | The current thread already owns the mutex.

DESCRIPTION:

NOTES:

### 11.4.12. pthread_mutex_trylock - Poll to Lock a Mutex

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutex_trylock(
        pthread_mutex_t *mutex
    );

STATUS CODES:

`EINVAL` | The specified mutex is invalid.
`EINVAL` | The mutex has the protocol attribute of `PTHREAD_PRIO_PROTECT` and the priority of the calling thread is higher than the current priority ceiling.
`EBUSY` | The mutex is already locked.

DESCRIPTION:

NOTES:

### 11.4.13. pthread_mutex_timedlock - Lock a Mutex with Timeout

CALLING SEQUENCE:

    #include <pthread.h>
    #include <time.h>
    int pthread_mutex_timedlock(
        pthread_mutex_t       *mutex,
        const struct timespec *timeout
    );

STATUS CODES:

`EINVAL` | The specified mutex is invalid.
`EINVAL` | The nanoseconds field of timeout is invalid.
`EINVAL` | The mutex has the protocol attribute of `PTHREAD_PRIO_PROTECT` and the priority of the calling thread is higher than the current priority ceiling.
`EDEADLK` | The current thread already owns the mutex.
`ETIMEDOUT` | The calling thread was unable to obtain the mutex within the specified timeout period.

DESCRIPTION:

NOTES:

### 11.4.14. pthread_mutex_unlock - Unlock a Mutex

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutex_unlock(
        pthread_mutex_t *mutex
    );

STATUS CODES:

`EINVAL` | The specified mutex is invalid.

DESCRIPTION:

NOTES:

### 11.4.15. pthread_mutex_setprioceiling - Dynamically Set the Priority Ceiling

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutex_setprioceiling(
        pthread_mutex_t *mutex,
        int              prioceiling,
        int             *oldceiling
    );

STATUS CODES:

`EINVAL` | The oldceiling pointer parameter is invalid.
`EINVAL` | The prioceiling parameter is an invalid priority.
`EINVAL` | The specified mutex is invalid.

DESCRIPTION:

NOTES:

### 11.4.16. pthread_mutex_getprioceiling - Get the Current Priority Ceiling

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_mutex_getprioceiling(
        pthread_mutex_t *mutex,
        int             *prioceiling
    );

STATUS CODES:

`EINVAL` | The prioceiling pointer parameter is invalid.
`EINVAL` | The specified mutex is invalid.

DESCRIPTION:

NOTES: