| # Boost.Threads ## Mutex Concept

Introduction
Locking Strategies

Recursive
Checked
Unchecked
Unspecified

Scheduling Policies

FIFO
Priority Driven
Unspecified

Concept Requirements

Mutex Concept
TryMutex Concept
TimedMutex Concept

Models

## Introduction

A mutex (short for mutual-exclusion) object is used to serializes access to a resource shared between multiple threads. The Mutex concept, with TryMutex and TimedMutex refinements, formalize the requirements. A model that implements Mutex and its refinements has two states: locked and unlocked. Before using a shared resource, a thread locks a Boost.Threads mutex object (an object whose type is a model of Mutex or one of it's refinements), insuring thread-safe access to the shared resource. When use of the shared resource is complete, the thread unlocks the mutex object, allowing another thread to acquire the lock and use the shared resource.

Traditional C thread APIs, like POSIX threads or the Windows thread APIs, expose functions to lock and unlock a mutex object. This is dangerous since it's easy to forget to unlock a locked mutex. When the flow of control is complex, with multiple return points, the likelihood of forgetting to unlock a mutex object would become even greater. When exceptions are thrown, it becomes nearly impossible to ensure that the mutex object is unlocked properly when using these traditional API's. The result is deadlock.

Many C++ threading libraries use a pattern known as Scoped Locking [Schmidt 00] to free the programmer from the need to explicitly lock and unlock mutex objects. With this pattern, a Lock concept is employed where the lock object's constructor locks the associated mutex object and the destructor automatically does the unlocking. The Boost.Threads library takes this pattern to the extreme in that Lock concepts are the only way to lock and unlock a mutex object: lock and unlock functions are not exposed by any Boost.Threads mutex objects. This helps to ensure safe usage patterns, especially when code throws exceptions.

## Locking Strategies

Every mutex object follows one of several locking strategies. These strategies define the semantics for the locking operation when the calling thread already owns a lock on the mutex object.

### Recursive

With a recursive locking strategy when a thread attempts to acquire a lock on the mutex object for which it already owns a lock, the operation is successful. Note the distinction between a thread, which may have multiple locks outstanding on a recursive mutex object, and a lock object, which even for a recursive mutex object cannot have any of its lock functions called multiple times without first calling unlock.

Internally a lock count is maintained and the owning thread must unlock the mutex model the same number of times that it's locked it before the mutex object's state returns to unlocked. Since mutex objects in Boost.Threads expose locking functionality only through lock concepts, a thread will always unlock a mutex object the same number of times that it locked it. This helps to eliminate a whole set of errors typically found in traditional C style thread APIs.

Classes recursive_mutex, recursive_try_mutex and recursive_timed_mutex use this locking strategy.

### Checked

With a checked locking strategy when a thread attempts to acquire a lock on the mutex object for which the thread already owns a lock, the operation will fail with some sort of error indication. Further, attempts by a thread to unlock a mutex object that was not locked by the thread will also return some sort of error indication. In Boost.Threads, an exception of type lock_error would be thrown in these cases.

Boost.Threads does not currently provide any mutex objects that use this strategy.

### Unchecked

With an unchecked locking strategy when a thread attempts to acquire a lock on a mutex object for which the thread already owns a lock the operation will deadlock. In general this locking strategy is less safe than a checked or recursive strategy, but it's also a faster strategy and so is employed by many libraries.

Boost.Threads does not currently provide any mutex objects that use this strategy.

### Unspecified

With an unspecified locking strategy, when a thread attempts to acquire a lock on a mutex object for which the thread already owns a lock the operation results in undefined behavior.

In general a mutex object with an unspecified locking strategy is unsafe, and it requires programmer discipline to use the mutex object properly. However, this strategy allows an implementation to be as fast as possible with no restrictions on its implementation. This is especially true for portable implementations that wrap the native threading support of a platform. For this reason, the classes mutex, try_mutex and timed_mutex use this locking strategy despite the lack of safety.

## Scheduling Policies

Every mutex object follows one of several scheduling policies. These policies define the semantics when the mutex object is unlocked and there is more than one thread waiting to acquire a lock. In other words, the policy defines which waiting thread shall acquire the lock.

### FIFO

With a FIFO scheduling policy, threads waiting for the lock will acquire it in a first come first serve order (or First In First Out). This can help prevent a high priority thread from starving lower priority threads that are also waiting on the mutex object's lock.

### Priority Driven

With a Priority Driven scheduling policy, the thread with the highest priority acquires the lock. Note that this means that low-priority threads may never acquire the lock if the mutex object has high contention and there is always at least one high-priority thread waiting. This is known as thread starvation. When multiple threads of the same priority are waiting on the mutex object's lock one of the other scheduling priorities will determine which thread shall acquire the lock.

### Unspecified

The mutex object does not specify a scheduling policy. In order to ensure portability, all Boost.Threads mutex models use an unspecified scheduling policy.

## Concept Requirements

### Mutex Concept

A Mutex object has two states: locked and unlocked. Mutex object state can only be determined by an object meeting the ScopedLock requirements and constructed for the Mutex object.

A Mutex is NonCopyable.

For a Mutex type M and an object m of that type, the following expressions must be well-formed and have the indicated effects.

Expression | Effects
`M m;` | Constructs a mutex object m. Post-condition: m is unlocked.
`(&m)->~M();` | Precondition: m is unlocked. Destroys a mutex object m.
`M::scoped_lock` | A model of ScopedLock.

### TryMutex Concept

A TryMutex is a refinement of Mutex. For a TryMutex type M and an object m of that type, the following expressions must be well-formed and have the indicated effects.

Expression | Effects
`M::scoped_try_lock` | A model of ScopedTryLock.

### TimedMutex Concept

A TimedMutex is a refinement of TryMutex. For a TimedMutex type M and an object m of that type, the following expressions must be well-formed and have the indicated effects.

Expression | Effects
`M::scoped_timed_lock` | A model of ScopedTimedLock.

## Models

Boost.Threads currently supplies six models of Mutex.

Concept | Refines | Models
Mutex |  | mutex recursive_mutex
TryMutex | Mutex | try_mutex recursive_try_mutex
TimedMutex | TryMutex | timed_mutex recursive_timed_mutex

Revised 05 November, 2001

© Copyright William E. Kempf 2001-2002. All Rights Reserved.

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