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• Grab a lock device (the piece that says “LOCK LACES®” on it). Press down on the top of the lock to open the passages, and thread the ends of the laces through the passages with your other hand. Once the laces have been threaded through, press down on the top of the lock again and slide the lock to the tongue of the shoe.
• The US Department of Justice is pleased to share its Environmental Justice Strategy and its Environmental Justice Guidance updated in 2014. The Department initially prepared these documents to implement the Department’s commitments in the area of environmental justice following the issuance of Executive Order 12898 on February 11, 1994.

A lock is a device used for raising and lowering boats, ships and other watercraft between stretches of water of different levels on river and canal waterways.The distinguishing feature of a lock is a fixed chamber in which the water level can be varied; whereas in a caisson lock, a boat lift, or on a canal inclined plane, it is the chamber itself (usually then called a caisson) that rises.

The correctness of a concurrent program should not depend on accidents of timing.

Since race conditions caused by concurrent manipulation of shared mutable data are disastrous bugs — hard to discover, hard to reproduce, hard to debug — we need a way for concurrent modules that share memory to synchronize with each other.

Locks are one synchronization technique.A lock is an abstraction that allows at most one thread to own it at a time.Holding a lock is how one thread tells other threads: “I’m changing this thing, don’t touch it right now.”

Locks have two operations:

• acquire allows a thread to take ownership of a lock.If a thread tries to acquire a lock currently owned by another thread, it blocks until the other thread releases the lock.At that point, it will contend with any other threads that are trying to acquire the lock.At most one thread can own the lock at a time.

• release relinquishes ownership of the lock, allowing another thread to take ownership of it.

Using a lock also tells the compiler and processor that you’re using shared memory concurrently, so that registers and caches will be flushed out to shared storage.This avoids the problem of reordering, ensuring that the owner of a lock is always looking at up-to-date data.

Bank account example

Our first example of shared memory concurrency was a bank with cash machines.The diagram from that example is on the right.

The bank has several cash machines, all of which can read and write the same account objects in memory.

Of course, without any coordination between concurrent reads and writes to the account balances, things went horribly wrong.

To solve this problem with locks, we can add a lock that protects each bank account.Now, before they can access or update an account balance, cash machines must first acquire the lock on that account.

In the diagram to the right, both A and B are trying to access account 1.Suppose B acquires the lock first.Then A must wait to read and write the balance until B finishes and releases the lock.This ensures that A and B are synchronized, but another cash machine C is able to run independently on a different account (because that account is protected by a different lock).

The lock statement acquires the mutual-exclusion lock for a given object, executes a statement block, and then releases the lock. While a lock is held, the thread that holds the lock can again acquire and release the lock. Any other thread is blocked from acquiring the lock and waits until the lock is released.

The lock statement is of the form

Balance Lock Code Jazz

where x is an expression of a reference type. It's precisely equivalent to

Since the code uses a try...finally block, the lock is released even if an exception is thrown within the body of a lock statement.

You can't use the await operator in the body of a lock statement.

Guidelines

When you synchronize thread access to a shared resource, lock on a dedicated object instance (for example, private readonly object balanceLock = new object();) or another instance that is unlikely to be used as a lock object by unrelated parts of the code. Avoid using the same lock object instance for different shared resources, as it might result in deadlock or lock contention. In particular, avoid using the following as lock objects:

• this, as it might be used by the callers as a lock.
• Type instances, as those might be obtained by the typeof operator or reflection.
• string instances, including string literals, as those might be interned.

Hold a lock for as short time as possible to reduce lock contention.

White Balance Lock Canon Eos 70d

Example

The following example defines an Account class that synchronizes access to its private balance field by locking on a dedicated balanceLock instance. Using the same instance for locking ensures that the balance field cannot be updated simultaneously by two threads attempting to call the Debit or Credit methods simultaneously.

C# language specification

For more information, see The lock statement section of the C# language specification.

See also