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This article focuses on "what are the security risks of multithreading in iOS development?" interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what are the hidden dangers of multithreading in iOS development?"

I. solution

Solution: use thread synchronization technology (synchronization, that is, coordinated pace, in a predetermined order)

The common thread synchronization technology is: locking

1 、 OSSpinLock

OSSpinLock is called a "spin lock". The thread waiting for the lock will be in a busy-wait state, occupying CPU resources all the time.

At present, it is no longer secure, and priority inversion problems may occur.

If the thread waiting for the lock has a higher priority, it will always occupy CPU resources, and the low-priority thread will not be able to release the lock.

Need to import header file # import

2 、 os_unfair_lock

Os_unfair_lock is used to replace unsafe OSSpinLock and is only supported from iOS10.

From the perspective of the underlying call, the thread waiting for the os_unfair_lock lock will be dormant, not busy, etc.

Need to import header file # import

3 、 pthread_mutex

Mutex is called a "mutex", and the thread waiting for the lock is dormant.

Need to import header file # import

Pthread_mutex- normal lock

Pthread_mutex- recursive lock

Pthread_mutex- condition

4 、 NSLock

NSLock is the encapsulation of mutex common lock.

5 、 NSRecursiveLock

NSRecursiveLock is also the encapsulation of mutex recursive lock. API is basically the same as NSLock.

6 、 NSCondition

NSCondition is an encapsulation of mutex and cond

7 、 NSConditionLock

NSConditionLock is a further encapsulation of NSCondition, and specific condition values can be set.

8 、 dispatch_semaphore

Semaphore is called "semaphore"

The initial value of the semaphore, which can be used to control the maximum number of concurrent access by threads

The initial value of the semaphore is 1, which means that only one thread is allowed to access resources at the same time, ensuring thread synchronization.

9. Dispatch_queue (DISPATCH_QUEUE_SERIAL)

Thread synchronization can also be achieved by directly using the serial queue of GCD.

10, @ synchronized

@ synchronized is the encapsulation of mutex recursive locks

Source code view: objc-sync.mm file in objc4 (official address of Apple source code)

@ synchronized (obj) generates recursive locks corresponding to obj, and then adds and unlocks them.

II. Performance comparison of iOS thread synchronization schemes

Principles:

The performance of ordinary lock is better than that of recursive lock

The more advanced the language, the more logic it encapsulates, and the worse the performance (for all languages)

Actual test

Sort performance from high to low

Os_unfair_lock / / disadvantage: only supported by iOS10

OSSpinLock / / disadvantages: priority reversal may occur. Apple is no longer safe and does not recommend it.

Dispatch_semaphore / / recommended

Pthread_mutex / / advantages: cross-platform mutexes (common locks) are recommended

Dispatch_queue (DISPATCH_QUEUE_SERIAL) / / c

NSLock / / oc

NSCondition / / oc

Pthread_mutex (recursive) / / Recursive lock

NSRecursiveLock / / oc

NSConditionLock / / oc

@ synchronized / / Recursive lock oc

III. Selection of spin lock and mutex lock

Spin lock: waiting state is busy, etc.

Mutex: waiting state is dormant

1. When is it more cost-effective to use a spin lock?

It is expected that the thread will have a short wait for the lock.

Locked code (critical section) is often called, but competition rarely occurs

CPU resources are not tight.

Multicore processor

2. When is it cost-effective to use mutexes?

It is expected that the thread will wait a long time for the lock.

Single core processor

The critical region has IO operation.

The code of the critical area is complex or the amount of circulation is large.

The competition in the critical area is very fierce.

Read-write lock

Scene:

Only one thread can write at a time

Allow multiple threads to read at the same time

At the same time, both write and read operations are not allowed.

The above scenario is a typical "multi-read and single-write", which is often used for reading and writing data such as files. The implementation schemes in iOS are as follows:

1. Read-write lock: pthread_rwlock

The thread waiting for the lock will go to sleep

2 、 dispatch_barrier_async

The concurrent queue passed in by this function must be created by itself through dispatch_queue_cretate

If you pass in a serial or a global concurrent queue, then this function is equivalent to the effect of the dispatch_async function

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