Say you do
MyLock *lock = [[MyLock new] autorelease];
#synchronized(lock) {
NSLog(#"Hello World");
//some very long process
}
In the main thread. Does that mean till //some very long process is done, the main thread is locked? If some other thread call
//Update on the main thread
dispatch_sync(dispatch_get_main_queue(), ^{
//Do some updates
});
That some updates will never be called? Am I correct?
If the code in the first code snippet never finishes, the second one won't be called, regardless of the #synchronized statement. The thread is blocked by the code that you're executing. The #synchronized statement is to synchronize data access between multiple threads and to be useful, it requires that all participating threads actually use the statement. It will not "magically" lock access to the data structure, unless all participating threads "agree" on it.
You don't use #synchronized to ensure that only one method executes on a given (single) thread, that is the case anyhow.
To give you a concrete example of its use, let's say you have an NSMutableArray that you want to protect from getting modified from different threads at the same time (which could lead to data corruption). In that case, you could always access it in a #synchronized block with the same lock token.
Example:
//Thread 1:
#synchronized (myArray) {
[myArray addObject:#"foo"];
}
//Thread 2:
#synchronized (myArray) {
[myArray removeObjectAtIndex:0];
}
This will ensure that the blocks of code that are enclosed by the #synchronized will never execute simultaneously. When one thread enters the block, other threads wait until it finishes, but only if they also use the same #synchronized statement. If you forget to use it on one thread, it doesn't help at all if you used it on the other.
The short answer is no. I think you dont understand the concept of locking. You should read more about syncchronization for example here:
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Multithreading/ThreadSafety/ThreadSafety.html
You have to synchronize using the same locking object (same instance!) in every case when you access the code, which you are trying to protect. You can store the locking object as property of a class.
In your case:
self.lock = [[MyLock new] autorelease]; //in init method initialize retain or strong lock property
...
#synchronized(self.lock) {
NSLog(#"Hello World");
//some very long process
}
//Update on the main thread
dispatch_sync(dispatch_get_main_queue(), ^{
#synchronized(self.lock) {
NSLog(#"Hello World");
//some very long process
}
});
If you can use as the locking object, the object which your are trying to protect.
Related
I've been trying to fix this crash for almost a week. The application crashes without any exception or stack-trace. The application does not crash in any way while running through instruments in zombie mode.
I have a method that gets called on a different thread.
The solution that fixed the crash was replacing
[self.mutableArray removeAllObjects];
with
dispatch_async(dispatch_get_main_queue(), ^{
[self.searchResult removeAllObjects];
});
I thought it might be a timing issue, so I tried to synchronize it, but it still crashed:
#synchronized(self)
{
[self.searchResult removeAllObjects];
}
Here is the code
- (void)populateItems
{
// Cancel if already exists
[self.searchThread cancel];
self.searchThread = [[NSThread alloc] initWithTarget:self
selector:#selector(populateItemsinBackground)
object:nil];
[self.searchThread start];
}
- (void)populateItemsinBackground
{
#autoreleasepool
{
if ([[NSThread currentThread] isCancelled])
[NSThread exit];
[self.mutableArray removeAllObjects];
// Populate data here into mutable array
for (loop here)
{
if ([[NSThread currentThread] isCancelled])
[NSThread exit];
// Add items to mutableArray
}
}
}
Is this problem with NSMutableArray not being thread-safe?
No.
It is not thread safe and if you need to modify your mutable array from another thread you should use NSLock to ensure everything goes as planned:
NSLock *arrayLock = [[NSLock alloc] init];
[...]
[arrayLock lock]; // NSMutableArray isn't thread-safe
[myMutableArray addObject:#"something"];
[myMutableArray removeObjectAtIndex:5];
[arrayLock unlock];
As others already said, NSMutableArray is not thread safe. In case anyone want to achieve more than removeAllObject in a thread-safe environment, I will give another solution using GCD besides the one using lock. What you have to do is to synchronize the read/update(replace/remove) actions.
First get the global concurrent queue:
dispatch_queue_t concurrent_queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
For read:
- (id)objectAtIndex:(NSUInteger)index {
__block id obj;
dispatch_sync(self.concurrent_queue, ^{
obj = [self.searchResult objectAtIndex:index];
});
return obj;
}
For insert:
- (void)insertObject:(id)obj atIndex:(NSUInteger)index {
dispatch_barrier_async(self.concurrent_queue, ^{
[self.searchResult insertObject:obj atIndex:index];
});
}
From Apple Doc about dispatch_barrier_async:
When the barrier block reaches the front of a private concurrent queue, it is not executed immediately. Instead, the queue waits until its currently executing blocks finish executing. At that point, the barrier block executes by itself. Any blocks submitted after the barrier block are not executed until the barrier block completes.
Similar for remove:
- (void)removeObjectAtIndex:(NSUInteger)index {
dispatch_barrier_async(self.concurrent_queue, ^{
[self.searchResult removeObjectAtIndex:index];
});
}
EDIT: Actually I found another simpler way today to synchronize access to a resource by using a serial queue provided by GCD.
From Apple Doc Concurrency Programming Guide > Dispatch Queues:
Serial queues are useful when you want your tasks to execute in a specific order. A serial queue executes only one task at a time and always pulls tasks from the head of the queue. You might use a serial queue instead of a lock to protect a shared resource or mutable data structure. Unlike a lock, a serial queue ensures that tasks are executed in a predictable order. And as long as you submit your tasks to a serial queue asynchronously, the queue can never deadlock.
Create your serial queue:
dispatch_queue_t myQueue = dispatch_queue_create("com.example.MyQueue", NULL);
Dispatch tasks async to the serial queue:
dispatch_async(myQueue, ^{
obj = [self.searchResult objectAtIndex:index];
});
dispatch_async(myQueue, ^{
[self.searchResult removeObjectAtIndex:index];
});
Hope it helps!
As well as NSLock can also use #synchronized(condition-object) you just have to make sure every access of the array is wrapped in a #synchronized with the same object acting as the condition-object , if you only want to modify the contents of the same array instance then you can use the array itself as the condition-object, other wise you will have to use something else you know will not go away, the parent object, i.e self, is a good choice because it will always be the same one for the same array.
atomic in #property attributes will only make setting the array thread safe not modifying the contents, i.e. self.mutableArray = ... is thread safe but [self.mutableArray removeObject:] is not.
__weak typeof(self)weakSelf = self;
#synchronized (weakSelf.mutableArray) {
[weakSelf.mutableArray removeAllObjects];
}
Since serial queues were mentioned: With a mutable array, just asking "is it thread safe" isn't enough. For example, making sure that removeAllObjects doesn't crash is all good and fine, but if another thread tries to process the array at the same time, it will either process the array before or after all elements are removed, and you really have to think what the behaviour should be.
Creating one class + object that is responsible for this array, creating a serial queue for it, and doing all operations through the class on that serial queue is the easiest way to get things right without making your brain hurt through synchronisation problems.
All the NSMutablexxx classes are not thread-safe. Operations including get,insert,remove,add and replace should be used with NSLock.This is a list of thread-safe and thread-unsafe classes given by apple: Thread Safety Summary
Almost NSMutable classes object is not thread safe.
I need an advice from you guys on the design of my app here, basically I would like to know if it will work as I expect ? As the multi-threading is quite tricky thing I would like to hear from you.
Basically my task is very simple -I've SomeBigSingletonClass - big singleton class, which has two methods someMethodOne and someMethodTwo
These methods should be invoked periodically (timer based) and in separate threads.
But there should be only one instance of each thread at the moment, e.g. there should be only one running someMethodOne at any time and the same for someMethodTwo.
What I've tried
GCD - Did implementation with GCD but it lacks very important feature, it does not provide means to check if there is any running task at the moment, i.e. I was not able to check if there is only one running instance of let say someMethodOne method.
NSThread - It does provide good functionality but I'm pretty sure that new high level technologies like NSOperation and GCD will make it more simple to maintain my code. So I decided to give-up with NSThread.
My Solution with NSOperation
How I plan to implement the two thread invokation
#implementation SomeBigSingletonClass
- (id)init
{
...
// queue is an iVar
queue = [[NSOperationQueue alloc] init];
// As I'll have maximum two running threads
[queue setMaxConcurrentOperationCount:2];
...
}
+ (SomeBigSingletonClass *)sharedInstance
{
static SomeBigSingletonClass *sharedInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedInstance = [[SomeBigSingletonClass alloc] init];
});
return sharedInstance;
}
- (void)someMethodOne
{
SomeMethodOneOperation *one = [[SomeMethodOneOperation alloc] init];
[queue addOperation:one];
}
- (void)someMethodTwo
{
SomeMethodTwoOperation *two = [[SomeMethodOneOperation alloc] init];
[queue addOperation:two];
}
#end
And finally my NSOperation inherited class will look like this
#implementation SomeMethodOneOperation
- (id)init
{
if (![super init]) return nil;
return self;
}
- (void)main {
// Check if the operation is not running
if (![self isExecuting]) {
[[SomeBigSingletonClass sharedInstance] doMethodOneStuff];
}
}
#end
And the same for SomeMethodTwoOperation operation class.
If you are using NSOperation, you can achieve what you want be creating your own NSOperationQueue and setting numberOfConcurrentOperations to 1.
You could have also maybe used an #synchronized scope with your class as your lock object.
EDIT: clarification---
What I am proposing:
Queue A (1 concurrent operation--used to perform SomeMethodOneOperation SomeMethodTwoOperation once at a time)
Queue B (n concurrent operations--used for general background operation performing)
EDIT 2: Updated code illustrating approach to run maximum operation one and operation two, with max one each of operation one and operation two executing at any given time.
-(void)enqueueMethodOne
{
static NSOperationQueue * methodOneQueue = nil ;
static dispatch_once_t onceToken ;
dispatch_once(&onceToken, ^{
queue = [ [ NSOperationQueue alloc ] init ] ;
queue = 1 ;
});
[ queue addOperation:[ NSBlockOperation blockOperationWithBlock:^{
... do method one ...
} ] ];
}
-(void)enqueueMethodTwo
{
static NSOperationQueue * queue = nil ;
static dispatch_once_t onceToken ;
dispatch_once(&onceToken, ^{
queue = [ [ NSOperationQueue alloc ] init ] ;
queue = 1 ;
});
[ queue addOperation:[ NSBlockOperation blockOperationWithBlock:^{
... do method two ...
} ] ];
}
EDIT 3:
per our discussion:
I pointed out that isExecuting is a member variable and refers only to the state of the operation being queried, not if any instance of that class is executing
therefore Deimus' solution won't work to keep multiple instances of operation one running simultaneously for example
Sorry, I'm late to the party. If your methods are called back based on timers, and you want them to execute concurrently with respect to one another, but synchronous with respect to themselves, might I suggest using GCD timers.
Basically, you have two timers, one which executes methodOne, and the other executes methodTwo. Since you pass blocks to the GCD timers, you don't even have to use methods, especially if you want to make sure other code does not call those methods when they are not supposed to run.
If you schedule the timers onto a concurrent queue, then both timers could possibly be running at the same time on different threads. However, the timer itself will only run when it is scheduled. Here is an example I just hacked up... you can easily use it with a singleton...
First, a helper function to create a timer that takes a block which will be called when the timer fires. The block passes the object, so it can be referenced by the block without creating a retain cycle. If we use self as the parameter name, the code in the block can look just like other code...
static dispatch_source_t setupTimer(Foo *fooIn, NSTimeInterval timeout, void (^block)(Foo * self)) {
// Create a timer that uses the default concurrent queue.
// Thus, we can create multiple timers that can run concurrently.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
uint64_t timeoutNanoSeconds = timeout * NSEC_PER_SEC;
dispatch_source_set_timer(timer,
dispatch_time(DISPATCH_TIME_NOW, timeoutNanoSeconds),
timeoutNanoSeconds,
0);
// Prevent reference cycle
__weak Foo *weakFoo = fooIn;
dispatch_source_set_event_handler(timer, ^{
// It is possible that the timer is running in another thread while Foo is being
// destroyed, so make sure it is still there.
Foo *strongFoo = weakFoo;
if (strongFoo) block(strongFoo);
});
return timer;
}
Now, the basic class implementation. If you don't want to expose methodOne and methodTwo, there is no reason to even create them, especially if they are simple, as you can just put that code directly in the block.
#implementation Foo {
dispatch_source_t timer1_;
dispatch_source_t timer2_;
}
- (void)methodOne {
NSLog(#"methodOne");
}
- (void)methodTwo {
NSLog(#"methodTwo");
}
- (id)initWithTimeout1:(NSTimeInterval)timeout1 timeout2:(NSTimeInterval)timeout2 {
if (self = [super init]) {
timer1_ = setupTimer(self, timeout1, ^(Foo *self) {
// Do "methodOne" work in this block... or call it.
[self methodOne];
});
timer2_ = setupTimer(self, timeout2, ^(Foo *self) {
// Do "methodOne" work in this block... or call it.
[self methodTwo];
});
dispatch_resume(timer1_);
dispatch_resume(timer2_);
}
return self;
}
- (void)dealloc {
dispatch_source_cancel(timer2_);
dispatch_release(timer2_);
dispatch_source_cancel(timer1_);
dispatch_release(timer1_);
}
#end
EDIT
In response to the comments (with more detail to hopefully explain why the block will not be executed concurrently, and why missed timers are coalesced into one).
You do not need to check for it being run multiple times. Straight from the documentation...
Dispatch sources are not reentrant. Any events received while the
dispatch source is suspended or while the event handler block is
currently executing are coalesced and delivered after the dispatch
source is resumed or the event handler block has returned.
That means when a GCD dispatch_source timer block is dispatched, it will not be dispatched again until the one that is already running completes. You do nothing, and the library itself will make sure the block is not executed multiple times concurrently.
If that block takes longer than the timer interval, then the "next" timer call will wait until the one that is running completes. Also, all the events that would have been delivered are coalesced into one single event.
You can call
unsigned numEventsFired = dispatch_source_get_data(timer);
from within your handler to get the number of events that have fired since the last time the handler was executed (e.g., if your handler ran through 4 timer firings, this would be 4 - but you would still get all this firings in this one event -- you would not receive separate events for them).
For example, let's say your interval timer is 1 second, and your timer happens to take 5 seconds to run. That timer will not fire again until the current block is done. Furthermore, all those timers will be coalesced into one, so you will get one call into your block, not 5.
Now, having said all that, I should caution you about what I think may be a bug. Now, I rarely lay bugs at the feet of library code, but this one is repeatable, and seems to go against the documentation. So, if it's not a bug, it's an undocumented feature. However, it is easy to get around.
When using timers, I have noticed that coalesced timers will most certainly be coalesced. That means, if your timer handler is running, and 5 timers fired while it was running, the block will be called immediately, representing those missed 5 events. However, as soon as that one is done, the block will be executed again, just once, no matter how many timer events were missed before.
It's easy to identify these, though, because dispatch_source_get_data(timer) will return 0, which means that no timer events have fired since the last time the block was called.
Thus, I have grown accustomed to adding this code as the first line of my timer handlers...
if (dispatch_source_get_data(timer) == 0) return;
We had a bug, and it destroys the looks of our UI, some of the UI elements overlap, or has been added to the subview twice. the bug is hardly reproduced so its hard to fix it. Now I thought of the reason, and probably the method that changes the UI are being called twice at the same time. And I was correct, I tried to create the bug programatically.
We have a bug which is caused by a method being accessed by different threads at the same time. To emulate this problem, and better understand it. see codes posted below.
When I do this, updatePresence Method call, my program works perfectly
ViewController.m
-(void)loadConversationScreen{
[conversationController updatePresence];
}
But when I do this, something goes wrong with my program
ViewController.m
-(void)loadConversationScreen{
[conversationController performSelectorInBackground:#selector(updatePresence) withObject:nil];
[conversationController updatePresence];
}
This is because the method is being accessed at the same time and and the instance of my UIView is being accessed/changed also at the same time.
How do I PROPERLY stop 2 threads from using a method at the same time?
How do I properly handle it in IOS(if there is no proper way, what are the work arounds), are there built in locks or somekind?
My app should support ios 4.0 and up
Advance thanks to all for your help.
The best thread lock for you is #sycnhronized(object) {}. This means only one thread can enter the code at a time. The object passed in is used to perform the lock; only one thread can enter a block protected by a particular object's synchronized at a time. The others will wait. This can be any Objective-C object, even a NSString.
Typically, you'd use whatever object you're trying to protect from multiple threads. You probably want #synchronized(self) {}:
-(void)updateVariables {
#synchronized(self) {
_foo = 1;
_bar = 2;
}
}
#sycnhronized is re-entrant in the sense that the same thread can call #sycnhronized as deeply as it wants, for instance:
- (void)a {
#synchronized(self) {
// entered "immediately" if called from b, where the #synchronized has
// already been called
_foo = _foo + 1;
}
}
- (void)b {
#synchronized(self) {
[self a];
}
}
For posterity and because I already typed it before reading your clarification, if you really cared only about updating the UI, you'd want to force your call over to the main thread instead like this:
- (void)someTask {
dispatch_async( dispatch_get_main_queue(), ^{
[self updateUI];
});
}
- (void)updateUI {
NSAssert( [NSThread isMainThread], #"called from non-main thread" );
// do UI updates here
}
As warrenm said you shouldn't update your UIView from a different thread than the Main thread (UI thread). Still, you asked if there is any workaround for what's going on. To be honest, you should try to, instead of blocking the access of the second thread to your method, understand why the methods is called twice. This is more a logical problem than anything else and you should try to fix that, instead of trying a shortcut.
Im using XMPPFramework and in it's code there's a method like this:
- (NSDictionary *)occupants
{
if (dispatch_get_current_queue() == moduleQueue)
{
return occupants;
}
else
{
__block NSDictionary *result;
dispatch_sync(moduleQueue, ^{//IT BLOCKS HERE, WITHOUT MESSAGE
result = [occupants copy];
});
return [result autorelease];
}
}
[EDIT]
It blocks inconsistently, not always, since the app is not doing anything I pause it and I see the thread has stopped there, and it never continues to execute.
What is wrong? Any ideas?
Thanks
The behavior you explain perfectly matches with the one that appears when you try to send perform an operation on main thread via GCD while being on the main thread. So you should check if moduleQueue is the main queue, then this is it. Try checking if it is the main queue if it is, skip the dispatch_sync block.
Blocks sometimes need to retain variables to ensure they are available when they execute. If you use a local variable inside a block, you should initialise it to zero where you declare it outside the block.
I have a set of methods that all synchronize to the class object (can't use self, because multiple instances of this object could be used in multiple threads). Some of those methods call other methods in the class that also synchronize on the class object. Somehow this works and does not cause the deadlock I would expect it to.
I would assume that testA would be blocked from running because testB already has a lock on the class object, but this apparently isn't the case.
Is it something special that #synchronized is doing or is this a feature of the underlying mutex locks?
Example code that works:
- (NSUInteger)testA
{
#synchronized(self.class)
{
NSLog(#"Doing something in A");
}
return 1;
}
- (void)testB
{
#synchronized(self.class)
{
NSLog(#"Doing something in B");
NSLog(#"A returned: %i", [self testA]);
}
}
When calling testB the output is:
Doing something in B
Doing something in A
A returned: 1
Mauricio's comment is correct. From TOCPL:
The Objective-C synchronization feature supports recursive and reentrant code. A thread can use a single semaphore several times in a recursive manner; other threads are blocked from using it until the thread releases all the locks obtained with it; that is, every #synchronized() block is exited normally or through an exception.