When a thread has acquired the lock and execute the following code, Could the thread will unlock the lock it has acquired just with the return statement? some code like this.
static pthread_mutex_t mutex;
int foo()
{
pthread_mutex_lock(mutex);
.........
execute some code here and some errors happen
return -1;// but without pthread_mutex_unlock
pthread_mutex_unlock(mutext)
return 0;
}
Some errors happens before pthread_mutex_unlock statement and the thread returns to the callee. Will the thread give back the mutext lock for other threads without executing pthread_mutex_unlock?
No, the lock is not automatically released. This is why, in C++ code, it is common to use Resource Aquisition is Initialization (RAII), which takes advantage of construction/destruction to ensure that each call to the lock function has a corresponding call to unlock. If you are writing pure C code, though, you will need to make sure that you unlock the mutex, even in error situations, before returning.
Note that you can make your coding a little bit easier by doing the following:
static inline int some_function_critical_section_unsynchronized(void) {
// ...
}
int some_function(void) {
int status = 0;
pthread_mutex_lock(mutex);
status = some_function_critical_section_unsynchronized();
pthread_mutex_unlock(mutex);
return status;
}
In other words, if you can separate the logic into smaller functions, you may be able to tease out the locking code from your logic. Of course, sometimes this is not possible (like when coding in this fashion would make the critical section too large, and for performance, the less readable form is required).
If you can use C++, I would strongly suggest using boost::thread and boost::scoped_lock to ensure that the acquired mutex is automatically freed when its usage has gone out of scope.
No, it will not automatically unlock the mutex. You must explicitly call pthread_mutex_unlock() in the error path, if the mutex has been locked by the function.
Related
TL;DR
Why does std::condition_variable::wait needs a mutex as one of its variables?
Answer 1
You may look a the documentation and quote that:
wait... Atomically releases lock
But that's not a real reason. That's just validate my question even more: why does it need it in the first place?
Answer 2
predicate is most likely query the state of a shared resource and it must be lock guarded.
OK. fair.
Two questions here
Is it always true that predicate query the state of a shared resource? I assume yes. I t doesn't make sense to me to implement it otherwise
What if I do not pass any predicate (it is optional)?
Using predicate - lock makes sense
int i = 0;
void waits()
{
std::unique_lock<std::mutex> lk(cv_m);
cv.wait(lk, []{return i == 1;});
std::cout << i;
}
Not Using predicate - why can't we lock after the wait?
int i = 0;
void waits()
{
cv.wait(lk);
std::unique_lock<std::mutex> lk(cv_m);
std::cout << i;
}
Notes
I know that there are no harmful implications to this practice. I just don't know how to explain to my self why it was design this way?
Question
If predicate is optional and is not passed to wait, why do we need the lock?
When using a condition variable to wait for a condition, a thread performs the following sequence of steps:
It determines that the condition is not currently true.
It starts waiting for some other thread to make the condition true. This is the wait call.
For example, the condition might be that a queue has elements in it, and a thread might see that the queue is empty and wait for another thread to put things in the queue.
If another thread were to intercede between these two steps, it could make the condition true and notify on the condition variable before the first thread actually starts waiting. In this case, the waiting thread would not receive the notification, and it might never stop waiting.
The purpose of requiring the lock to be held is to prevent other threads from interceding like this. Additionally, the lock must be unlocked to allow other threads to do whatever we're waiting for, but it can't happen before the wait call because of the notify-before-wait problem, and it can't happen after the wait call because we can't do anything while we're waiting. It has to be part of the wait call, so wait has to know about the lock.
Now, you might look at the notify_* methods and notice that those methods don't require the lock to be held, so there's nothing actually stopping another thread from notifying between steps 1 and 2. However, a thread calling notify_* is supposed to hold the lock while performing whatever action it does to make the condition true, which is usually enough protection.
TL;DR
If predicate is optional and is not passed to wait, why do we need the lock?
condition_variable is designed to wait for a certain condition to come true, not to wait just for a notification. So to "catch" the "moment" when the condition becomes true you need to check the condition and wait for the notification. And to avoid a race condition you need those two to be a single atomic operation.
Purpose Of condition_variable:
Enable a program to implement this: do some action when a condition C holds.
Intended Protocol:
Condition producer changes state of the world from !C to C.
Condition consumer waits for C to happen and takes the action while/after condition C holds.
Simplification:
For simplicity (to limit number of cases to think of) let's assume that C never switches back to !C. Let's also forget about spurious wakeups. Even with this assumptions we'll see that the lock is necessary.
Naive Approach:
Let's have two threads with an essential code summarized like this:
void producer() {
_condition = true;
_condition_variable.notify_all();
}
void consumer() {
if (!_condition) {
_condition_variable.wait();
}
action();
}
The Problem:
The problem here is a race condition. A problematic interleaving of the threads is following:
The consumer reads condition, checks it to be false and decides to wait.
A thread scheduler interrupts consumer and resumes producer.
The producer updates condition to become true and invokes notify_all().
The consumer is resumed.
The consumer actually does wait(), but is never notified and waken up (a liveness hazard).
So without locking the consumer may miss the event of the condition becoming true.
Solution:
Disclaimer: this code still does not handle spurious wakeups and possibility of condition becoming false again.
void producer() {
{ std::unique_lock<std::mutex> l(_mutex);
_condition = true;
}
_condition_variable.notify_all();
}
void consumer() {
{ std::unique_lock<std::mutex> l(_mutex);
if (!_condition) {
_condition_variable.wait(l);
}
}
action();
}
Here we check condition, release lock and start waiting as a single atomic operation, preventing the race condition mentioned before.
See Also
Why Lock condition await must hold the lock
You need a std::unique_lock when using std::condition_variable for the same reason you need a std::FILE* when using std::fwrite and for the same reason a BasicLockable is necessary when using std::unique_lock itself.
The feature std::fwrite gives you, entire the reason it exists, is to write to files. So you have to give it a file. The feature std::unique_lock provides you is RAII locking and unlocking of a mutex (or another BasicLockable, like std::shared_mutex, etc.) so you have to give it something to lock and unlock.
The feature std::condition_variable provides, the entire reason it exists, is the atomically waiting and unlocking a lock (and completing a wait and locking). So you have to give it something to lock.
Why would someone want that is a separate question that has been discussed already. For example:
When is a condition variable needed, isn't a mutex enough?
Conditional Variable vs Semaphore
Advantages of using condition variables over mutex
And so on.
As has been explained, the pred parameter is optional, but having some sort of a predicate and testing it isn't. Or, in other words, not having a predicate doesn't make any sense inn a manner similar to how having a condition variable without a lock doesn't making any sense.
The reason you have a lock is because you have shared state you need to protect from simultaneous access. Some function of that shared state is the predicate.
If you don't have a predicate and you don't have a lock you really don't need a condition variable just like if you don't have a file you really don't need fwrite.
A final point is that the second code snippet you wrote is very broken. Obviously it won't compile as you define the lock after you try to pass it as an argument to condition_variable::wait(). You probably meant something like:
std::mutex mtx_cv;
std::condition_variable cv;
...
{
std::unique_lock<std::mutex> lk(mtx_cv);
cv.wait(lk);
lk.lock(); // throws std::system_error with an error code of std::errc::resource_deadlock_would_occur
}
The reason this is wrong is very simple. condition_variable::wait's effects are (from [thread.condition.condvar]):
Effects:
— Atomically calls lock.unlock() and blocks on *this.
— When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
— The function will unblock when signaled by a call to notify_one() or a call to notify_all(), or spuriously
After the return from wait() the lock is locked, and unique_lock::lock() throws an exception if it has already locked the mutex it wraps ([thread.lock.unique.locking]).
Again, why would someone want coupling waiting and locking the way std::condition_variable does is a separate question, but given that it does - you cannot, by definition, lock a std::condition_variable's std::unique_lock after std::condition_variable::wait has returned.
It's not stated in the documentation (and could be implemented differently) but conceptually you can imagine the condition variable has another mutex to both protect its own data but also coordinate the condition, waiting and notification with modification of the consumer code data (e.g. queue.size()) affecting the test.
So when you call wait(...) the following (logically) happens.
Precondition: The consumer code holds the lock (CCL) controlling the consumer condition data (CCD).
The condition is checked, if true, execution in the consumer code continues still holding the lock.
If false, it first acquires its own lock (CVL), adds the current thread to the waiting thread collection releases the consumer lock and puts itself to waiting and releases its own lock (CVL).
That final step is tricky because it needs to sleep the thread and release the CVL at the same time or in that order or in a way that threads notified just before going to wait are able to (somehow) not go to wait.
The step of acquiring the CVL before releasing the CCD is key. Any parallel thread trying to update the CCD and notify will be blocked either by the CCL or CVL. If the CCL was released before acquiring the CVL a parallel thread could acquire the CCL, change the data and then notify before the the to-be-waiting thread is added to the waiters.
A parallel thread acquires the CCL, modifies the data to make the condition true (or at least worth testing) and then notifies. Notification acquires the the CVL and identifies a blocked thread (or threads) if any to unwait. The unwaited threads then seek to acquire the CCL and may block there but won't leave wait and re-perform the test until they've acquired it.
Notification must acquire the CVL to make sure threads that have found the test false have been added to the waiters.
It's OK (possibly preferable for performance) to notify without holding the CCL because the hand-off between the CCL and CVL in the wait code is ensuring the ordering.
It may be preferrable because notifying when holding the CCL may mean all the unwaited threads just unwait to block (on the CCL) while the thread modifying the data is still holding the lock.
Notice that even if the CCD is atomic you must modify it holding the CCL or that Lock CVL, unlock CCL step won't ensure the total ordering required to make sure notifications aren't sent when threads are in the process of going to wait.
The standard only talks about atomicity of operations and another implementation may have a way of blocking notification before completing the 'add to waiters' step has completed following a failed test. The C++ Standard is careful to not dictate an implementation.
In all that, to answer some of the specific questions.
Must the state be shared? Sort of. There could be an external condition like a file being in a directory and the wait is timed to re-try after a time-period. You can decide for yourself whether you consider the file system or even just the wall-clock to be shared state.
Must there be any state? Not necessarily. A thread can wait on notification.
That could be tricky to coordinate because there has to be enough sequencing to stop the other thread notifying out of turn. The commonest solution is to have some boolean flag set by the notifying thread so the notified thread knows if it missed it. The normal use of void wait(std::unique_lock<std::mutex>& lk) is when the predicate is checked outside:
std::unique_lock<std::mutex> ulk(ccd_mutex)
while(!condition){
cv.wait(ulk);
}
Where the notifying thread uses:
{
std::lock_guard<std::mutex> guard(ccd_mutex);
condition=true;
}
cv.notify();
The reason is that in some times the waiting-thread holds the m_mutex:
#include <mutex>
#include <condition_variable>
void CMyClass::MyFunc()
{
std::unique_lock<std::mutex> guard(m_mutex);
// do something (on the protected resource)
m_condiotion.wait(guard, [this]() {return !m_bSpuriousWake; });
// do something else (on the protected resource)
guard.unluck();
// do something else than else
}
and a thread should never go to sleep while holding a m_mutex. One doesn't want to lock everybody out, while sleeping. So, atomically: {guard is unlocked and the thread go to sleep}. Once it waked up by the other-thread (m_condiotion.notify_one(), let's say) guard is locked again, and then the thread continue.
Reference (video)
Because if not so, there's a race condition before the waiting thread noticing the change of the shared state and the wait() call.
Assume we got a shared state of type std::atomic state_, there's still a fair chance for the waiting thread to miss a notification:
T1(waiting) | T2(notification)
---------------------------------------------- * ---------------------------
1) for (int i = state_; i != 0; i = state_) { |
2) | state_ = 0;
3) | cv.notify();
4) cv.wait(); |
5) }
6) // go on with the satisfied condition... |
Note that the wait() call failed to notice the latest value of state_ and may keep waiting forever.
I was writing a multithreading code using pthread_cond in conjuction with mutexes, which made me wonder:
is the signal one time, so if the signal is sent before the other thread is waiting for it, the other thread will keep waiting indefinitely?
Since cond_wait() unlocks the mutex, is it a thumb rule to write this statement JUST before mutex_unlock(), (I realise this makes the latter redundant, but I do that just for clarity) or are there many scenarios where you would want to write the function outside the mutex lock?
Make this your mantra:
Only ever wait for something ...
Waiting should almost always look like this:
if (pthread_mutex_lock(...) != 0) {
/* something terrible happened, panic */
}
while (test-condition) {
pthread_cond_wait(...)
}
pthread_mutex_unlock(...)
If the exclusive check of test-condition fails, and so a context enters pthread_cond_wait the associated mutex will be atomically unlocked.
This means another context can enter code that looks like:
if (pthread_mutex_lock(...) != 0) {
/* panic */
}
test-condition = false;
pthread_cond_signal(...);
pthread_mutex_unlock(...);
Changing the predicate and atomically waking the first context that is in the call to pthread_cond_wait, which in turn checks the predicate test-condition and can jump past the loop.
If we just look at the waiting code again:
if (pthread_mutex_lock(...) != 0) {
/* something terrible happened, panic */
}
while (test-condition) {
pthread_cond_wait(...)
}
pthread_mutex_unlock(...)
Between the call to wait and unlock, there is always exclusivity; Either because the mutex was acquired exclusively (the predicated wait loop was not entered), or because before returning from a call topthread_cond_wait the mutex was re-acquired atomically.
Synchronization is hard to get right, and is costly for a multi-threaded application; One should attempt to keep critical sections simple to squeeze the margins for error to their minimum size.
Another important thing to do is check the return values of all these pthread_* calls; The return value is important information about state that you always need to know, and nearly always need to act upon.
Some useful man pages (for return values):
pthread_mutex_lock
pthread_cond_wait
pthread_cond_signal
On a multi threading system, if two threads want to work on a shared memory after locking a mutex.
Thread A:
pthread_mutex_lock(&mutex)
....... //Memory corruption or Assert and thread exits
pthread_mutex_unlock(&mutex)
Thread B:
pthread_mutex_lock(&mutex)
.......
pthread_mutex_unlock(&mutex)
If Thread A acquires the mutex first and exits due to memory corruption or assert, Thread B will be waiting forever causing a deadlock.
Is there a way/method i can use to come out from this kind of deadlock, once it happened?
Is there any other safer method similar to mutex that I can use?
You can set the ROBUST attribute on a mutex. With a robust mutex, if the thread that acquired it exits for some reason without unlocking it, the mutex enters a special state where the next thread that attempts to lock it will get EOWNERDEAD.
It is then the responsibility of that thread to cleanup any inconsistent state. If recovery is possible, the thread shall call pthread_mutex_consistent(3) any time before pthread_mutex_unlock(3), so that the other threads can use it as before. If recovery is not possible, the mutex should be unlocked without calling pthread_mutex_consistent(3), causing it to enter an unusable state where the only permissible operation is to destroy it.
Note that the mutex is locked even if EOWNERDEAD was returned (I think it's the only condition under which pthread_mutex_lock(3) returns with an error but locks the mutex).
To set the ROBUST attribute, use pthread_mutexattr_setrobust(3) after initializing the mutex attributes instance. Remember that this must be done before initializing the mutex. So, something like:
pthread_mutex_t mutex;
pthread_mutexattr_t mutex_attrs;
if (pthread_mutexattr_init(&mutex_attrs) != 0) {
/* Handle error... */
}
if (pthread_mutexattr_setrobust(&mutex_attrs, PTHREAD_MUTEX_ROBUST) != 0) {
/* Handle error... */
}
if (pthread_mutex_init(&mutex, &mutex_attrs) != 0) {
/* Handle error... */
}
Then you can use it like:
int lock_res = pthread_mutex_lock(&mutex);
if (lock_res == EOWNERDEAD) {
/* Someone died before unlocking the mutex
* We assume there's no cleanup to do
*/
if (pthread_mutex_consistent(&mutex) != 0) {
/* Handle error... */
}
} else if (lock_res != 0) {
/* Some other error, handle it here */
}
/* mutex is locked here, do stuff... */
if (pthread_mutex_unlock(&mutex) != 0) {
/* Handle error */
}
For more info you can see the manpage for pthread_mutex_consistent(3) and pthread_mutex_getrobust(3) / pthread_mutex_setrobust(3)
If you want threads to clean up after themselves, you generally need to ask for it or do it yourself.
You can for example use pthread_cleanup_push() and pthread_cleanup_pop() to ensure cleanup happens if the thread exits using pthread_exit() or if it is cancelled (by way of pthread_cancel()).
The downsides of this API are that each pair of calls must be in the same function and in the same lexical nesting level (e.g. In your thread's entry function).
I can't tell if you're allowed to cancel a thread from within a signal handler, so you might not be able to handle this using that API, and these types of errors and assertions typically make the entire process exit (Although, presumably if you're implementing your own assert variant, it could call pthread_exit()).
There do exist other lock types that unlock automatically (e.g. flock() locks, or robust mutexes as shown by Filipe Gonçalves) when the owner dies, but you have little guarantee of state consistency in cases where someone lost a lock and will need to clean up after them
I have a multithreaded app that has to read some data often, and occasionally that data is updated. I have problems with writing by using unique_lock and problems with reading by using upgrade_lock
There is examples of my problems:
void unlock(){
test.stream = 0;
test.mtx.unlock();
}
void lock_mtx(int i){
boost::unique_lock<boost::shared_mutex> lock(test.mtx);
test.stream = i;
boost::this_thread::sleep_for(boost::chrono::milliseconds(10000));
unlock();
boost::this_thread::sleep_for(boost::chrono::milliseconds(1000));
}
When I destruct lock , mutex is already unlocked by this thread, and sometimes it is locked by another thread, but destructor make it free again. After destruction of lock (in the first thread) third thread take mutex and I have two writers at the one moment
void lock_mtx(int i){
boost::upgrade_lock<boost::shared_mutex> lock(test.mtx);
read_from_locked();
boost::this_thread::sleep_for(boost::chrono::milliseconds(5000));
boost::upgrade_to_unique_lock<boost::shared_mutex> uniqueLock(lock);
write_to_locked();
boost::this_thread::sleep_for(boost::chrono::milliseconds(10000));
}
The second problem, is that when some thread takes upgrade_lock, other threads can't read shared objects
Both of problems occur in MS VisualStudio 2013 and Windows8 x64
void lock_mtx(int i)
{
{
boost::unique_lock<boost::shared_mutex> lock(test.mtx);
test.stream = i;
boost::this_thread::sleep_for(boost::chrono::milliseconds(10000));
}
boost::this_thread::sleep_for(boost::chrono::milliseconds(1000));
}
The purpose of unique_lock and lock_guard is to automatically unlock the mutex when it goes out of scope. This pattern is known as RAII.
Rule of thumb: Never manually call lock()/unlock() on your (Basic|Shared)Lockable objects. It's an antipattern because
It's extremely hard to get right (think of exception safety)
It usually indicates a code smell (locks being held across different method calls). If you even need this, consider making the RAII lock guard (lock_guard or unique_lock) a member of the containing class, or return the unique_lock so the caller has the option to explicitly adopt the lock, or to just let it be automatically released by the guard.
A mutex is a programming concept that is frequently used to solve multi-threading problems. My question to the community:
What is a mutex and how do you use it?
When I am having a big heated discussion at work, I use a rubber chicken which I keep in my desk for just such occasions. The person holding the chicken is the only person who is allowed to talk. If you don't hold the chicken you cannot speak. You can only indicate that you want the chicken and wait until you get it before you speak. Once you have finished speaking, you can hand the chicken back to the moderator who will hand it to the next person to speak. This ensures that people do not speak over each other, and also have their own space to talk.
Replace Chicken with Mutex and person with thread and you basically have the concept of a mutex.
Of course, there is no such thing as a rubber mutex. Only rubber chicken. My cats once had a rubber mouse, but they ate it.
Of course, before you use the rubber chicken, you need to ask yourself whether you actually need 5 people in one room and would it not just be easier with one person in the room on their own doing all the work. Actually, this is just extending the analogy, but you get the idea.
A Mutex is a Mutually exclusive flag. It acts as a gate keeper to a section of code allowing one thread in and blocking access to all others. This ensures that the code being controlled will only be hit by a single thread at a time. Just be sure to release the mutex when you are done. :)
Mutual Exclusion. Here's the Wikipedia entry on it.
The point of a mutex is to synchronize two threads. When you have two threads attempting to access a single resource, the general pattern is to have the first block of code attempting access to set the mutex before entering the code. When the second code block attempts access, it sees that the mutex is set and waits until the first block of code is complete (and unsets the mutex), then continues.
Specific details of how this is accomplished obviously varies greatly by programming language.
When you have a multi-threaded application, the different threads sometimes share a common resource, such as a variable or similar. This shared source often cannot be accessed at the same time, so a construct is needed to ensure that only one thread is using that resource at a time.
The concept is called "mutual exclusion" (short Mutex), and is a way to ensure that only one thread is allowed inside that area, using that resource etc.
How to use them is language specific, but is often (if not always) based on a operating system mutex.
Some languages doesn't need this construct, due to the paradigm, for example functional programming (Haskell, ML are good examples).
What is a Mutex?
The mutex (In fact, the term mutex is short for mutual exclusion) also known as spinlock is the simplest synchronization tool that is used to protect critical regions and thus prevent race conditions. That is a thread must acquire a lock before entering into a critical section (In critical section multi threads share a common variable, updating a table, writing a file and so on), it releases the lock when it leaves critical section.
What is a Race Condition?
A race condition occurs when two or more threads can access shared data and they try to change it at the same time. Because the thread scheduling algorithm can swap between threads at any time, you don't know the order in which the threads will attempt to access the shared data. Therefore, the result of the change in data is dependent on the thread scheduling algorithm, i.e. both threads are "racing" to access/change the data.
Real life example:
When I am having a big heated discussion at work, I use a rubber
chicken which I keep in my desk for just such occasions. The person
holding the chicken is the only person who is allowed to talk. If you
don't hold the chicken you cannot speak. You can only indicate that
you want the chicken and wait until you get it before you speak. Once
you have finished speaking, you can hand the chicken back to the
moderator who will hand it to the next person to speak. This ensures
that people do not speak over each other, and also have their own
space to talk.
Replace Chicken with Mutex and person with thread and you basically have the concept of a mutex.
#Xetius
Usage in C#:
This example shows how a local Mutex object is used to synchronize access to a protected resource. Because each calling thread is blocked until it acquires ownership of the mutex, it must call the ReleaseMutex method to release ownership of the thread.
using System;
using System.Threading;
class Example
{
// Create a new Mutex. The creating thread does not own the mutex.
private static Mutex mut = new Mutex();
private const int numIterations = 1;
private const int numThreads = 3;
static void Main()
{
// Create the threads that will use the protected resource.
for(int i = 0; i < numThreads; i++)
{
Thread newThread = new Thread(new ThreadStart(ThreadProc));
newThread.Name = String.Format("Thread{0}", i + 1);
newThread.Start();
}
// The main thread exits, but the application continues to
// run until all foreground threads have exited.
}
private static void ThreadProc()
{
for(int i = 0; i < numIterations; i++)
{
UseResource();
}
}
// This method represents a resource that must be synchronized
// so that only one thread at a time can enter.
private static void UseResource()
{
// Wait until it is safe to enter.
Console.WriteLine("{0} is requesting the mutex",
Thread.CurrentThread.Name);
mut.WaitOne();
Console.WriteLine("{0} has entered the protected area",
Thread.CurrentThread.Name);
// Place code to access non-reentrant resources here.
// Simulate some work.
Thread.Sleep(500);
Console.WriteLine("{0} is leaving the protected area",
Thread.CurrentThread.Name);
// Release the Mutex.
mut.ReleaseMutex();
Console.WriteLine("{0} has released the mutex",
Thread.CurrentThread.Name);
}
}
// The example displays output like the following:
// Thread1 is requesting the mutex
// Thread2 is requesting the mutex
// Thread1 has entered the protected area
// Thread3 is requesting the mutex
// Thread1 is leaving the protected area
// Thread1 has released the mutex
// Thread3 has entered the protected area
// Thread3 is leaving the protected area
// Thread3 has released the mutex
// Thread2 has entered the protected area
// Thread2 is leaving the protected area
// Thread2 has released the mutex
MSDN Reference Mutex
There are some great answers here, here is another great analogy for explaining what mutex is:
Consider single toilet with a key. When someone enters, they take the key and the toilet is occupied. If someone else needs to use the toilet, they need to wait in a queue. When the person in the toilet is done, they pass the key to the next person in queue. Make sense, right?
Convert the toilet in the story to a shared resource, and the key to a mutex. Taking the key to the toilet (acquire a lock) permits you to use it. If there is no key (the lock is locked) you have to wait. When the key is returned by the person (release the lock) you're free to acquire it now.
In C#, the common mutex used is the Monitor. The type is 'System.Threading.Monitor'. It may also be used implicitly via the 'lock(Object)' statement. One example of its use is when constructing a Singleton class.
private static readonly Object instanceLock = new Object();
private static MySingleton instance;
public static MySingleton Instance
{
lock(instanceLock)
{
if(instance == null)
{
instance = new MySingleton();
}
return instance;
}
}
The lock statement using the private lock object creates a critical section. Requiring each thread to wait until the previous is finished. The first thread will enter the section and initialize the instance. The second thread will wait, get into the section, and get the initialized instance.
Any sort of synchronization of a static member may use the lock statement similarly.
To understand MUTEX at first you need to know what is "race condition" and then only you will understand why MUTEX is needed. Suppose you have a multi-threading program and you have two threads. Now, you have one job in the job queue. The first thread will check the job queue and after finding the job it will start executing it. The second thread will also check the job queue and find that there is one job in the queue. So, it will also assign the same job pointer. So, now what happens, both the threads are executing the same job. This will cause a segmentation fault. This is the example of a race condition.
The solution to this problem is MUTEX. MUTEX is a kind of lock which locks one thread at a time. If another thread wants to lock it, the thread simply gets blocked.
The MUTEX topic in this pdf file link is really worth reading.
Mutexes are useful in situations where you need to enforce exclusive access to a resource accross multiple processes, where a regular lock won't help since it only works accross threads.
Mutex: Mutex stands for Mutual Exclusion. It means only one process/thread can enter into critical section at a given time. In concurrent programming multiple threads/process updating the shared resource (any variable, shared memory etc.) may lead to some unexpected result. ( As the result depends upon the which thread/process gets the first access).
In order to avoid such an unexpected result we need some synchronization mechanism, which ensures that only one thread/process gets access to such a resource at a time.
pthread library provides support for Mutex.
typedef union
{
struct __pthread_mutex_s
{
***int __lock;***
unsigned int __count;
int __owner;
#ifdef __x86_64__
unsigned int __nusers;
#endif
int __kind;
#ifdef __x86_64__
short __spins;
short __elision;
__pthread_list_t __list;
# define __PTHREAD_MUTEX_HAVE_PREV 1
# define __PTHREAD_SPINS 0, 0
#else
unsigned int __nusers;
__extension__ union
{
struct
{
short __espins;
short __elision;
# define __spins __elision_data.__espins
# define __elision __elision_data.__elision
# define __PTHREAD_SPINS { 0, 0 }
} __elision_data;
__pthread_slist_t __list;
};
#endif
This is the structure for mutex data type i.e pthread_mutex_t.
When mutex is locked, __lock set to 1. When it is unlocked __lock set to 0.
This ensure that no two processes/threads can access the critical section at same time.