What is a mutex

Question

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

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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 controled will only be hit by a single thread at a time   Just be sure to release the mutex when you are done

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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

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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

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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

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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

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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

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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

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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  quot racing quot  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  lt  numThreads  i                          Thread newThread   new Thread new ThreadStart ThreadProc                newThread Name   String Format  quot Thread 0  quot   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  lt  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  quot  0  is requesting the mutex quot                              Thread CurrentThread Name           mut WaitOne             Console WriteLine  quot  0  has entered the protected area quot                              Thread CurrentThread Name               Place code to access non-reentrant resources here              Simulate some work          Thread Sleep 500            Console WriteLine  quot  0  is leaving the protected area quot                Thread CurrentThread Name               Release the Mutex          mut ReleaseMutex            Console WriteLine  quot  0  has released the mutex quot                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

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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

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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

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