Difference between binary semaphore and mutex

Question

Is there any difference between a binary semaphore and mutex or are they essentially the same

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http   www geeksforgeeks org archives 9102  discusses in details   Mutex is locking mechanism used to synchronize access to a resource  Semaphore is signaling mechanism    Its up to to programmer if he she wants to use  binary semaphore in place of mutex

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binary semaphore  is a programming language circumvent to use a   semaphore   like   mutex    Apparently there are two very big differences    The way you call each one of them  The maximum length of the  identifier

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Since none of the above answer clears the confusion  here is one which cleared my confusion      Strictly speaking  a mutex is a locking mechanism used to   synchronize access to a resource  Only one task  can be a thread or   process based on OS abstraction  can acquire the mutex  It means there   will be ownership associated with mutex  and only the owner can   release the lock  mutex        Semaphore is signaling mechanism     I am done  you can carry on    kind of signal   For example  if you are listening songs  assume it as   one task  on your mobile and at the same time your friend called you    an interrupt will be triggered upon which an interrupt service routine    ISR  will signal the call processing task to wakeup    Source  http   www geeksforgeeks org mutex-vs-semaphore

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Modified question is - What s the difference between A mutex and a  binary  semaphore in  Linux    Ans  Following are the differences     i  Scope     The scope of mutex is within a process address space which has created it and is used for synchronization of threads  Whereas semaphore can be used across process space and hence it can be used for interprocess synchronization   ii  Mutex is lightweight and faster than semaphore  Futex is even faster   iii  Mutex can be acquired by same thread successfully multiple times with condition that it should release it same number of times  Other thread trying to acquire will block  Whereas in case of semaphore if same process tries to acquire it again it blocks as it can be acquired only once

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On Windows  there are two differences between mutexes and binary semaphores    A mutex can only be released by the thread which has ownership  i e  the thread which previously called the Wait function   or which took ownership when creating it   A semaphore can be released by any thread  A thread can call a wait function repeatedly on a mutex without blocking  However  if you call a wait function twice on a binary semaphore without releasing the semaphore in between  the thread will block

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The concept was clear to me after going over above posts  But there were some lingering questions  So  I wrote this small piece of code    When we try to give a semaphore without taking it  it goes through  But  when you try to give a mutex without taking it  it fails  I tested this on a Windows platform  Enable USE MUTEX to run the same code using a MUTEX    include  lt stdio h gt   include  lt windows h gt   define xUSE MUTEX 1  define MAX SEM COUNT 1  DWORD WINAPI Thread no 1  LPVOID lpParam    DWORD WINAPI Thread no 2  LPVOID lpParam     HANDLE Handle Of Thread 1   0  HANDLE Handle Of Thread 2   0  int Data Of Thread 1   1  int Data Of Thread 2   2  HANDLE ghMutex   NULL  HANDLE ghSemaphore   NULL    int main void      ifdef USE MUTEX     ghMutex   CreateMutex  NULL  FALSE  NULL       if  ghMutex     NULL                 printf  CreateMutex error   d n   GetLastError             return 1         else        Create a semaphore with initial and max counts of MAX SEM COUNT     ghSemaphore   CreateSemaphore NULL MAX SEM COUNT MAX SEM COUNT NULL       if  ghSemaphore    NULL                 printf  CreateSemaphore error   d n   GetLastError             return 1         endif        Create thread 1      Handle Of Thread 1   CreateThread  NULL  0 Thread no 1   amp Data Of Thread 1  0  NULL         if   Handle Of Thread 1    NULL                printf  Create first thread problem  n            return 1                sleep for 5 seconds         Sleep 5   1000          Create thread 2        Handle Of Thread 2   CreateThread  NULL  0 Thread no 2   amp Data Of Thread 2  0  NULL         if   Handle Of Thread 2    NULL                printf  Create second thread problem  n            return 1                Sleep for 20 seconds     Sleep 20   1000        printf  Out of the program  n        return 0      int my critical section code HANDLE thread handle      ifdef USE MUTEX     if thread handle    Handle Of Thread 1                   get the lock            WaitForSingleObject ghMutex  INFINITE           printf  Thread 1 holding the mutex  n           else        get the semaphore        if thread handle    Handle Of Thread 1                WaitForSingleObject ghSemaphore  INFINITE           printf  Thread 1 holding semaphore  n           endif      if thread handle    Handle Of Thread 1                   sleep for 10 seconds            Sleep 10   1000    ifdef USE MUTEX         printf  Thread 1 about to release mutex  n     else         printf  Thread 1 about to release semaphore  n     endif           else                  sleep for 3 secconds            Sleep 3   1000           ifdef USE MUTEX        release the lock       if  ReleaseMutex ghMutex                 printf  Release Mutex error in thread  d  error    d n    thread handle    Handle Of Thread 1   1 2  GetLastError            else     if   ReleaseSemaphore ghSemaphore 1 NULL                        printf  ReleaseSemaphore error in thread  d  error    d n   thread handle    Handle Of Thread 1   1 2   GetLastError            endif      return 0     DWORD WINAPI Thread no 1  LPVOID lpParam           my critical section code Handle Of Thread 1       return 0      DWORD WINAPI Thread no 2  LPVOID lpParam          my critical section code Handle Of Thread 2       return 0      The very fact that semaphore lets you signal  it is done using a resource   even though it never owned the resource  makes me think there is a very loose coupling between owning and signaling in the case of semaphores

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Myth   Couple of article says that  binary semaphore and mutex are same  or  Semaphore with value 1 is mutex  but the basic difference is Mutex can be released only by thread that had acquired it  while you can signal semaphore from any other thread  Key Points      A thread can acquire more than one lock  Mutex       A mutex can be locked more than once only if its a recursive mutex  here lock and unlock for mutex should be same     If a thread which had already locked a mutex  tries to lock the mutex again  it will enter into the waiting list of that mutex  which results in deadlock       Binary semaphore and mutex are similar but not same      Mutex is costly operation due to protection protocols associated with it      Main aim of mutex is achieve atomic access or lock on resource

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At a theoretical level  they are no different semantically   You can implement a mutex using semaphores or vice versa  see here for an example    In practice  the implementations are different and they offer slightly different services  The practical difference  in terms of the system services surrounding them  is that the implementation of a mutex is aimed at being a more lightweight synchronisation mechanism   In oracle-speak  mutexes are known as latches and semaphores are known as waits  At the lowest level  they use some sort of atomic test and set mechanism   This reads the current value of a memory location  computes some sort of conditional and writes out a value at that location in a single instruction that cannot be interrupted   This means that you can acquire a mutex and test to see if anyone else had it before you  A typical mutex implementation has a process or thread executing the test-and-set instruction and evaluating whether anything else had set the mutex    A key point here is that there is no interaction with the scheduler  so we have no idea  and don t care  who has set the lock   Then we either give up our time slice and attempt it again when the task is re-scheduled or execute a spin-lock   A spin lock is an algorithm like  Count down from 5000       i  Execute the test-and-set instruction     ii  If the mutex is clear  we have acquired it in the previous instruction          so we can exit the loop    iii  When we get to zero  give up our time slice   When we have finished executing our protected code  known as a critical section  we just set the mutex value to zero or whatever means  clear    If multiple tasks are attempting to acquire the mutex then the next task that happens to be scheduled after the mutex is released will get access to the resource   Typically you would use mutexes to control a synchronised resource where exclusive access is only needed for very short periods of time  normally to make an update to a shared data structure  A semaphore is a synchronised data structure  typically using a mutex  that has a count and some system call wrappers that interact with the scheduler in a bit more depth than the mutex libraries would   Semaphores are incremented and decremented and used to block tasks until something else is ready   See Producer Consumer Problem for a simple example of this   Semaphores are initialised to some value - a binary semaphore is just a special case where the semaphore is initialised to 1   Posting to a semaphore has the effect of waking up a waiting process  A basic semaphore algorithm looks like   somewhere in the program startup  Initialise the semaphore to its start-up value   Acquiring a semaphore    i   synchronised  Attempt to decrement the semaphore value   ii  If the value would be less than zero  put the task on the tail of the list of tasks waiting on the semaphore and give up the time slice   Posting a semaphore    i   synchronised  Increment the semaphore value   ii  If the value is greater or equal to the amount requested in the post at the front of the queue  take that task off the queue and make it runnable     iii  Repeat  ii  for all tasks until the posted value is exhausted or there are no more tasks waiting   In the case of a binary semaphore the main practical difference between the two is the nature of the system services surrounding the actual data structure  EDIT  As evan has rightly pointed out  spinlocks will slow down a single processor machine   You would only use a spinlock on a multi-processor box because on a single processor the process holding the mutex will never reset it while another task is running   Spinlocks are only useful on multi-processor architectures

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They are NOT the same thing   They are used for different purposes  While both types of semaphores have a full empty state and use the same API  their usage is very different     Mutual Exclusion Semaphores Mutual Exclusion semaphores are used to protect shared resources  data structure  file  etc       A Mutex semaphore is  owned  by the task that takes it   If Task B attempts to semGive a mutex currently held by Task A  Task B s call will return an error and fail   Mutexes always use the following sequence        - SemTake   - Critical Section   - SemGive  Here is a simple example      Thread A                     Thread B    Take Mutex      access data                                 Take Mutex   lt    Will block             Give Mutex                     access data   lt    Unblocks                                                                       Give Mutex   Binary Semaphore Binary Semaphore address a totally different question      Task B is pended waiting for something to happen  a sensor being tripped for example   Sensor Trips and an Interrupt Service Routine runs  It needs to notify a task of the trip  Task B should run and take appropriate actions for the sensor trip  Then go back to waiting        Task A                      Task B                                Take BinSemaphore    lt    wait for something    Do Something Noteworthy    Give BinSemaphore           do something     lt    unblocks   Note that with a binary semaphore  it is OK for B to take the semaphore and A to give it  Again  a binary semaphore is NOT protecting a resource from access  The act of Giving and Taking a semaphore are fundamentally decoupled  It typically makes little sense for the same task to so a give and a take on the same binary semaphore

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On Windows  there are two differences between mutexes and binary semaphores    A mutex can only be released by the thread which has ownership  i e  the thread which previously called the Wait function   or which took ownership when creating it   A semaphore can be released by any thread  A thread can call a wait function repeatedly on a mutex without blocking  However  if you call a wait function twice on a binary semaphore without releasing the semaphore in between  the thread will block

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Almost all of the above said it right  Let me also try my bit to clarify if somebody still has a doubt    Mutex -  used for serialization Semaphore-  synchronization    Purpose of both are different however  same functionality could be achieved through both of them with careful programming   Standard Example-  producer consumer problem   initial value of SemaVar 0  Producer                           Consumer ---                                SemaWait  - gt decrement SemaVar    produce data --- SemaSignal SemaVar or SemaVar    --- gt consumer unblocks as SemVar is 1 now    Hope I could clarify

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The concept was clear to me after going over above posts  But there were some lingering questions  So  I wrote this small piece of code    When we try to give a semaphore without taking it  it goes through  But  when you try to give a mutex without taking it  it fails  I tested this on a Windows platform  Enable USE MUTEX to run the same code using a MUTEX    include  lt stdio h gt   include  lt windows h gt   define xUSE MUTEX 1  define MAX SEM COUNT 1  DWORD WINAPI Thread no 1  LPVOID lpParam    DWORD WINAPI Thread no 2  LPVOID lpParam     HANDLE Handle Of Thread 1   0  HANDLE Handle Of Thread 2   0  int Data Of Thread 1   1  int Data Of Thread 2   2  HANDLE ghMutex   NULL  HANDLE ghSemaphore   NULL    int main void      ifdef USE MUTEX     ghMutex   CreateMutex  NULL  FALSE  NULL       if  ghMutex     NULL                 printf  CreateMutex error   d n   GetLastError             return 1         else        Create a semaphore with initial and max counts of MAX SEM COUNT     ghSemaphore   CreateSemaphore NULL MAX SEM COUNT MAX SEM COUNT NULL       if  ghSemaphore    NULL                 printf  CreateSemaphore error   d n   GetLastError             return 1         endif        Create thread 1      Handle Of Thread 1   CreateThread  NULL  0 Thread no 1   amp Data Of Thread 1  0  NULL         if   Handle Of Thread 1    NULL                printf  Create first thread problem  n            return 1                sleep for 5 seconds         Sleep 5   1000          Create thread 2        Handle Of Thread 2   CreateThread  NULL  0 Thread no 2   amp Data Of Thread 2  0  NULL         if   Handle Of Thread 2    NULL                printf  Create second thread problem  n            return 1                Sleep for 20 seconds     Sleep 20   1000        printf  Out of the program  n        return 0      int my critical section code HANDLE thread handle      ifdef USE MUTEX     if thread handle    Handle Of Thread 1                   get the lock            WaitForSingleObject ghMutex  INFINITE           printf  Thread 1 holding the mutex  n           else        get the semaphore        if thread handle    Handle Of Thread 1                WaitForSingleObject ghSemaphore  INFINITE           printf  Thread 1 holding semaphore  n           endif      if thread handle    Handle Of Thread 1                   sleep for 10 seconds            Sleep 10   1000    ifdef USE MUTEX         printf  Thread 1 about to release mutex  n     else         printf  Thread 1 about to release semaphore  n     endif           else                  sleep for 3 secconds            Sleep 3   1000           ifdef USE MUTEX        release the lock       if  ReleaseMutex ghMutex                 printf  Release Mutex error in thread  d  error    d n    thread handle    Handle Of Thread 1   1 2  GetLastError            else     if   ReleaseSemaphore ghSemaphore 1 NULL                        printf  ReleaseSemaphore error in thread  d  error    d n   thread handle    Handle Of Thread 1   1 2   GetLastError            endif      return 0     DWORD WINAPI Thread no 1  LPVOID lpParam           my critical section code Handle Of Thread 1       return 0      DWORD WINAPI Thread no 2  LPVOID lpParam          my critical section code Handle Of Thread 2       return 0      The very fact that semaphore lets you signal  it is done using a resource   even though it never owned the resource  makes me think there is a very loose coupling between owning and signaling in the case of semaphores

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The answer may depend on the target OS   For example  at least one RTOS implementation I m familiar with will allow multiple sequential  get  operations against a single OS mutex  so long as they re all from within the same thread context   The multiple gets must be replaced by an equal number of puts before another thread will be allowed to get the mutex   This differs from binary semaphores  for which only a single get is allowed at a time  regardless of thread contexts   The idea behind this type of mutex is that you protect an object by only allowing a single context to modify the data at a time   Even if the thread gets the mutex and then calls a function that further modifies the object  and gets puts the protector mutex around its own operations   the operations should still be safe because they re all happening under a single thread         mutexGet        Other threads can no longer get the mutex          Make changes to the protected object                  objectModify        Also gets puts the mutex   Only allowed from this thread context          Make more changes to the protected object                  mutexPut        Finally allows other threads to get the mutex      Of course  when using this feature  you must be certain that all accesses within a single thread really are safe   I m not sure how common this approach is  or whether it applies outside of the systems with which I m familiar   For an example of this kind of mutex  see the ThreadX RTOS

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Since none of the above answer clears the confusion  here is one which cleared my confusion      Strictly speaking  a mutex is a locking mechanism used to   synchronize access to a resource  Only one task  can be a thread or   process based on OS abstraction  can acquire the mutex  It means there   will be ownership associated with mutex  and only the owner can   release the lock  mutex        Semaphore is signaling mechanism     I am done  you can carry on    kind of signal   For example  if you are listening songs  assume it as   one task  on your mobile and at the same time your friend called you    an interrupt will be triggered upon which an interrupt service routine    ISR  will signal the call processing task to wakeup    Source  http   www geeksforgeeks org mutex-vs-semaphore

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A mutex can be released only by the thread that had acquired it   A binary semaphore can be signaled by any thread  or process     so semaphores are more suitable for some synchronization problems like producer-consumer   On Windows  binary semaphores are more like event objects than mutexes

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Mutex work on blocking critical region  But Semaphore work on count

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You can clearly remember difference by this   Mutex lock   is for protecting critical region  Mutex can t be used across processes  only used in single process  Semaphore  is for signalling availability of a resource  Semaphore can be used both across processes and across processes

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Mutex are used for   Locking Mechanisms    one process at a time can use a shared resource  whereas  Semaphores are used for   Signaling Mechanisms    like  I am done   now can continue

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As many folks here have mentioned  a mutex is used to protect a critical piece of code  AKA critical section   You will acquire the mutex  lock   enter critical section  and release mutex  unlock  all in the same thread   While using a semaphore  you can make a thread wait on a semaphore  say thread A   until another thread  say thread B completes whatever task  and then sets the Semaphore for thread A to stop the wait  and continue its task

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While a binary semaphore may be used as a mutex  a mutex is a more specific use-case  in that only the process that locked the mutex is supposed to unlock it  This ownership constraint makes it possible to provide protection against    Accidental release Recursive Deadlock Task Death Deadlock   These constraints are not always present because they degrade the speed  During the development of your code  you can enable these checks  temporarily    e g  you can enable Error check attribute in your mutex  Error checking mutexes return EDEADLK if you try to lock the same one twice and EPERM if you unlock a mutex that isn t yours   pthread mutex t mutex  pthread mutexattr t attr  pthread mutexattr init   amp attr   pthread mutexattr settype   amp attr  PTHREAD MUTEX ERRORCHECK NP   pthread mutex init   amp mutex   amp attr     Once initialised we can place these checks in our code like this   if pthread mutex unlock  amp mutex   EPERM   printf  Unlock failed Mutex not owned by this thread n

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As many folks here have mentioned  a mutex is used to protect a critical piece of code  AKA critical section   You will acquire the mutex  lock   enter critical section  and release mutex  unlock  all in the same thread   While using a semaphore  you can make a thread wait on a semaphore  say thread A   until another thread  say thread B completes whatever task  and then sets the Semaphore for thread A to stop the wait  and continue its task

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Best Solution  The only difference is   1 Mutex -  lock and unlock are under the ownership of a thread that locks the mutex   2 Semaphore -  No ownership i e  if one thread calls semwait s  any other thread can call sempost s  to remove the lock

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You can clearly remember difference by this   Mutex lock   is for protecting critical region  Mutex can t be used across processes  only used in single process  Semaphore  is for signalling availability of a resource  Semaphore can be used both across processes and across processes

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Though mutex  amp  semaphores are used as synchronization primitives  there is a big difference between them  In the case of mutex  only the thread that locked or acquired the mutex can unlock it  In the case of a semaphore  a thread waiting on a semaphore can be signaled by a different thread  Some operating system supports using mutex  amp  semaphores between process  Typically usage is creating in shared memory

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A mutex can be released only by the thread that had acquired it   A binary semaphore can be signaled by any thread  or process     so semaphores are more suitable for some synchronization problems like producer-consumer   On Windows  binary semaphores are more like event objects than mutexes

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Best Solution  The only difference is   1 Mutex -  lock and unlock are under the ownership of a thread that locks the mutex   2 Semaphore -  No ownership i e  if one thread calls semwait s  any other thread can call sempost s  to remove the lock

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You obviously use mutex to lock a data in one thread getting accessed by another thread at the same time   Assume that you have just called lock   and in the process of accessing data   This means that you don   t expect any other thread  or another instance of the same thread-code  to access the same data locked by the same mutex   That is  if it is the same thread-code getting executed on a different thread instance  hits the lock  then the lock   should block the control flow there   This applies to a thread that uses a different thread-code  which is also accessing the same data and which is also locked by the same mutex   In this case   you are still in the process of accessing the data and you may take  say  another 15 secs to reach the mutex unlock  so that the other thread that is getting blocked in mutex lock would unblock and would allow the control to access the data   Do you at any cost allow yet another thread to just unlock the same mutex  and in turn  allow the thread that is already waiting  blocking  in the mutex lock to unblock and access the data   Hope you got what I am saying here  As per  agreed upon universal definition       with    mutex    this can   t happen   No other thread can unlock the lock in your thread with    binary-semaphore    this can happen  Any other thread can unlock the lock in your thread   So  if you are very particular about using binary-semaphore instead of mutex  then you should be very careful in    scoping    the locks and unlocks  I mean that every control-flow that hits every lock should hit an unlock call  also there shouldn   t be any    first unlock     rather it should be always    first lock

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binary semaphore  is a programming language circumvent to use a   semaphore   like   mutex    Apparently there are two very big differences    The way you call each one of them  The maximum length of the  identifier

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Nice articles on the topic    MUTEX VS  SEMAPHORES     PART 1  SEMAPHORES MUTEX VS  SEMAPHORES     PART 2  THE MUTEX MUTEX VS  SEMAPHORES     PART 3  FINAL PART   MUTUAL EXCLUSION PROBLEMS   From part 2      The mutex is similar to the principles   of the binary semaphore with one   significant difference  the principle   of ownership  Ownership is the simple   concept that when a task locks    acquires  a mutex only it can unlock    release  it  If a task tries to   unlock a mutex it hasn   t locked  thus   doesn   t own  then an error condition   is encountered and  most importantly    the mutex is not unlocked  If the   mutual exclusion object doesn t have   ownership then  irrelevant of what it   is called  it is not a mutex

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Apart from the fact that mutexes have an owner  the two objects may be optimized for different usage  Mutexes are designed to be held only for a short time  violating this can cause poor performance and unfair scheduling  For example  a running thread may be permitted to acquire a mutex  even though another thread is already blocked on it  Semaphores may provide more fairness  or fairness can be forced using several condition variables

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Mutex and binary semaphore are both of the same usage  but in reality  they are different   In case of mutex  only the thread which have locked it can unlock it  If any other thread comes to lock it  it will wait    In case of semaphone  that s not the case  Semaphore is not tied up with a particular thread ID

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At a theoretical level  they are no different semantically   You can implement a mutex using semaphores or vice versa  see here for an example    In practice  the implementations are different and they offer slightly different services  The practical difference  in terms of the system services surrounding them  is that the implementation of a mutex is aimed at being a more lightweight synchronisation mechanism   In oracle-speak  mutexes are known as latches and semaphores are known as waits  At the lowest level  they use some sort of atomic test and set mechanism   This reads the current value of a memory location  computes some sort of conditional and writes out a value at that location in a single instruction that cannot be interrupted   This means that you can acquire a mutex and test to see if anyone else had it before you  A typical mutex implementation has a process or thread executing the test-and-set instruction and evaluating whether anything else had set the mutex    A key point here is that there is no interaction with the scheduler  so we have no idea  and don t care  who has set the lock   Then we either give up our time slice and attempt it again when the task is re-scheduled or execute a spin-lock   A spin lock is an algorithm like  Count down from 5000       i  Execute the test-and-set instruction     ii  If the mutex is clear  we have acquired it in the previous instruction          so we can exit the loop    iii  When we get to zero  give up our time slice   When we have finished executing our protected code  known as a critical section  we just set the mutex value to zero or whatever means  clear    If multiple tasks are attempting to acquire the mutex then the next task that happens to be scheduled after the mutex is released will get access to the resource   Typically you would use mutexes to control a synchronised resource where exclusive access is only needed for very short periods of time  normally to make an update to a shared data structure  A semaphore is a synchronised data structure  typically using a mutex  that has a count and some system call wrappers that interact with the scheduler in a bit more depth than the mutex libraries would   Semaphores are incremented and decremented and used to block tasks until something else is ready   See Producer Consumer Problem for a simple example of this   Semaphores are initialised to some value - a binary semaphore is just a special case where the semaphore is initialised to 1   Posting to a semaphore has the effect of waking up a waiting process  A basic semaphore algorithm looks like   somewhere in the program startup  Initialise the semaphore to its start-up value   Acquiring a semaphore    i   synchronised  Attempt to decrement the semaphore value   ii  If the value would be less than zero  put the task on the tail of the list of tasks waiting on the semaphore and give up the time slice   Posting a semaphore    i   synchronised  Increment the semaphore value   ii  If the value is greater or equal to the amount requested in the post at the front of the queue  take that task off the queue and make it runnable     iii  Repeat  ii  for all tasks until the posted value is exhausted or there are no more tasks waiting   In the case of a binary semaphore the main practical difference between the two is the nature of the system services surrounding the actual data structure  EDIT  As evan has rightly pointed out  spinlocks will slow down a single processor machine   You would only use a spinlock on a multi-processor box because on a single processor the process holding the mutex will never reset it while another task is running   Spinlocks are only useful on multi-processor architectures

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The Toilet example is an enjoyable analogy      Mutex       Is a key to a toilet  One person can   have the key - occupy the toilet - at   the time  When finished  the person   gives  frees  the key to the next   person in the queue       Officially   Mutexes are typically   used to serialise access to a section   of  re-entrant code that cannot be   executed concurrently by more than one   thread  A mutex object only allows one   thread into a controlled section    forcing other threads which attempt to   gain access to that section to wait   until the first thread has exited from   that section   Ref  Symbian Developer   Library       A mutex is really a semaphore with   value 1        Semaphore       Is the number of free identical toilet   keys  Example  say we have four   toilets with identical locks and keys    The semaphore count - the count of   keys - is set to 4 at beginning  all   four toilets are free   then the count   value is decremented as people are   coming in  If all toilets are full    ie  there are no free keys left  the   semaphore count is 0  Now  when eq    one person leaves the toilet    semaphore is increased to 1  one free   key   and given to the next person in   the queue       Officially   A semaphore restricts the   number of simultaneous users of a   shared resource up to a maximum   number  Threads can request access to   the resource  decrementing the   semaphore   and can signal that they   have finished using the resource    incrementing the semaphore    Ref    Symbian Developer Library

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Mutexes have ownership   unlike  semaphores    Although  any thread   within  the  scope  of a mutex  can get an unlocked mutex and lock access to the same critical section of  code only the thread that locked a mutex should unlock it

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They are NOT the same thing   They are used for different purposes  While both types of semaphores have a full empty state and use the same API  their usage is very different     Mutual Exclusion Semaphores Mutual Exclusion semaphores are used to protect shared resources  data structure  file  etc       A Mutex semaphore is  owned  by the task that takes it   If Task B attempts to semGive a mutex currently held by Task A  Task B s call will return an error and fail   Mutexes always use the following sequence        - SemTake   - Critical Section   - SemGive  Here is a simple example      Thread A                     Thread B    Take Mutex      access data                                 Take Mutex   lt    Will block             Give Mutex                     access data   lt    Unblocks                                                                       Give Mutex   Binary Semaphore Binary Semaphore address a totally different question      Task B is pended waiting for something to happen  a sensor being tripped for example   Sensor Trips and an Interrupt Service Routine runs  It needs to notify a task of the trip  Task B should run and take appropriate actions for the sensor trip  Then go back to waiting        Task A                      Task B                                Take BinSemaphore    lt    wait for something    Do Something Noteworthy    Give BinSemaphore           do something     lt    unblocks   Note that with a binary semaphore  it is OK for B to take the semaphore and A to give it  Again  a binary semaphore is NOT protecting a resource from access  The act of Giving and Taking a semaphore are fundamentally decoupled  It typically makes little sense for the same task to so a give and a take on the same binary semaphore

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I think most of the answers here were confusing especially those saying that mutex can be released only by the process that holds it but semaphore can be signaled by ay process  The above line is kind of vague in terms of semaphore  To understand we should know that there are two kinds of semaphore one is called counting semaphore and the other is called a binary semaphore  In counting semaphore handles access to n number of resources where n can be defined before the use  Each semaphore has a count variable  which keeps the count of the number of resources in use  initially  it is set to n  Each process that wishes to uses a resource performs a wait   operation on the semaphore  thereby decrementing the count   When a process releases a resource  it performs a release   operation  incrementing the count   When the count becomes 0  all the resources are being used  After that  the process waits until the count becomes more than 0  Now here is the catch only the process that holds the resource can increase the count no other process can increase the count only the processes holding a resource can increase the count and the process waiting for the semaphore again checks and when it sees the resource available it decreases the count again  So in terms of binary semaphore  only the process holding the semaphore can increase the count  and count remains zero until it stops using the semaphore and increases the count and other process gets the chance to access the semaphore   The main difference between binary semaphore and mutex is that semaphore is a signaling mechanism and mutex is a locking mechanism  but binary semaphore seems to function like mutex that creates confusion  but both are different concepts suitable for a different kinds of work

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While a binary semaphore may be used as a mutex  a mutex is a more specific use-case  in that only the process that locked the mutex is supposed to unlock it  This ownership constraint makes it possible to provide protection against    Accidental release Recursive Deadlock Task Death Deadlock   These constraints are not always present because they degrade the speed  During the development of your code  you can enable these checks  temporarily    e g  you can enable Error check attribute in your mutex  Error checking mutexes return EDEADLK if you try to lock the same one twice and EPERM if you unlock a mutex that isn t yours   pthread mutex t mutex  pthread mutexattr t attr  pthread mutexattr init   amp attr   pthread mutexattr settype   amp attr  PTHREAD MUTEX ERRORCHECK NP   pthread mutex init   amp mutex   amp attr     Once initialised we can place these checks in our code like this   if pthread mutex unlock  amp mutex   EPERM   printf  Unlock failed Mutex not owned by this thread n

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Diff between Binary Semaphore and Mutex  OWNERSHIP  Semaphores can be signalled  posted  even from a non current owner  It means you can simply post from any other thread  though you are not the owner   Semaphore is a public property in process  It can be simply posted by a non owner thread  Please Mark this difference in BOLD letters  it mean a lot

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The basic issue is concurrency  There is more than one flow of control  Think about two processes using a shared memory  Now only one process can access the shared memory at a time  If more than one process accesses the shared memory at a time  the contents of shared memory would get corrupted  It is like a railroad track  Only one train can run on it  else there would be an accident So there is a signalling mechanism  which a driver checks  If the signal is green  the train can go and if it is red it has to wait to use the track  Similarly in case of shared memory  there is a binary semaphore  If the semaphore is 1  a process acquires it  makes it 0  and goes ahead and accesses it  If the semaphore is 0  the process waits  The functionality the binary semaphore has to provide is mutual exclusion  or mutex  in short  so that only one of the many concurrent entities  process or thread  mutually excludes others  It is a plus that we have counting semaphores  which help in synchronizing multiple instances of a resource   Mutual exclusion is the basic functionality provided by semaphores  Now in the context of threads  we might have a different name and syntax for it  But the underlying concept is the same  how to keep integrity of code and data in concurrent programming  In my opinion  things like ownership  and associated checks are refinements provided by implementations

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Diff between Binary Semaphore and Mutex  OWNERSHIP  Semaphores can be signalled  posted  even from a non current owner  It means you can simply post from any other thread  though you are not the owner   Semaphore is a public property in process  It can be simply posted by a non owner thread  Please Mark this difference in BOLD letters  it mean a lot

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The Toilet example is an enjoyable analogy      Mutex       Is a key to a toilet  One person can   have the key - occupy the toilet - at   the time  When finished  the person   gives  frees  the key to the next   person in the queue       Officially   Mutexes are typically   used to serialise access to a section   of  re-entrant code that cannot be   executed concurrently by more than one   thread  A mutex object only allows one   thread into a controlled section    forcing other threads which attempt to   gain access to that section to wait   until the first thread has exited from   that section   Ref  Symbian Developer   Library       A mutex is really a semaphore with   value 1        Semaphore       Is the number of free identical toilet   keys  Example  say we have four   toilets with identical locks and keys    The semaphore count - the count of   keys - is set to 4 at beginning  all   four toilets are free   then the count   value is decremented as people are   coming in  If all toilets are full    ie  there are no free keys left  the   semaphore count is 0  Now  when eq    one person leaves the toilet    semaphore is increased to 1  one free   key   and given to the next person in   the queue       Officially   A semaphore restricts the   number of simultaneous users of a   shared resource up to a maximum   number  Threads can request access to   the resource  decrementing the   semaphore   and can signal that they   have finished using the resource    incrementing the semaphore    Ref    Symbian Developer Library

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At a theoretical level  they are no different semantically   You can implement a mutex using semaphores or vice versa  see here for an example    In practice  the implementations are different and they offer slightly different services  The practical difference  in terms of the system services surrounding them  is that the implementation of a mutex is aimed at being a more lightweight synchronisation mechanism   In oracle-speak  mutexes are known as latches and semaphores are known as waits  At the lowest level  they use some sort of atomic test and set mechanism   This reads the current value of a memory location  computes some sort of conditional and writes out a value at that location in a single instruction that cannot be interrupted   This means that you can acquire a mutex and test to see if anyone else had it before you  A typical mutex implementation has a process or thread executing the test-and-set instruction and evaluating whether anything else had set the mutex    A key point here is that there is no interaction with the scheduler  so we have no idea  and don t care  who has set the lock   Then we either give up our time slice and attempt it again when the task is re-scheduled or execute a spin-lock   A spin lock is an algorithm like  Count down from 5000       i  Execute the test-and-set instruction     ii  If the mutex is clear  we have acquired it in the previous instruction          so we can exit the loop    iii  When we get to zero  give up our time slice   When we have finished executing our protected code  known as a critical section  we just set the mutex value to zero or whatever means  clear    If multiple tasks are attempting to acquire the mutex then the next task that happens to be scheduled after the mutex is released will get access to the resource   Typically you would use mutexes to control a synchronised resource where exclusive access is only needed for very short periods of time  normally to make an update to a shared data structure  A semaphore is a synchronised data structure  typically using a mutex  that has a count and some system call wrappers that interact with the scheduler in a bit more depth than the mutex libraries would   Semaphores are incremented and decremented and used to block tasks until something else is ready   See Producer Consumer Problem for a simple example of this   Semaphores are initialised to some value - a binary semaphore is just a special case where the semaphore is initialised to 1   Posting to a semaphore has the effect of waking up a waiting process  A basic semaphore algorithm looks like   somewhere in the program startup  Initialise the semaphore to its start-up value   Acquiring a semaphore    i   synchronised  Attempt to decrement the semaphore value   ii  If the value would be less than zero  put the task on the tail of the list of tasks waiting on the semaphore and give up the time slice   Posting a semaphore    i   synchronised  Increment the semaphore value   ii  If the value is greater or equal to the amount requested in the post at the front of the queue  take that task off the queue and make it runnable     iii  Repeat  ii  for all tasks until the posted value is exhausted or there are no more tasks waiting   In the case of a binary semaphore the main practical difference between the two is the nature of the system services surrounding the actual data structure  EDIT  As evan has rightly pointed out  spinlocks will slow down a single processor machine   You would only use a spinlock on a multi-processor box because on a single processor the process holding the mutex will never reset it while another task is running   Spinlocks are only useful on multi-processor architectures

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A mutex can be released only by the thread that had acquired it   A binary semaphore can be signaled by any thread  or process     so semaphores are more suitable for some synchronization problems like producer-consumer   On Windows  binary semaphores are more like event objects than mutexes

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Their synchronization semantics are very different    mutexes allow serialization of access to a given resource i e  multiple threads wait for a lock  one at a time and as previously said  the thread owns the lock until it is done  only this particular thread can unlock it  a binary semaphore is a counter with value 0 and 1  a task blocking on it until any task does a sem post  The semaphore advertises that a resource is available  and it provides the mechanism to wait until it is signaled as being available    As such one can see a mutex as a token passed from task to tasks and a semaphore as traffic red-light  it signals someone that it can proceed

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Modified question is - What s the difference between A mutex and a  binary  semaphore in  Linux    Ans  Following are the differences     i  Scope     The scope of mutex is within a process address space which has created it and is used for synchronization of threads  Whereas semaphore can be used across process space and hence it can be used for interprocess synchronization   ii  Mutex is lightweight and faster than semaphore  Futex is even faster   iii  Mutex can be acquired by same thread successfully multiple times with condition that it should release it same number of times  Other thread trying to acquire will block  Whereas in case of semaphore if same process tries to acquire it again it blocks as it can be acquired only once

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MUTEX  Until recently  the only sleeping lock in the kernel was the semaphore  Most users of semaphores instantiated a semaphore with a count of one and treated them as a mutual exclusion lock   a sleeping version of the spin-lock  Unfortunately  semaphores are rather generic and do not impose any usage constraints  This makes them useful for managing exclusive access in obscure situations  such as complicated dances between the kernel and userspace  But it also means that simpler locking is harder to do  and the lack of enforced rules makes any sort of automated debugging or constraint enforcement impossible  Seeking a simpler sleeping lock  the kernel developers introduced the mutex Yes  as you are now accustomed to  that is a confusing name  Let   s clarify The term    mutex    is a generic name to refer to any sleeping lock that enforces mutual exclusion  such as a semaphore with a usage count of one  In recent Linux kernels  the proper noun    mutex    is now also a specific type of sleeping lock that implements mutual exclusion That is  a mutex is a mutex   The simplicity and efficiency of the mutex come from the additional constraints it imposes on its users over and above what the semaphore requires  Unlike a semaphore  which implements the most basic of behaviour in accordance with Dijkstra   s original design  the mutex has a stricter  narrower use case  n Only one task can hold the mutex at a time  That is  the usage count on a mutex is always one     Whoever locked a mutex must unlock it  That is  you cannot lock a mutex in one context and then unlock it in another  This means that the mutex isn   t suitable for more complicated synchronizations between kernel and user-space  Most use cases  however  cleanly lock and unlock from the same context  Recursive locks and unlocks are not allowed  That is  you cannot recursively acquire the same mutex  and you cannot unlock an unlocked mutex  A process cannot exit while holding a mutex  A mutex cannot be acquired by an interrupt handler or bottom half  even with mutex trylock    A mutex can be managed only via the official API  It must be initialized via the methods described in this section and cannot be copied  hand initialized  or reinitialized     1  Linux Kernel Development  Third Edition Robert Love

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Mutex  Mutexes are typically used to serialise access to a section of re-entrant code that cannot be executed concurrently by more than one thread  A mutex object only allows one thread into a controlled section  forcing other threads which attempt to gain access to that section to wait until the first thread has exited from that section Proper use of a mutex is to protect a shared resource can have a dangerous unintended side effect  Any two RTOS tasks that operate at different priorities and coordinate via a mutex  create the opportunity for priority inversion  Mutex works in user space   Semaphore  Semaphore is a signalling mechanism  Semaphore restricts the number of simultaneous users of a shared resource up to a maximum number  Threads can request access to the resource  decrementing the semaphore  and can signal that they have finished using the resource  incrementing the semaphore   It allows number of thread to access shared resources The correct use of a semaphore is for signaling from one task to another semaphores can also be used to signal from an interrupt service routine  ISR  to a task  Signaling a semaphore is a non-blocking RTOS behavior and thus ISR safe   Because this technique eliminates the error-prone need to disable interrupts at the task level This works in kernel space

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In windows the difference is as below  MUTEX  process which successfully executes wait has to execute a signal and vice versa  BINARY SEMAPHORES  Different processes can execute wait or signal operation on a semaphore

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Mutexes have ownership   unlike  semaphores    Although  any thread   within  the  scope  of a mutex  can get an unlocked mutex and lock access to the same critical section of  code only the thread that locked a mutex should unlock it

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Mutex work on blocking critical region  But Semaphore work on count

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Mutex  Suppose we have critical section thread T1 wants to access it then it follows below steps  T1    Lock Use Critical Section Unlock   Binary semaphore  It works based on signaling wait and signal  wait s  decrease  s  value by one usually  s  value is initialize with value  1   signal s  increases  s  value by one  if  s  value is 1 means no one is using critical section  when value is 0 means critical section is in use  suppose thread T2 is using critical section then it follows below steps  T2      wait s   initially s value is one after calling wait it s value decreased by one i e 0 Use critical section signal s     now s value is increased and it become 1   Main difference between Mutex and Binary semaphore is in Mutext if thread lock the critical section then it has to unlock critical section no other thread can unlock it  but in case of Binary semaphore if one thread locks critical section using wait s  function then value of s become  0  and no one can access it until value of  s  become 1 but suppose some other thread calls signal s  then value of  s  become 1 and it allows other function to use critical section  hence in Binary semaphore thread doesn t have ownership

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Apart from the fact that mutexes have an owner  the two objects may be optimized for different usage  Mutexes are designed to be held only for a short time  violating this can cause poor performance and unfair scheduling  For example  a running thread may be permitted to acquire a mutex  even though another thread is already blocked on it  Semaphores may provide more fairness  or fairness can be forced using several condition variables

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Their synchronization semantics are very different    mutexes allow serialization of access to a given resource i e  multiple threads wait for a lock  one at a time and as previously said  the thread owns the lock until it is done  only this particular thread can unlock it  a binary semaphore is a counter with value 0 and 1  a task blocking on it until any task does a sem post  The semaphore advertises that a resource is available  and it provides the mechanism to wait until it is signaled as being available    As such one can see a mutex as a token passed from task to tasks and a semaphore as traffic red-light  it signals someone that it can proceed

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Mutex work on blocking critical region  But Semaphore work on count

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I think most of the answers here were confusing especially those saying that mutex can be released only by the process that holds it but semaphore can be signaled by ay process  The above line is kind of vague in terms of semaphore  To understand we should know that there are two kinds of semaphore one is called counting semaphore and the other is called a binary semaphore  In counting semaphore handles access to n number of resources where n can be defined before the use  Each semaphore has a count variable  which keeps the count of the number of resources in use  initially  it is set to n  Each process that wishes to uses a resource performs a wait   operation on the semaphore  thereby decrementing the count   When a process releases a resource  it performs a release   operation  incrementing the count   When the count becomes 0  all the resources are being used  After that  the process waits until the count becomes more than 0  Now here is the catch only the process that holds the resource can increase the count no other process can increase the count only the processes holding a resource can increase the count and the process waiting for the semaphore again checks and when it sees the resource available it decreases the count again  So in terms of binary semaphore  only the process holding the semaphore can increase the count  and count remains zero until it stops using the semaphore and increases the count and other process gets the chance to access the semaphore   The main difference between binary semaphore and mutex is that semaphore is a signaling mechanism and mutex is a locking mechanism  but binary semaphore seems to function like mutex that creates confusion  but both are different concepts suitable for a different kinds of work

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Mutex are used for   Locking Mechanisms    one process at a time can use a shared resource  whereas  Semaphores are used for   Signaling Mechanisms    like  I am done   now can continue

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The Toilet example is an enjoyable analogy      Mutex       Is a key to a toilet  One person can   have the key - occupy the toilet - at   the time  When finished  the person   gives  frees  the key to the next   person in the queue       Officially   Mutexes are typically   used to serialise access to a section   of  re-entrant code that cannot be   executed concurrently by more than one   thread  A mutex object only allows one   thread into a controlled section    forcing other threads which attempt to   gain access to that section to wait   until the first thread has exited from   that section   Ref  Symbian Developer   Library       A mutex is really a semaphore with   value 1        Semaphore       Is the number of free identical toilet   keys  Example  say we have four   toilets with identical locks and keys    The semaphore count - the count of   keys - is set to 4 at beginning  all   four toilets are free   then the count   value is decremented as people are   coming in  If all toilets are full    ie  there are no free keys left  the   semaphore count is 0  Now  when eq    one person leaves the toilet    semaphore is increased to 1  one free   key   and given to the next person in   the queue       Officially   A semaphore restricts the   number of simultaneous users of a   shared resource up to a maximum   number  Threads can request access to   the resource  decrementing the   semaphore   and can signal that they   have finished using the resource    incrementing the semaphore    Ref    Symbian Developer Library

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The answer may depend on the target OS   For example  at least one RTOS implementation I m familiar with will allow multiple sequential  get  operations against a single OS mutex  so long as they re all from within the same thread context   The multiple gets must be replaced by an equal number of puts before another thread will be allowed to get the mutex   This differs from binary semaphores  for which only a single get is allowed at a time  regardless of thread contexts   The idea behind this type of mutex is that you protect an object by only allowing a single context to modify the data at a time   Even if the thread gets the mutex and then calls a function that further modifies the object  and gets puts the protector mutex around its own operations   the operations should still be safe because they re all happening under a single thread         mutexGet        Other threads can no longer get the mutex          Make changes to the protected object                  objectModify        Also gets puts the mutex   Only allowed from this thread context          Make more changes to the protected object                  mutexPut        Finally allows other threads to get the mutex      Of course  when using this feature  you must be certain that all accesses within a single thread really are safe   I m not sure how common this approach is  or whether it applies outside of the systems with which I m familiar   For an example of this kind of mutex  see the ThreadX RTOS

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The basic issue is concurrency  There is more than one flow of control  Think about two processes using a shared memory  Now only one process can access the shared memory at a time  If more than one process accesses the shared memory at a time  the contents of shared memory would get corrupted  It is like a railroad track  Only one train can run on it  else there would be an accident So there is a signalling mechanism  which a driver checks  If the signal is green  the train can go and if it is red it has to wait to use the track  Similarly in case of shared memory  there is a binary semaphore  If the semaphore is 1  a process acquires it  makes it 0  and goes ahead and accesses it  If the semaphore is 0  the process waits  The functionality the binary semaphore has to provide is mutual exclusion  or mutex  in short  so that only one of the many concurrent entities  process or thread  mutually excludes others  It is a plus that we have counting semaphores  which help in synchronizing multiple instances of a resource   Mutual exclusion is the basic functionality provided by semaphores  Now in the context of threads  we might have a different name and syntax for it  But the underlying concept is the same  how to keep integrity of code and data in concurrent programming  In my opinion  things like ownership  and associated checks are refinements provided by implementations

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Mutex work on blocking critical region  But Semaphore work on count

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A Mutex controls access to a single shared resource  It provides operations to acquire   access to that resource and release   it when done   A Semaphore controls access to a shared pool of resources  It provides operations to Wait   until one of the resources in the pool becomes available  and Signal   when it is given back to the pool   When number of resources a Semaphore protects is greater than 1  it is called a Counting Semaphore  When it controls one resource  it is called a Boolean Semaphore  A boolean semaphore is equivalent to a mutex   Thus a Semaphore is a higher level abstraction than Mutex  A Mutex can be implemented using a Semaphore but not the other way around

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A Mutex controls access to a single shared resource  It provides operations to acquire   access to that resource and release   it when done   A Semaphore controls access to a shared pool of resources  It provides operations to Wait   until one of the resources in the pool becomes available  and Signal   when it is given back to the pool   When number of resources a Semaphore protects is greater than 1  it is called a Counting Semaphore  When it controls one resource  it is called a Boolean Semaphore  A boolean semaphore is equivalent to a mutex   Thus a Semaphore is a higher level abstraction than Mutex  A Mutex can be implemented using a Semaphore but not the other way around

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Mutex and binary semaphore are both of the same usage  but in reality  they are different   In case of mutex  only the thread which have locked it can unlock it  If any other thread comes to lock it  it will wait    In case of semaphone  that s not the case  Semaphore is not tied up with a particular thread ID

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Myth   Couple of article says that  binary semaphore and mutex are same  or  Semaphore with value 1 is mutex  but the basic difference is Mutex can be released only by thread that had acquired it  while you can signal semaphore from any other thread  Key Points      A thread can acquire more than one lock  Mutex       A mutex can be locked more than once only if its a recursive mutex  here lock and unlock for mutex should be same     If a thread which had already locked a mutex  tries to lock the mutex again  it will enter into the waiting list of that mutex  which results in deadlock       Binary semaphore and mutex are similar but not same      Mutex is costly operation due to protection protocols associated with it      Main aim of mutex is achieve atomic access or lock on resource

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You obviously use mutex to lock a data in one thread getting accessed by another thread at the same time   Assume that you have just called lock   and in the process of accessing data   This means that you don   t expect any other thread  or another instance of the same thread-code  to access the same data locked by the same mutex   That is  if it is the same thread-code getting executed on a different thread instance  hits the lock  then the lock   should block the control flow there   This applies to a thread that uses a different thread-code  which is also accessing the same data and which is also locked by the same mutex   In this case   you are still in the process of accessing the data and you may take  say  another 15 secs to reach the mutex unlock  so that the other thread that is getting blocked in mutex lock would unblock and would allow the control to access the data   Do you at any cost allow yet another thread to just unlock the same mutex  and in turn  allow the thread that is already waiting  blocking  in the mutex lock to unblock and access the data   Hope you got what I am saying here  As per  agreed upon universal definition       with    mutex    this can   t happen   No other thread can unlock the lock in your thread with    binary-semaphore    this can happen  Any other thread can unlock the lock in your thread   So  if you are very particular about using binary-semaphore instead of mutex  then you should be very careful in    scoping    the locks and unlocks  I mean that every control-flow that hits every lock should hit an unlock call  also there shouldn   t be any    first unlock     rather it should be always    first lock

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The answer may depend on the target OS   For example  at least one RTOS implementation I m familiar with will allow multiple sequential  get  operations against a single OS mutex  so long as they re all from within the same thread context   The multiple gets must be replaced by an equal number of puts before another thread will be allowed to get the mutex   This differs from binary semaphores  for which only a single get is allowed at a time  regardless of thread contexts   The idea behind this type of mutex is that you protect an object by only allowing a single context to modify the data at a time   Even if the thread gets the mutex and then calls a function that further modifies the object  and gets puts the protector mutex around its own operations   the operations should still be safe because they re all happening under a single thread         mutexGet        Other threads can no longer get the mutex          Make changes to the protected object                  objectModify        Also gets puts the mutex   Only allowed from this thread context          Make more changes to the protected object                  mutexPut        Finally allows other threads to get the mutex      Of course  when using this feature  you must be certain that all accesses within a single thread really are safe   I m not sure how common this approach is  or whether it applies outside of the systems with which I m familiar   For an example of this kind of mutex  see the ThreadX RTOS

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On Windows  there are two differences between mutexes and binary semaphores    A mutex can only be released by the thread which has ownership  i e  the thread which previously called the Wait function   or which took ownership when creating it   A semaphore can be released by any thread  A thread can call a wait function repeatedly on a mutex without blocking  However  if you call a wait function twice on a binary semaphore without releasing the semaphore in between  the thread will block

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At a theoretical level  they are no different semantically   You can implement a mutex using semaphores or vice versa  see here for an example    In practice  the implementations are different and they offer slightly different services  The practical difference  in terms of the system services surrounding them  is that the implementation of a mutex is aimed at being a more lightweight synchronisation mechanism   In oracle-speak  mutexes are known as latches and semaphores are known as waits  At the lowest level  they use some sort of atomic test and set mechanism   This reads the current value of a memory location  computes some sort of conditional and writes out a value at that location in a single instruction that cannot be interrupted   This means that you can acquire a mutex and test to see if anyone else had it before you  A typical mutex implementation has a process or thread executing the test-and-set instruction and evaluating whether anything else had set the mutex    A key point here is that there is no interaction with the scheduler  so we have no idea  and don t care  who has set the lock   Then we either give up our time slice and attempt it again when the task is re-scheduled or execute a spin-lock   A spin lock is an algorithm like  Count down from 5000       i  Execute the test-and-set instruction     ii  If the mutex is clear  we have acquired it in the previous instruction          so we can exit the loop    iii  When we get to zero  give up our time slice   When we have finished executing our protected code  known as a critical section  we just set the mutex value to zero or whatever means  clear    If multiple tasks are attempting to acquire the mutex then the next task that happens to be scheduled after the mutex is released will get access to the resource   Typically you would use mutexes to control a synchronised resource where exclusive access is only needed for very short periods of time  normally to make an update to a shared data structure  A semaphore is a synchronised data structure  typically using a mutex  that has a count and some system call wrappers that interact with the scheduler in a bit more depth than the mutex libraries would   Semaphores are incremented and decremented and used to block tasks until something else is ready   See Producer Consumer Problem for a simple example of this   Semaphores are initialised to some value - a binary semaphore is just a special case where the semaphore is initialised to 1   Posting to a semaphore has the effect of waking up a waiting process  A basic semaphore algorithm looks like   somewhere in the program startup  Initialise the semaphore to its start-up value   Acquiring a semaphore    i   synchronised  Attempt to decrement the semaphore value   ii  If the value would be less than zero  put the task on the tail of the list of tasks waiting on the semaphore and give up the time slice   Posting a semaphore    i   synchronised  Increment the semaphore value   ii  If the value is greater or equal to the amount requested in the post at the front of the queue  take that task off the queue and make it runnable     iii  Repeat  ii  for all tasks until the posted value is exhausted or there are no more tasks waiting   In the case of a binary semaphore the main practical difference between the two is the nature of the system services surrounding the actual data structure  EDIT  As evan has rightly pointed out  spinlocks will slow down a single processor machine   You would only use a spinlock on a multi-processor box because on a single processor the process holding the mutex will never reset it while another task is running   Spinlocks are only useful on multi-processor architectures

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Nice articles on the topic    MUTEX VS  SEMAPHORES     PART 1  SEMAPHORES MUTEX VS  SEMAPHORES     PART 2  THE MUTEX MUTEX VS  SEMAPHORES     PART 3  FINAL PART   MUTUAL EXCLUSION PROBLEMS   From part 2      The mutex is similar to the principles   of the binary semaphore with one   significant difference  the principle   of ownership  Ownership is the simple   concept that when a task locks    acquires  a mutex only it can unlock    release  it  If a task tries to   unlock a mutex it hasn   t locked  thus   doesn   t own  then an error condition   is encountered and  most importantly    the mutex is not unlocked  If the   mutual exclusion object doesn t have   ownership then  irrelevant of what it   is called  it is not a mutex

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The answer may depend on the target OS   For example  at least one RTOS implementation I m familiar with will allow multiple sequential  get  operations against a single OS mutex  so long as they re all from within the same thread context   The multiple gets must be replaced by an equal number of puts before another thread will be allowed to get the mutex   This differs from binary semaphores  for which only a single get is allowed at a time  regardless of thread contexts   The idea behind this type of mutex is that you protect an object by only allowing a single context to modify the data at a time   Even if the thread gets the mutex and then calls a function that further modifies the object  and gets puts the protector mutex around its own operations   the operations should still be safe because they re all happening under a single thread         mutexGet        Other threads can no longer get the mutex          Make changes to the protected object                  objectModify        Also gets puts the mutex   Only allowed from this thread context          Make more changes to the protected object                  mutexPut        Finally allows other threads to get the mutex      Of course  when using this feature  you must be certain that all accesses within a single thread really are safe   I m not sure how common this approach is  or whether it applies outside of the systems with which I m familiar   For an example of this kind of mutex  see the ThreadX RTOS

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Mutex  Suppose we have critical section thread T1 wants to access it then it follows below steps  T1    Lock Use Critical Section Unlock   Binary semaphore  It works based on signaling wait and signal  wait s  decrease  s  value by one usually  s  value is initialize with value  1   signal s  increases  s  value by one  if  s  value is 1 means no one is using critical section  when value is 0 means critical section is in use  suppose thread T2 is using critical section then it follows below steps  T2      wait s   initially s value is one after calling wait it s value decreased by one i e 0 Use critical section signal s     now s value is increased and it become 1   Main difference between Mutex and Binary semaphore is in Mutext if thread lock the critical section then it has to unlock critical section no other thread can unlock it  but in case of Binary semaphore if one thread locks critical section using wait s  function then value of s become  0  and no one can access it until value of  s  become 1 but suppose some other thread calls signal s  then value of  s  become 1 and it allows other function to use critical section  hence in Binary semaphore thread doesn t have ownership

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A mutex can be released only by the thread that had acquired it   A binary semaphore can be signaled by any thread  or process     so semaphores are more suitable for some synchronization problems like producer-consumer   On Windows  binary semaphores are more like event objects than mutexes

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Mutex is used to protect the sensitive code and data  semaphore is used to synchronization You also can have practical use with protect the sensitive code  but there might be a risk that release the protection by the other thread by operation V So The main difference between bi-semaphore and mutex is the ownership For instance by toilet   Mutex is like that one can enter the toilet and lock the door  no one else can enter until the man get out  bi-semaphore is like that one can  enter the toilet and lock the door  but someone else could enter by asking the administrator to open the door  it s ridiculous

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MUTEX  Until recently  the only sleeping lock in the kernel was the semaphore  Most users of semaphores instantiated a semaphore with a count of one and treated them as a mutual exclusion lock   a sleeping version of the spin-lock  Unfortunately  semaphores are rather generic and do not impose any usage constraints  This makes them useful for managing exclusive access in obscure situations  such as complicated dances between the kernel and userspace  But it also means that simpler locking is harder to do  and the lack of enforced rules makes any sort of automated debugging or constraint enforcement impossible  Seeking a simpler sleeping lock  the kernel developers introduced the mutex Yes  as you are now accustomed to  that is a confusing name  Let   s clarify The term    mutex    is a generic name to refer to any sleeping lock that enforces mutual exclusion  such as a semaphore with a usage count of one  In recent Linux kernels  the proper noun    mutex    is now also a specific type of sleeping lock that implements mutual exclusion That is  a mutex is a mutex   The simplicity and efficiency of the mutex come from the additional constraints it imposes on its users over and above what the semaphore requires  Unlike a semaphore  which implements the most basic of behaviour in accordance with Dijkstra   s original design  the mutex has a stricter  narrower use case  n Only one task can hold the mutex at a time  That is  the usage count on a mutex is always one     Whoever locked a mutex must unlock it  That is  you cannot lock a mutex in one context and then unlock it in another  This means that the mutex isn   t suitable for more complicated synchronizations between kernel and user-space  Most use cases  however  cleanly lock and unlock from the same context  Recursive locks and unlocks are not allowed  That is  you cannot recursively acquire the same mutex  and you cannot unlock an unlocked mutex  A process cannot exit while holding a mutex  A mutex cannot be acquired by an interrupt handler or bottom half  even with mutex trylock    A mutex can be managed only via the official API  It must be initialized via the methods described in this section and cannot be copied  hand initialized  or reinitialized     1  Linux Kernel Development  Third Edition Robert Love

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They are NOT the same thing   They are used for different purposes  While both types of semaphores have a full empty state and use the same API  their usage is very different     Mutual Exclusion Semaphores Mutual Exclusion semaphores are used to protect shared resources  data structure  file  etc       A Mutex semaphore is  owned  by the task that takes it   If Task B attempts to semGive a mutex currently held by Task A  Task B s call will return an error and fail   Mutexes always use the following sequence        - SemTake   - Critical Section   - SemGive  Here is a simple example      Thread A                     Thread B    Take Mutex      access data                                 Take Mutex   lt    Will block             Give Mutex                     access data   lt    Unblocks                                                                       Give Mutex   Binary Semaphore Binary Semaphore address a totally different question      Task B is pended waiting for something to happen  a sensor being tripped for example   Sensor Trips and an Interrupt Service Routine runs  It needs to notify a task of the trip  Task B should run and take appropriate actions for the sensor trip  Then go back to waiting        Task A                      Task B                                Take BinSemaphore    lt    wait for something    Do Something Noteworthy    Give BinSemaphore           do something     lt    unblocks   Note that with a binary semaphore  it is OK for B to take the semaphore and A to give it  Again  a binary semaphore is NOT protecting a resource from access  The act of Giving and Taking a semaphore are fundamentally decoupled  It typically makes little sense for the same task to so a give and a take on the same binary semaphore

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In windows the difference is as below  MUTEX  process which successfully executes wait has to execute a signal and vice versa  BINARY SEMAPHORES  Different processes can execute wait or signal operation on a semaphore

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They are NOT the same thing   They are used for different purposes  While both types of semaphores have a full empty state and use the same API  their usage is very different     Mutual Exclusion Semaphores Mutual Exclusion semaphores are used to protect shared resources  data structure  file  etc       A Mutex semaphore is  owned  by the task that takes it   If Task B attempts to semGive a mutex currently held by Task A  Task B s call will return an error and fail   Mutexes always use the following sequence        - SemTake   - Critical Section   - SemGive  Here is a simple example      Thread A                     Thread B    Take Mutex      access data                                 Take Mutex   lt    Will block             Give Mutex                     access data   lt    Unblocks                                                                       Give Mutex   Binary Semaphore Binary Semaphore address a totally different question      Task B is pended waiting for something to happen  a sensor being tripped for example   Sensor Trips and an Interrupt Service Routine runs  It needs to notify a task of the trip  Task B should run and take appropriate actions for the sensor trip  Then go back to waiting        Task A                      Task B                                Take BinSemaphore    lt    wait for something    Do Something Noteworthy    Give BinSemaphore           do something     lt    unblocks   Note that with a binary semaphore  it is OK for B to take the semaphore and A to give it  Again  a binary semaphore is NOT protecting a resource from access  The act of Giving and Taking a semaphore are fundamentally decoupled  It typically makes little sense for the same task to so a give and a take on the same binary semaphore

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Though mutex  amp  semaphores are used as synchronization primitives  there is a big difference between them  In the case of mutex  only the thread that locked or acquired the mutex can unlock it  In the case of a semaphore  a thread waiting on a semaphore can be signaled by a different thread  Some operating system supports using mutex  amp  semaphores between process  Typically usage is creating in shared memory

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Mutex is used to protect the sensitive code and data  semaphore is used to synchronization You also can have practical use with protect the sensitive code  but there might be a risk that release the protection by the other thread by operation V So The main difference between bi-semaphore and mutex is the ownership For instance by toilet   Mutex is like that one can enter the toilet and lock the door  no one else can enter until the man get out  bi-semaphore is like that one can  enter the toilet and lock the door  but someone else could enter by asking the administrator to open the door  it s ridiculous

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Mutex  Mutexes are typically used to serialise access to a section of re-entrant code that cannot be executed concurrently by more than one thread  A mutex object only allows one thread into a controlled section  forcing other threads which attempt to gain access to that section to wait until the first thread has exited from that section Proper use of a mutex is to protect a shared resource can have a dangerous unintended side effect  Any two RTOS tasks that operate at different priorities and coordinate via a mutex  create the opportunity for priority inversion  Mutex works in user space   Semaphore  Semaphore is a signalling mechanism  Semaphore restricts the number of simultaneous users of a shared resource up to a maximum number  Threads can request access to the resource  decrementing the semaphore  and can signal that they have finished using the resource  incrementing the semaphore   It allows number of thread to access shared resources The correct use of a semaphore is for signaling from one task to another semaphores can also be used to signal from an interrupt service routine  ISR  to a task  Signaling a semaphore is a non-blocking RTOS behavior and thus ISR safe   Because this technique eliminates the error-prone need to disable interrupts at the task level This works in kernel space

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The Toilet example is an enjoyable analogy      Mutex       Is a key to a toilet  One person can   have the key - occupy the toilet - at   the time  When finished  the person   gives  frees  the key to the next   person in the queue       Officially   Mutexes are typically   used to serialise access to a section   of  re-entrant code that cannot be   executed concurrently by more than one   thread  A mutex object only allows one   thread into a controlled section    forcing other threads which attempt to   gain access to that section to wait   until the first thread has exited from   that section   Ref  Symbian Developer   Library       A mutex is really a semaphore with   value 1        Semaphore       Is the number of free identical toilet   keys  Example  say we have four   toilets with identical locks and keys    The semaphore count - the count of   keys - is set to 4 at beginning  all   four toilets are free   then the count   value is decremented as people are   coming in  If all toilets are full    ie  there are no free keys left  the   semaphore count is 0  Now  when eq    one person leaves the toilet    semaphore is increased to 1  one free   key   and given to the next person in   the queue       Officially   A semaphore restricts the   number of simultaneous users of a   shared resource up to a maximum   number  Threads can request access to   the resource  decrementing the   semaphore   and can signal that they   have finished using the resource    incrementing the semaphore    Ref    Symbian Developer Library

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http   www geeksforgeeks org archives 9102  discusses in details   Mutex is locking mechanism used to synchronize access to a resource  Semaphore is signaling mechanism    Its up to to programmer if he she wants to use  binary semaphore in place of mutex

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On Windows  there are two differences between mutexes and binary semaphores    A mutex can only be released by the thread which has ownership  i e  the thread which previously called the Wait function   or which took ownership when creating it   A semaphore can be released by any thread  A thread can call a wait function repeatedly on a mutex without blocking  However  if you call a wait function twice on a binary semaphore without releasing the semaphore in between  the thread will block

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Almost all of the above said it right  Let me also try my bit to clarify if somebody still has a doubt    Mutex -  used for serialization Semaphore-  synchronization    Purpose of both are different however  same functionality could be achieved through both of them with careful programming   Standard Example-  producer consumer problem   initial value of SemaVar 0  Producer                           Consumer ---                                SemaWait  - gt decrement SemaVar    produce data --- SemaSignal SemaVar or SemaVar    --- gt consumer unblocks as SemVar is 1 now    Hope I could clarify

[operating-system] Difference between binary semaphore and mutex

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