Mutex

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Mutex
You are encouraged to solve this task according to the task description, using any language you may know.

A mutex (abbreviated Mutually Exclusive access) is a synchronization object, a variant of semaphore with k=1. A mutex is said to be seized by a task decreasing k. It is released when the task restores k. Mutexes are typically used to protect a shared resource from concurrent access. A task seizes (or acquires) the mutex, then accesses the resource, and after that releases the mutex.

A mutex is a low-level synchronization primitive exposed to deadlocking. A deadlock can occur with just two tasks and two mutexes (if each task attempts to acquire both mutexes, but in the opposite order). Entering the deadlock is usually aggravated by a race condition state, which leads to sporadic hangups, which are very difficult to track down.

Contents

[edit] Variants of mutexes

[edit] Global and local mutexes

Usually the OS provides various implementations of mutexes corresponding to the variants of tasks available in the OS. For example, system-wide mutexes can be used by processes. Local mutexes can be used only by threads etc. This distinction is maintained because, depending on the hardware, seizing a global mutex might be a thousand times slower than seizing a local one.

[edit] Reentrant mutex

A reentrant mutex can be seized by the same task multiple times. Each seizing of the mutex is matched by releasing it, in order to allow another task to seize it.

[edit] Read write mutex

A read write mutex can be seized at two levels for read and for write. The mutex can be seized for read by any number of tasks. Only one task may seize it for 'write. Read write mutexes are usually used to protect resources which can be accessed in mutable and immutable ways. Immutable (read) access is granted concurrently for many tasks because they do not change the resource state. Read write mutexes can be reentrant, global or local. Further, promotion operations may be provided. That's when a task that has seized the mutex for write releases it while keeping seized for read. Note that the reverse operation is potentially deadlocking and requires some additional access policy control.

[edit] Deadlock prevention

There exists a simple technique of deadlock prevention when mutexes are seized in some fixed order. This is discussed in depth in the Dining philosophers problem.

[edit] Sample implementations / APIs

[edit] Ada

Ada provides higher-level concurrency primitives, which are complete in the sense that they also allow implementations of the lower-level ones, like mutexes. Here is an implementation of a plain non-reentrant mutex based on protected objects.

The mutex interface:

protected type Mutex is
entry Seize;
procedure Release;
private
Owned : Boolean := False;
end Mutex;

The implementation of:

protected body Mutex is
entry Seize when not Owned is
begin
Owned := True;
end Seize;
procedure Release is
begin
Owned := False;
end Release;
end Mutex;

Here the entry Seize has a queue of the tasks waiting for the mutex. The entry's barrier is closed when Owned is true. So any task calling to the entry will be queued. When the barrier is open the first task from the queue executes the entry and Owned becomes true closing the barrier again. The procedure Release simply sets Owned to false. Both Seize and Release are protected actions whose execution causes reevaluation of all barriers, in this case one of Seize.

Use:

declare
M : Mutex;
begin
M.Seize; -- Wait infinitely for the mutex to be free
... -- Critical code
M.Release; -- Release the mutex
...
select
M.Seize; -- Wait no longer than 0.5s
or delay 0.5;
raise Timed_Out;
end select;
... -- Critical code
M.Release; -- Release the mutex
end;

It is also possible to implement mutex as a monitor task.

[edit] BBC BASIC

      REM Create mutex:
SYS "CreateMutex", 0, 0, 0 TO hMutex%
 
REM Wait to acquire mutex:
REPEAT
SYS "WaitForSingleObject", hMutex%, 1 TO res%
UNTIL res% = 0
 
REM Release mutex:
SYS "ReleaseMutex", hMutex%
 
REM Free mutex:
SYS "CloseHandle", hMutex%

[edit] C

[edit] Win32

Works with: Win32

To create a mutex operating system "object":

HANDLE hMutex = CreateMutex(NULL, FALSE, NULL);

To lock the mutex:

WaitForSingleObject(hMutex, INFINITE);

To unlock the mutex

ReleaseMutex(hMutex);

When the program is finished with the mutex:

CloseHandle(hMutex);

[edit] POSIX

Works with: POSIX

Creating a mutex:

#include <pthread.h>
 
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

Or:

pthread_mutex_t mutex;
pthread_mutex_init(&mutex, NULL);

Locking:

int error = pthread_mutex_lock(&mutex);

Unlocking:

int error = pthread_mutex_unlock(&mutex);

Trying to lock (but do not wait if it can't)

int error = pthread_mutex_trylock(&mutex);

[edit] C++

[edit] Win32

Works with: Win32
See C example

[edit] POSIX

Works with: POSIX
See C example

[edit] D

 
class Synced
{
public:
synchronized int func (int input)
{
num += input;
return num;
}
private:
static num = 0;
}
 

Keep in mind that synchronized used as above works on a per-class-instance basis. This is described in [[1]].

The following example tries to illustrate the problem:

import tango.core.Thread, tango.io.Stdout, tango.util.log.Trace;
 
class Synced {
public synchronized int func (int input) {
Trace.formatln("in {} at func enter: {}", input, foo);
// stupid loop to consume some time
int arg;
for (int i = 0; i < 1000*input; ++i) {
for (int j = 0; j < 10_000; ++j) arg += j;
}
foo += input;
Trace.formatln("in {} at func exit: {}", input, foo);
return arg;
}
private static int foo;
}
 
void main(char[][] args) {
SimpleThread[] ht;
Stdout.print( "Starting application..." ).newline;
 
for (int i=0; i < 3; i++) {
Stdout.print( "Starting thread for: " )(i).newline;
ht ~= new SimpleThread(i+1);
ht[i].start();
}
 
// wait for all threads
foreach( s; ht )
s.join();
}
 
class SimpleThread : Thread
{
private int d_id;
this (int id) {
super (&run);
d_id = id;
}
 
void run() {
auto tested = new Synced;
Trace.formatln ("in run() {}", d_id);
tested.func(d_id);
}
}

Every created thread creates its own Synced object, and because the monitor created by synchronized statement is created for every object, each thread can enter the func() method.

To resolve that either func() could be done static (static member functions are synchronized per-class basis) or synchronized block should be used like here:

 
class Synced {
public int func (int input) {
synchronized(Synced.classinfo) {
// ...
foo += input;
// ...
}
return arg;
}
private static int foo;
}
 

[edit] E

E's approach to concurrency is to never block, in favor of message passing/event queues/callbacks. Therefore, it is unidiomatic to use a mutex at all, and incorrect, or rather unsafe, to use a mutex which blocks the calling thread. That said, here is a mutex written in E.

def makeMutex() {
 
# The mutex is available (released) if available is resolved, otherwise it
# has been seized/locked. The specific value of available is irrelevant.
var available := null
 
# The interface to the mutex is a function, taking a function (action)
# to be executed.
def mutex(action) {
# By assigning available to our promise here, the mutex remains
# unavailable to the /next/ caller until /this/ action has gotten
# its turn /and/ resolved its returned value.
available := Ref.whenResolved(available, fn _ { action <- () })
}
return mutex
}

This implementation of a mutex is designed to have a very short implementation as well as usage in E. The mutex object is a function which takes a function action to be executed once the mutex is available. The mutex is unavailable until the return value of action resolves. This interface has been chosen over lock and unlock operations to reduce the hazard of unbalanced lock/unlock pairs, and because it naturally fits into E code.

Usage example:

Creating the mutex:
 
? def mutex := makeMutex()
# value: <mutex>
 
Creating the shared resource:
 
? var value := 0
# value: 0
 
Manipulating the shared resource non-atomically so as to show a problem:
 
? for _ in 0..1 {
> when (def v := (&value) <- get()) -> {
> (&value) <- put(v + 1)
> }
> }
 
? value
# value: 1
 
The value has been incremented twice, but non-atomically, and so is 1 rather
than the intended 2.
 
? value := 0
# value: 0
 
This time, we use the mutex to protect the action.
 
? for _ in 0..1 {
> mutex(fn {
> when (def v := (&value) <- get()) -> {
> (&value) <- put(v + 1)
> }
> })
> }
 
? value
# value: 2

when blocks and Ref.whenResolved return a promise for the result of the deferred action, so the mutex here waits for the gratuitously complicated increment to complete before becoming available for the next action.

[edit] Erlang

Erlang has no mutexes so this is a super simple one, hand built to allow 3 slowly printing processes to print until done before the next one starts.

 
-module( mutex ).
 
-export( [task/0] ).
 
task() ->
Mutex = erlang:spawn( fun() -> loop() end ),
[erlang:spawn(fun() -> random:seed( X, 0, 0 ), print(Mutex, X, 3) end) || X <- lists:seq(1, 3)].
 
 
 
loop() ->
receive
{acquire, Pid} ->
Pid ! {access, erlang:self()},
receive
{release, Pid} -> loop()
end
end.
 
mutex_acquire( Pid ) ->
Pid ! {acquire, erlang:self()},
receive
{access, Pid} -> ok
end.
 
mutex_release( Pid ) -> Pid ! {release, erlang:self()}.
 
print( _Mutex, _N, 0 ) -> ok;
print( Mutex, N, M ) ->
timer:sleep( random:uniform(100) ),
mutex_acquire( Mutex ),
io:fwrite( "Print ~p: ", [N] ),
[print_slow(X) || X <- lists:seq(1, 3)],
io:nl(),
mutex_release( Mutex ),
print( Mutex, N, M - 1 ).
 
print_slow( X ) ->
io:fwrite( " ~p", [X] ),
timer:sleep( 100 ).
 
Output:
27> mutex:task().
Print 2:  1 2 3    
Print 1:  1 2 3    
Print 3:  1 2 3    
Print 2:  1 2 3    
Print 1:  1 2 3    
Print 3:  1 2 3    
Print 2:  1 2 3    
Print 1:  1 2 3    
Print 3:  1 2 3    

[edit] Go

[edit] sync.Mutex

Translation of: E

Go has mutexes, and here is an example use of a mutex, somewhat following the example of E. This code defines a slow incrementer, that reads a variable, then a significant amount of time later, writes an incremented value back to the variable. Two incrementers are started concurrently. Without the mutex, one would overwrite the other and the result would be 1. Using a mutex, as shown here, one waits for the other and the result is 2.

package main
 
import (
"fmt"
"sync"
"time"
)
 
var value int
var m sync.Mutex
var wg sync.WaitGroup
 
func slowInc() {
m.Lock()
v := value
time.Sleep(1e8)
value = v+1
m.Unlock()
wg.Done()
}
 
func main() {
wg.Add(2)
go slowInc()
go slowInc()
wg.Wait()
fmt.Println(value)
}

Output:

2

Read-write mutex is provided by the sync.RWMutex type. For a code example using a RWMutex, see Atomic updates#RWMutex.

[edit] Channels

If a mutex is exactly what you need, sync.Mutex is there. As soon as things start getting complicated though, Go channels offer a much clearer alternative. As a gateway from mutexes to channels, here is the above program implemented with channels:

package main
 
import (
"fmt"
"time"
)
 
var value int
 
func slowInc(ch, done chan bool) {
// channel receive, used here to implement mutex lock.
// it will block until a value is available on the channel
<-ch
 
// same as above
v := value
time.Sleep(1e8)
value = v + 1
 
// channel send, equivalent to mutex unlock.
// makes a value available on channel
ch <- true
 
// channels can be used to signal completion too
done <- true
}
 
func main() {
ch := make(chan bool, 1) // ch used as a mutex
done := make(chan bool) // another channel used to signal completion
go slowInc(ch, done)
go slowInc(ch, done)
// a freshly created sync.Mutex starts out unlocked, but a freshly created
// channel is empty, which for us represents "locked." sending a value on
// the channel puts the value up for grabs, thus representing "unlocked."
ch <- true
<-done
<-done
fmt.Println(value)
}

The value passed on the channel is not accessed here, just as the internal state of a mutex is not accessed. Rather, it is only the effect of the value being available that is important. (Of course if you wanted to send something meaningful on the channel, a reference to the shared resource would be a good start...)

[edit] Haskell

Haskell has a slight variation on the mutex, namely the MVar. MVars, unlike mutexes, are containers. However, they are similar enough that MVar () is essentially a mutex. A MVar can be in two states: empty or full, only storing a value when full. There are 4 main ways to deal with MVars:

takeMVar :: MVar a -> IO a
putMVar :: MVar a -> a -> IO ()
tryTakeMVar :: MVar a -> IO (Maybe a)
tryPutMVar :: MVar a -> a -> IO Bool
 

takeMVar will attempt to fetch a value from the MVar, and will block while the MVar is empty. After using this, the MVar will be left empty. putMVar will attempt to put a value in a MVar, and will block while there already is a value in the MVar. This will leave the MVar full. The last two functions are non-blocking versions of takeMVar and putMVar, returning Nothing and False, respectively, if their blocking counterpart would have blocked.

For more information see the documentation.

[edit] Icon and Unicon

The following code uses features exclusive to Unicon.

 
 
x: = mutex() # create and return a mutex handle for sharing between threads needing to synchronize with each other
 
lock(x) # lock mutex x
 
trylock(x)) # non-blocking lock, succeeds only if there are no other thread already in the critical region
 
unlock(x) # unlock mutex x
 
 

[edit] Java

Works with: Java version 1.5+

Java 5 added a Semaphore class which can act as a mutex (as stated above, a mutex is "a variant of semaphore with k=1").

import java.util.concurrent.Semaphore;
 
public class VolatileClass{
public Semaphore mutex = new Semaphore(1); //also a "fair" boolean may be passed which,
//when true, queues requests for the lock
public void needsToBeSynched(){
//...
}
//delegate methods could be added for acquiring and releasing the mutex
}

Using the mutex:

public class TestVolitileClass throws Exception{
public static void main(String[] args){
VolatileClass vc = new VolatileClass();
vc.mutex.acquire(); //will wait automatically if another class has the mutex
//can be interrupted similarly to a Thread
//use acquireUninterruptibly() to avoid that
vc.needsToBeSynched();
vc.mutex.release();
}
}

Java also has the synchronized keyword, which allows almost any object to be used to enforce mutual exclusion.

public class Main {
static Object mutex = new Object();
static int i = 0;
 
public void addAndPrint()
{
System.out.print("" + i + " + 1 = ");
i++;
System.out.println("" + i);
}
 
public void subAndPrint()
{
System.out.print("" + i + " - 1 = ");
i--;
System.out.println("" + i);
}
 
 
public static void main(String[] args){
final Main m = new Main();
new Thread() {
public void run()
{
while (true) { synchronized(m.mutex) { m.addAndPrint(); } }
}
}.start();
new Thread() {
public void run()
{
while (true) { synchronized(m.mutex) { m.subAndPrint(); } }
}
}.start();
}
}

The "synchronized" keyword actually is a form of monitor, which was a later-proposed solution to the same problems that mutexes and semaphores were designed to solve. More about synchronization may be found on Sun's website - http://java.sun.com/docs/books/tutorial/essential/concurrency/sync.html , and more about monitors may be found in any decent operating systems textbook.

[edit] Logtalk

Logtalk provides a synchronized/0 directive for synchronizing all object (or category) predicates using the same implicit mutex and a synchronized/1 directive for synchronizing a set of predicates using the same implicit mutex. Follow an usage example of the synchronized/1 directive (inspired by the Erlang example). Works when using SWI-Prolog, XSB, or YAP as the backend compiler.

 
:- object(slow_print).
 
:- threaded.
 
:- public(start/0).
 
:- private([slow_print_abc/0, slow_print_123/0]).
:- synchronized([slow_print_abc/0, slow_print_123/0]).
 
start :-
% launch two threads, running never ending goals
threaded((
repeat_abc,
repeat_123
)).
 
repeat_abc :-
repeat, slow_print_abc, fail.
 
repeat_123 :-
repeat, slow_print_123, fail.
 
slow_print_abc :-
write(a), thread_sleep(0.2),
write(b), thread_sleep(0.2),
write(c), nl.
 
slow_print_123 :-
write(1), thread_sleep(0.2),
write(2), thread_sleep(0.2),
write(3), nl.
 
:- end_object.
 

Sample output:

 
?- slow_print::start.
abc
123
abc
123
abc
123
abc
123
abc
...
 

[edit] Objective-C

NSLock *m = [[NSLock alloc] init];
 
[m lock]; // locks in blocking mode
 
if ([m tryLock]) { // acquire a lock -- does not block if not acquired
// lock acquired
} else {
// already locked, does not block
}
 
[m unlock];

Reentrant mutex is provided by the NSRecursiveLock class.

Objective-C also has @synchronized() blocks, like Java.

[edit] Objeck

Objeck provides a simple way to lock a section of code. Please refer to the programer's guide for addition information.

 
m := ThreadMutex->New("lock a");
# section locked
critical(m) {
...
}
# section unlocked
 

[edit] OCaml

OCaml provides a built-in Mutex module.
It is very simple, there are four functions:

let m = Mutex.create() in
Mutex.lock m; (* locks in blocking mode *)
 
if (Mutex.try_lock m)
then ... (* did the lock *)
else ... (* already locked, do not block *)
 
Mutex.unlock m;

[edit] Oz

Oz has "locks" which are local, reentrant mutexes.

Creating a mutex:

declare L = {Lock.new}

The only way to acquire a mutex is to use the lock syntax. This ensures that releasing a lock can never be forgotten. Even if an exception occurs, the lock will be released.

lock L then
{System.show exclusive}
end

To make it easier to work with objects, classes can be marked with the property locking. Instances of such classes have their own internal lock and can use a variant of the lock syntax:

class Test
prop locking
 
meth test
lock
{Show exclusive}
end
end
end

[edit] Perl

Code demonstrating shared resources and simple locking. Resource1 and Resource2 represent some limited resources that must be exclusively used and released by each thread. Each thread reports how many of each is available; if it goes below zero, something is wrong. Try comment out either of the "lock $lock*" line to see what happens without locking.

use Thread qw'async';
use threads::shared;
 
my ($lock1, $lock2, $resource1, $resource2) :shared = (0) x 4;
 
sub use_resource {
{ # curly provides lexical scope, exiting which causes lock to release
lock $lock1;
$resource1 --; # acquire resource
sleep(int rand 3); # artifical delay to pretend real work
$resource1 ++; # release resource
print "In thread ", threads->tid(), ": ";
print "Resource1 is $resource1\n";
}
{
lock $lock2;
$resource2 --;
sleep(int rand 3);
$resource2 ++;
print "In thread ", threads->tid(), ": ";
print "Resource2 is $resource2\n";
}
}
 
# create 9 threads and clean up each after they are done.
for ( map async{ use_resource }, 1 .. 9) {
$_->join
}

[edit] Perl 6

my $lock = Lock.new;
 
$lock.protect: { your-ad-here() }

Locks are reentrant. You may explicitly lock and unlock them, but the syntax above guarantees the lock will be unlocked on scope exit, even if by thrown exception or other exotic control flow. That being said, direct use of locks is discouraged in Perl 6 in favor of promises, channels, and supplies, which offer better composable semantics.

[edit] PicoLisp

PicoLisp uses several mechanisms of interprocess communication, mainly within the same process family (children of the same parent process) for database synchronization (e.g. 'lock', 'sync' or 'tell'.

For a simple synchronization of unrelated PicoLisp processes the 'acquire' / 'release' function pair can be used.

[edit] PureBasic

PureBasic has the following Mutex functions;

MyMutex=CreateMutex()
Result = TryLockMutex(MyMutex)
LockMutex(MyMutex)
UnlockMutex(MyMutex)
FreeMutex(MyMutex)

Example

Declare ThreadedTask(*MyArgument)
Define Mutex
 
If OpenConsole()
Define thread1, thread2, thread3
 
Mutex = CreateMutex()
thread1 = CreateThread(@ThreadedTask(), 1): Delay(5)
thread2 = CreateThread(@ThreadedTask(), 2): Delay(5)
thread3 = CreateThread(@ThreadedTask(), 3)
WaitThread(thread1)
WaitThread(thread2)
WaitThread(thread3)
 
PrintN(#CRLF$+"Press ENTER to exit"): Input()
FreeMutex(Mutex)
CloseConsole()
EndIf
 
Procedure ThreadedTask(*MyArgument)
Shared Mutex
Protected a, b
For a = 1 To 3
LockMutex(Mutex)
; Without Lock-/UnLockMutex() here the output from the parallel threads would be all mixed.
; Reading/Writing to shared memory resources are a common use for Mutextes i PureBasic
PrintN("Thread "+Str(*MyArgument)+": Print 3 numbers in a row:")
For b = 1 To 3
Delay(75)
PrintN("Thread "+Str(*MyArgument)+" : "+Str(b))
Next
UnlockMutex(Mutex)
Next
EndProcedure

[edit] Python

Demonstrating semaphores. Note that semaphores can be considered as a multiple version of mutex; while a mutex allows a singular exclusive access to code or resources, a semaphore grants access to number of threads up to certain value.
import threading
from time import sleep
 
# res: max number of resources. If changed to 1, it functions
# identically to a mutex/lock object
res = 2
sema = threading.Semaphore(res)
 
class res_thread(threading.Thread):
def run(self):
global res
n = self.getName()
for i in range(1, 4):
# acquire a resource if available and work hard
# for 2 seconds. if all res are occupied, block
# and wait
sema.acquire()
res = res - 1
print n, "+ res count", res
sleep(2)
 
# after done with resource, return it to pool and flag so
res = res + 1
print n, "- res count", res
sema.release()
 
# create 4 threads, each acquire resorce and work
for i in range(1, 5):
t = res_thread()
t.start()

[edit] Racket

Racket has semaphores which can be used as mutexes in the usual way. With other language features this can be used to implement new features -- for example, here is how we would implement a protected-by-a-mutex function:

 
(define foo
(let ([sema (make-semaphore 1)])
(lambda (x)
(dynamic-wind (λ() (semaphore-wait sema))
(λ() (... do something ...))
(λ() (semaphore-post sema))))))

and it is now easy to turn this into a macro for definitions of such functions:

 
(define-syntax-rule (define/atomic (name arg ...) E ...)
(define name
(let ([sema (make-semaphore 1)])
(lambda (arg ...)
(dynamic-wind (λ() (semaphore-wait sema))
(λ() E ...)
(λ() (semaphore-post sema)))))))
;; this does the same as the above now:
(define/atomic (foo x)
(... do something ...))

But more than just linguistic features, Racket has many additional synchronization tools in its VM. Some notable examples: OS semaphore for use with OS threads, green threads, lightweight OS threads, and heavyweight OS threads, synchronization channels, thread mailboxes, CML-style event handling, generic synchronizeable event objects, non-blocking IO, etc, etc.

[edit] Ruby

Ruby's standard library includes a mutex_m module that can be mixed-in to a class.

require 'mutex_m'
 
class SomethingWithMutex
include Mutex_m
...
end

Individual objects can be extended with the module too

an_object = Object.new
an_object.extend(Mutex_m)

An object with mutex powers can then:

# acquire a lock -- block execution until it becomes free
an_object.mu_lock
 
# acquire a lock -- return immediately even if not acquired
got_lock = an_object.mu_try_lock
 
# have a lock?
if an_object.mu_locked? then ...
 
# release the lock
an_object.mu_unlock
 
# wrap a lock around a block of code -- block execution until it becomes free
an_object.my_synchronize do
do critical stuff
end

[edit] Tcl

Tcl's mutexes have four functions.

package require Thread
 
# How to create a mutex
set m [thread::mutex create]
 
# This will block if the lock is already held unless the mutex is made recursive
thread::mutex lock $m
# Now locked...
thread::mutex unlock $m
# Unlocked again
 
# Dispose of the mutex
thread::mutex destroy $m

There are also read-write mutexes available.

set rw [thread::rwmutex create]
 
# Get and drop a reader lock
thread::rwmutex rlock $rw
thread::rwmutex unlock $rw
 
# Get and drop a writer lock
thread::rwmutex wlock $rw
thread::rwmutex unlock $rw
 
thread::rwmutex destroy $rw
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