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I am a student, currently studying operating system's concurrency - semaphore.
I read books and read articles about semaphores, mutex & semaphores... but can't seem to answer title's condition.
There exists a semaphore, and semaphore can be used as "Binary semaphore" and "Counting semaphore" which is classified by initial value.
I understand binary semaphore can prevent race condition by acting similarly as mutex(but two are not the same by various reasons.)
What i am curious about is that when we initialize the semaphore's value of more than or equal to 2, let's say n, then n values can enter the critical session. Then does this use of semaphore cause race condition?
I read articles about counting semaphores and it is considered that they are considered to keep track of the access to resources, and I'm confused about
do we not use counting semaphore like this, and is counting semaphore not used to solve concurrency problems?
added below because my questions weren't detailed.
For example, when there are 100 threads, and I initialize X=10, then initialize semaphore with sem_init(&s, 0, X), and if there is a critical session in threads' code flow, then doesn't it induce race condition because 10 threads are allowed to use resources and do through the threads' flow?
Semaphores prevent race conditions. Counting semaphores are used where there is more than one instance of the resource that they control available.
If they control access to a single resource, then a mutex semaphore will be used. If there are two resources that can be used then a counting semaphore of two will be used. If there are three, then a semaphore of three will be, and so on.
What i am curious about is that when we initialize the semaphore's value of more than or equal to 2, let's say n, then n values can enter the critical session. Then does this use of semaphore cause race condition?
You are talking about a counting semaphore which generally gets initialized to 0. I actually can't think of a use case where you'd want to initialize it to a value >0 because each waiting thread/task will cause the counting semaphore to increment as long as it's waiting. Also the increments are atomic instructions and will not cause concurrency problems.
Such a question.I am trying to understand how to use a semaphore. For exercise I took classical problem of readers / writers
with a cyclic memory buffer. I would like to discuss only the writers. If I initialize the semaphore with a count greater than 1,
I see that my writers can write to the same memory location. Then what is the meaning of the semaphore with the counter if it does
not guarantee synchronized access to a shared resource? It seems I should have for each memory cell the separate semaphore.
Well your use case is a special situation when the semaphore is initialized to 1 and behaves like a mutex. Obviously putting 2 would be an error as it would not be a correct lock anymore.
Nevertheless, semaphores a used in many other situations, for example, say you want to make sure that you do not have more than 5 thread running at a time.
You would setup the semaphore at 5, and each time you spawn a thread you do a down on it, and each time a thread finishes, you would do a up.
Trying to spawn the 6th thread would cause you to be 'stuck' in the down() until a thread eventually finishes at performs a up() that will unblock you.
Semaphore
Semaphores are a way to share a resource among multiple threads. In the Readers-writers problem, it is a way to guarantee consistency of the data, by preventing updates while it is being read, and preventing reads while it is being written to. It allows only one writer, and multiple concurrent readers.
Talking about semaphores is only useful if there are both readers and writers; In the case of an exclusive lock, where there can only be one thread who 'owns' the lock (has access to the resource), they are usually called Mutex (short for mutual exclusion).
Implementation
I implemented semaphores the other way around (due to CPU specifics): positive indicates how many readers there are, and a negative number indicates that there is one writer.
Initially the semaphore is 0, indicating no writer, and no readers.
Read Lock
Any time a reader wants to read, the semaphore must be 0 or positive, to support concurrent reads. If this is so, it is incremented. Positive numbers then, indicate that there are readers.
A reader would do a LOCK_READ, which succeeds, unless the semaphore is negative, indicating it is in the process of being written to and thus inconsistent. If this happens, the thread doing the read lock is suspended until the semaphore becomes 0 (or higher).
Write Lock
Any time a writer wants to write, the semaphore must be 0, because if it is positive, the readers may get partially updated (corrupt) data, and if it is negative, it is already locked for writing by another thread. If the semaphore indicates that the resource is not being accessed (0), the semaphore is decremented.
Unlocking
The unlocking is the reverse of the locking, except that there is no need to suspend the thread to unlock a resource. A read lock is lifted by decrementing the semaphore, and a write lock is lifted by incrementing the semaphore.
I've recently read up about Semaphores and get most of the logic.
Except for the fact that,
When let's say the value of Semaphore is 5, that means 5 threads can't enter the critical section, but how do we make sure these 5 threads don't try to access the same resource again causing a concurrency problem.
Is it something we are supposed to manage manually?
I think you got it backwards :)
You create the semaphore with the count for how many concurrent threads can enter it.
Say you have five resources for doing some work, you then create the semaphore with a count of five. What this means is that the first five threads that try to enter the semaphore with WaitOne get let in, decrementing the counter in the process.
When a thread exits the protected area with Release, it increments the counter again.
If a thread attempts to enter when the count is zero or below, that thread blocks until one of the thread already "in" the semaphore exists.
This way only five threads can be "in" the protected area at any one time.
To be clear: I do mostly embedded stuff, i.e. it's C and some kind of real-time kernel in microcontroller; but actually this question should be platform-independent.
I've read nice article by Michael Barr: Mutexes and Semaphores Demystified, as well as this related answer on StackOverflow. I understand clearly what binary semaphore is for, and what mutex is for. That's great.
But to be honest I never knew, and still can't understand, what so-called counting semaphore (i.e. semaphore with max count > 1) is for. In what cases should I use it?
Long time ago, before I've read aforementioned article by Michael Barr, I've told something like "you can use it when you have, like, a hotel room with certain number of beds. The number of beds is a maximum count for the semaphore, just like a number of keys for that room".
It probably sounds nicely, but actually I never had such a situation in my programming practice (and can't imagine any), and Michael Barr said this approach is just wrong, and he seems right.
Then, after I've read the article, I supposed it might probably be used when I have, say, some kind of FIFO buffer. Assume the buffer's capacity is 10 elements, and we have two tasks: A (the producer), and B (the consumer). Then:
Semaphore's max count should be set to 10;
When A wants to put data into buffer, it signals the semaphore.
When B wants to get the data from buffer, it waits the semaphore.
Well, but it doesn't work:
What if A tries to put new data to the FIFO, but there is no room? How would it wait for the place: should it call signal before putting new data (and signal then should be able to wait until max count < max count)? If so, semaphore will be signaled before data is actually put in the FIFO, this is wrong.
Semaphore is not enough for the proper synchronization: the FIFO itself needs to be synchronized as well. And then, it produces classic TOCTTOU problem: there is a period of time while semaphore is already either signaled or waited, but FIFO isn't yet modified.
So, when should I use that beast, the counting semaphore?
The 'classic' example is, indeed, a producer-consumer queue.
An unbounded queue requires one semaphore, (to count the queue entries), and a mutex-protected thread-safe queue, (or equivalent lock-free thread-safe queue). The semaphore is intialized to zero. Producers lock the mutex, push an object onto the queue, unlock the mutex and signal the semaphore. Consumers wait on the semaphore, lock the mutex, pop the object and unlock the mutex.
An bounded queue requires two semaphores, (one 'count' to count the entries, the other 'available' to count the free space), and a mutex-protected thread-safe queue, (or equivalent lock-free thread-safe queue). 'count' is initialized to zero and 'available' to the number of spaces free in an empty queue. Producers wait for 'available', lock the mutex, push an object onto the queue, unlock the mutex and signal 'count'. Consumers wait on 'count', lock the mutex, pop the object, unlock the mutex and signal 'available'.
This is a classic use for semaphores and had been around since forever, (well, since Dijkstra, anyway:). It's been tried billions of times, and it works fine for any number of producers/consumers.
There is no TOCTTOU issue, no corner-cases, no races.
The 'mutex' functionality may be provided by yet another semaphore, initialized to 1. This allows 'two semaphore' unbounded, and 'three semaphore' bounded implementations.
I supposed it might probably be used when I have, say, some kind of FIFO buffer. Assume the buffer's capacity is 10 elements, and we have two tasks: A (the producer), and B (the consumer). Then:
Semaphore's max count should be set to 10;
When A wants to put data into buffer, it signals the semaphore.
When B wants to get the data from buffer, it waits the semaphore.
This is not the way semaphores are used in the producer-consumer scenario. The standard solution is to use two counting semaphores, one for the empty slots (initialized to the number of available slots), and another for the filled slots (initialized to 0).
Producers try to allocate empty slots to put items in, so they start with wait-ing on the semaphore assigned to the empty slots. Consumers try to "allocate" (get hold of) filled slots, so they start with wait-ing on the semaphore assigned to the filled slots.
After finishing their work, they both signal the other semaphore since they transform slots from empty to filled and from filled to empty, respectively.
Standard solution scheme:
semaphore mutex = 1;
semaphore filled = 0;
semaphore empty = SIZE;
producer() {
while ( true) {
item = produceItem();
wait(empty);
wait(mutex);
putItemIntoBuffer( item);
signal(mutex);
signal(filled);
}
}
consumer() {
while ( true) {
wait( filled);
wait( mutex);
item = removeItemFromBuffer();
signal( mutex);
signal( empty);
consumeItem( item);
}
}
I think counting semaphores serve well in this situation.
Another, maybe simpler, example could be using a counting semaphore for avoiding deadlock in the Dining philosophers scenario. Since deadlock can occur only when all philosophers sit down simultaneously and pick their (say) left fork, deadlock can be avoided by not allowing all of them into the dining room at the same time. This can be achieved by a counting semaphore (enter) initialized to one less than the number of philosophers.
The protocol of one philosopher then becomes:
wait( enter)
wait( left_fork)
wait( right_fork)
eat()
signal( left_fork)
signal( right_fork)
signal( enter)
This ensures that all philosophers cannot be in the dining room at the same time.
Some of the more popular use cases of counting semaphores are -
Limiting the number of connections in a JDBC connection pool.
Network connection throttling.
Limiting concurrent access to resources such as a disk.
When should we use mutex and when should we use semaphore ?
Here is how I remember when to use what -
Semaphore:
Use a semaphore when you (thread) want to sleep till some other thread tells you to wake up. Semaphore 'down' happens in one thread (producer) and semaphore 'up' (for same semaphore) happens in another thread (consumer)
e.g.: In producer-consumer problem, producer wants to sleep till at least one buffer slot is empty - only the consumer thread can tell when a buffer slot is empty.
Mutex:
Use a mutex when you (thread) want to execute code that should not be executed by any other thread at the same time. Mutex 'down' happens in one thread and mutex 'up' must happen in the same thread later on.
e.g.: If you are deleting a node from a global linked list, you do not want another thread to muck around with pointers while you are deleting the node. When you acquire a mutex and are busy deleting a node, if another thread tries to acquire the same mutex, it will be put to sleep till you release the mutex.
Spinlock:
Use a spinlock when you really want to use a mutex but your thread is not allowed to sleep.
e.g.: An interrupt handler within OS kernel must never sleep. If it does the system will freeze / crash. If you need to insert a node to globally shared linked list from the interrupt handler, acquire a spinlock - insert node - release spinlock.
A mutex is a mutual exclusion object, similar to a semaphore but that only allows one locker at a time and whose ownership restrictions may be more stringent than a semaphore.
It can be thought of as equivalent to a normal counting semaphore (with a count of one) and the requirement that it can only be released by the same thread that locked it(a).
A semaphore, on the other hand, has an arbitrary count and can be locked by that many lockers concurrently. And it may not have a requirement that it be released by the same thread that claimed it (but, if not, you have to carefully track who currently has responsibility for it, much like allocated memory).
So, if you have a number of instances of a resource (say three tape drives), you could use a semaphore with a count of 3. Note that this doesn't tell you which of those tape drives you have, just that you have a certain number.
Also with semaphores, it's possible for a single locker to lock multiple instances of a resource, such as for a tape-to-tape copy. If you have one resource (say a memory location that you don't want to corrupt), a mutex is more suitable.
Equivalent operations are:
Counting semaphore Mutual exclusion semaphore
-------------------------- --------------------------
Claim/decrease (P) Lock
Release/increase (V) Unlock
Aside: in case you've ever wondered at the bizarre letters (P and V) used for claiming and releasing semaphores, it's because the inventor was Dutch. In that language:
Probeer te verlagen: means to try to lower;
Verhogen: means to increase.
(a) ... or it can be thought of as something totally distinct from a semaphore, which may be safer given their almost-always-different uses.
It is very important to understand that a mutex is not a semaphore with count 1!
This is the reason there are things like binary semaphores (which are really semaphores with count 1).
The difference between a Mutex and a Binary-Semaphore is the principle of ownership:
A mutex is acquired by a task and therefore must also be released by the same task.
This makes it possible to fix several problems with binary semaphores (Accidental release, recursive deadlock, and priority inversion).
Caveat: I wrote "makes it possible", if and how these problems are fixed is up to the OS implementation.
Because the mutex has to be released by the same task it is not very good for the synchronization of tasks. But if combined with condition variables you get very powerful building blocks for building all kinds of IPC primitives.
So my recommendation is: if you got cleanly implemented mutexes and condition variables (like with POSIX pthreads) use these.
Use semaphores only if they fit exactly to the problem you are trying to solve, don't try to build other primitives (e.g. rw-locks out of semaphores, use mutexes and condition variables for these)
There is a lot of misunderstanding between mutexes and semaphores. The best explanation I found so far is in this 3-Part article:
Mutex vs. Semaphores – Part 1: Semaphores
Mutex vs. Semaphores – Part 2: The Mutex
Mutex vs. Semaphores – Part 3 (final part): Mutual Exclusion Problems
While #opaxdiablo answer is totally correct I would like to point out that the usage scenario of both things is quite different. The mutex is used for protecting parts of code from running concurrently, semaphores are used for one thread to signal another thread to run.
/* Task 1 */
pthread_mutex_lock(mutex_thing);
// Safely use shared resource
pthread_mutex_unlock(mutex_thing);
/* Task 2 */
pthread_mutex_lock(mutex_thing);
// Safely use shared resource
pthread_mutex_unlock(mutex_thing); // unlock mutex
The semaphore scenario is different:
/* Task 1 - Producer */
sema_post(&sem); // Send the signal
/* Task 2 - Consumer */
sema_wait(&sem); // Wait for signal
See http://www.netrino.com/node/202 for further explanations
See "The Toilet Example" - http://pheatt.emporia.edu/courses/2010/cs557f10/hand07/Mutex%20vs_%20Semaphore.htm:
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
Mutex is to protect the shared resource.
Semaphore is to dispatch the threads.
Mutex:
Imagine that there are some tickets to sell. We can simulate a case where many people buy the tickets at the same time: each person is a thread to buy tickets. Obviously we need to use the mutex to protect the tickets because it is the shared resource.
Semaphore:
Imagine that we need to do a calculation as below:
c = a + b;
Also, we need a function geta() to calculate a, a function getb() to calculate b and a function getc() to do the calculation c = a + b.
Obviously, we can't do the c = a + b unless geta() and getb() have been finished.
If the three functions are three threads, we need to dispatch the three threads.
int a, b, c;
void geta()
{
a = calculatea();
semaphore_increase();
}
void getb()
{
b = calculateb();
semaphore_increase();
}
void getc()
{
semaphore_decrease();
semaphore_decrease();
c = a + b;
}
t1 = thread_create(geta);
t2 = thread_create(getb);
t3 = thread_create(getc);
thread_join(t3);
With the help of the semaphore, the code above can make sure that t3 won't do its job untill t1 and t2 have done their jobs.
In a word, semaphore is to make threads execute as a logicial order whereas mutex is to protect shared resource.
So they are NOT the same thing even if some people always say that mutex is a special semaphore with the initial value 1. You can say like this too but please notice that they are used in different cases. Don't replace one by the other even if you can do that.
Trying not to sound zany, but can't help myself.
Your question should be what is the difference between mutex and semaphores ?
And to be more precise question should be, 'what is the relationship between mutex and semaphores ?'
(I would have added that question but I'm hundred % sure some overzealous moderator would close it as duplicate without understanding difference between difference and relationship.)
In object terminology we can observe that :
observation.1 Semaphore contains mutex
observation.2 Mutex is not semaphore and semaphore is not mutex.
There are some semaphores that will act as if they are mutex, called binary semaphores, but they are freaking NOT mutex.
There is a special ingredient called Signalling (posix uses condition_variable for that name), required to make a Semaphore out of mutex.
Think of it as a notification-source. If two or more threads are subscribed to same notification-source, then it is possible to send them message to either ONE or to ALL, to wakeup.
There could be one or more counters associated with semaphores, which are guarded by mutex. The simple most scenario for semaphore, there is a single counter which can be either 0 or 1.
This is where confusion pours in like monsoon rain.
A semaphore with a counter that can be 0 or 1 is NOT mutex.
Mutex has two states (0,1) and one ownership(task).
Semaphore has a mutex, some counters and a condition variable.
Now, use your imagination, and every combination of usage of counter and when to signal can make one kind-of-Semaphore.
Single counter with value 0 or 1 and signaling when value goes to 1 AND then unlocks one of the guy waiting on the signal == Binary semaphore
Single counter with value 0 to N and signaling when value goes to less than N, and locks/waits when values is N == Counting semaphore
Single counter with value 0 to N and signaling when value goes to N, and locks/waits when values is less than N == Barrier semaphore (well if they dont call it, then they should.)
Now to your question, when to use what. (OR rather correct question version.3 when to use mutex and when to use binary-semaphore, since there is no comparison to non-binary-semaphore.)
Use mutex when
1. you want a customized behavior, that is not provided by binary semaphore, such are spin-lock or fast-lock or recursive-locks.
You can usually customize mutexes with attributes, but customizing semaphore is nothing but writing new semaphore.
2. you want lightweight OR faster primitive
Use semaphores, when what you want is exactly provided by it.
If you dont understand what is being provided by your implementation of binary-semaphore, then IMHO, use mutex.
And lastly read a book rather than relying just on SO.
I think the question should be the difference between mutex and binary semaphore.
Mutex = It is a ownership lock mechanism, only the thread who acquire the lock can release the lock.
binary Semaphore = It is more of a signal mechanism, any other higher priority thread if want can signal and take the lock.
All the above answers are of good quality,but this one's just to memorize.The name Mutex is derived from Mutually Exclusive hence you are motivated to think of a mutex lock as Mutual Exclusion between two as in only one at a time,and if I possessed it you can have it only after I release it.On the other hand such case doesn't exist for Semaphore is just like a traffic signal(which the word Semaphore also means).
As was pointed out, a semaphore with a count of one is the same thing as a 'binary' semaphore which is the same thing as a mutex.
The main things I've seen semaphores with a count greater than one used for is producer/consumer situations in which you have a queue of a certain fixed size.
You have two semaphores then. The first semaphore is initially set to be the number of items in the queue and the second semaphore is set to 0. The producer does a P operation on the first semaphore, adds to the queue. and does a V operation on the second. The consumer does a P operation on the second semaphore, removes from the queue, and then does a V operation on the first.
In this way the producer is blocked whenever it fills the queue, and the consumer is blocked whenever the queue is empty.
A mutex is a special case of a semaphore. A semaphore allows several threads to go into the critical section. When creating a semaphore you define how may threads are allowed in the critical section. Of course your code must be able to handle several accesses to this critical section.
I find the answer of #Peer Stritzinger the correct one.
I wanted to add to his answer the following quote from the book Programming with POSIX Threads by David R Butenhof. On page 52 of chapter 3 the author writes (emphasis mine):
You cannot lock a mutex when the calling thread already has that mutex locked. The result of attempting to do so may be an error return (EDEADLK), or it may be a self-deadlock, where the unfortunate thread waits forever. You cannot unlock a mutex that is unlocked, or that is locked by another thread. Locked mutexes are owned by the thread that locks them. If you need an "unowned" lock, use a semaphore. Section 6.6.6 discusses semaphores)
With this in mind, the following piece of code illustrates the danger of using a semaphore of size 1 as a replacement for a mutex.
sem = Semaphore(1)
counter = 0 // shared variable
----
Thread 1
for (i in 1..100):
sem.lock()
++counter
sem.unlock()
----
Thread 2
for (i in 1..100):
sem.lock()
++counter
sem.unlock()
----
Thread 3
sem.unlock()
thread.sleep(1.sec)
sem.lock()
If only for threads 1 and 2, the final value of counter should be 200. However, if by mistake that semaphore reference was leaked to another thread and called unlock, than you wouldn't get mutual exclusion.
With a mutex, this behaviour would be impossible by definition.
Binary semaphore and Mutex are different. From OS perspective, a binary semaphore and counting semaphore are implemented in the same way and a binary semaphore can have a value 0 or 1.
Mutex -> Can only be used for one and only purpose of mutual exclusion for a critical section of code.
Semaphore -> Can be used to solve variety of problems. A binary semaphore can be used for signalling and also solve mutual exclusion problem. When initialized to 0, it solves signalling problem and when initialized to 1, it solves mutual exclusion problem.
When the number of resources are more and needs to be synchronized, we can use counting semaphore.
In my blog, I have discussed these topics in detail.
https://designpatterns-oo-cplusplus.blogspot.com/2015/07/synchronization-primitives-mutex-and.html