Metered concurrency
You are encouraged to solve this task according to the task description, using any language you may know.
The goal of this task is to create a counting semaphore used to control the execution of a set of concurrent units. This task intends to demonstrate coordination of active concurrent units through the use of a passive concurrent unit. The operations for a counting semaphore are acquire, release, and count. Each active concurrent unit should attempt to acquire the counting semaphore before executing its assigned duties. In this case the active concurrent unit should report that it has acquired the semaphore. It should sleep for 2 seconds and then release the semaphore.
Ada
The interface for the counting semaphore is defined in an Ada package specification: <lang ada>package Semaphores is
protected type Counting_Semaphore(Max : Positive) is
entry Acquire;
procedure Release;
function Count return Natural;
private
Lock_Count : Natural := 0;
end Counting_Semaphore;
end Semaphores;</lang> The Acquire entry has a condition associated with it. A task can only execute the Acquire entry when Lock_Count is less than Max. This is the key to making this structure behave as a counting semaphore. This condition, and all the other aspects of Counting_Semaphore are contained in the package body. <lang ada>package body Semaphores is
------------------------ -- Counting_Semaphore -- ------------------------
protected body Counting_Semaphore is
-------------
-- Acquire --
-------------
entry Acquire when Lock_Count < Max is
begin
Lock_Count := Lock_Count + 1;
end Acquire;
-----------
-- Count --
-----------
function Count return Natural is
begin
return Lock_Count;
end Count;
-------------
-- Release --
-------------
procedure Release is
begin
if Lock_Count > 0 then
Lock_Count := Lock_Count - 1;
end if;
end Release;
end Counting_Semaphore;
end Semaphores;</lang> We now need a set of tasks to properly call an instance of Counting_Semaphore. <lang ada>with Semaphores; with Ada.Text_Io; use Ada.Text_Io;
procedure Semaphores_Main is
-- Create an instance of a Counting_Semaphore with Max set to 3 Lock : Semaphores.Counting_Semaphore(3);
-- Define a task type to interact with the Lock object declared above
task type Worker is
entry Start (Sleep : in Duration; Id : in Positive);
end Worker;
task body Worker is
Sleep_Time : Duration;
My_Id : Positive;
begin
accept Start(Sleep : in Duration; Id : in Positive) do
My_Id := Id;
Sleep_Time := Sleep;
end Start;
--Acquire the lock. The task will suspend until the Acquire call completes
Lock.Acquire;
Put_Line("Task #" & Positive'Image(My_Id) & " acquired the lock.");
-- Suspend the task for Sleep_Time seconds
delay Sleep_Time;
-- Release the lock. Release is unconditional and happens without suspension
Lock.Release;
end Worker;
-- Create an array of 5 Workers type Staff is array(Positive range 1..5) of Worker; Crew : Staff;
begin
for I in Crew'range loop
Crew(I).Start(2.0, I);
end loop;
end Semaphores_Main;</lang>
ALGOL 68
<lang algol68>SEMA sem = LEVEL 1;
PROC job = (INT n)VOID: (
printf(($" Job "d" acquired Semaphore ..."$,n)); TO 10000000 DO SKIP OD; printf(($" Job "d" releasing Semaphore"l$,n))
);
PAR (
( DOWN sem ; job(1) ; UP sem ) , ( DOWN sem ; job(2) ; UP sem ) , ( DOWN sem ; job(3) ; UP sem )
)</lang> Output:
Job 3 acquired Semaphore ... Job 3 releasing Semaphore Job 1 acquired Semaphore ... Job 1 releasing Semaphore Job 2 acquired Semaphore ... Job 2 releasing Semaphore
BBC BASIC
In BBC BASIC concurrency can only be achieved by timer events (short of running multiple processes). <lang bbcbasic> INSTALL @lib$+"TIMERLIB"
DIM tID%(6)
REM Two workers may be concurrent
DIM Semaphore%(2)
tID%(6) = FN_ontimer(11, PROCtimer6, 1)
tID%(5) = FN_ontimer(10, PROCtimer5, 1)
tID%(4) = FN_ontimer(11, PROCtimer4, 1)
tID%(3) = FN_ontimer(10, PROCtimer3, 1)
tID%(2) = FN_ontimer(11, PROCtimer2, 1)
tID%(1) = FN_ontimer(10, PROCtimer1, 1)
ON CLOSE PROCcleanup : QUIT
ON ERROR PRINT REPORT$ : PROCcleanup : END
sc% = 0
REPEAT
oldsc% = sc%
sc% = -SUM(Semaphore%())
IF sc%<>oldsc% PRINT "Semaphore count now ";sc%
WAIT 0
UNTIL FALSE
DEF PROCtimer1 : PROCtask(1) : ENDPROC
DEF PROCtimer2 : PROCtask(2) : ENDPROC
DEF PROCtimer3 : PROCtask(3) : ENDPROC
DEF PROCtimer4 : PROCtask(4) : ENDPROC
DEF PROCtimer5 : PROCtask(5) : ENDPROC
DEF PROCtimer6 : PROCtask(6) : ENDPROC
DEF PROCtask(n%)
LOCAL i%, temp%
PRIVATE delay%(), sem%()
DIM delay%(6), sem%(6)
IF delay%(n%) THEN
delay%(n%) -= 1
IF delay%(n%) = 0 THEN
SWAP Semaphore%(sem%(n%)),temp%
delay%(n%) = -1
PRINT "Task " ; n% " released semaphore"
ENDIF
ENDPROC
ENDIF
FOR i% = 1 TO DIM(Semaphore%(),1)
temp% = TRUE
SWAP Semaphore%(i%),temp%
IF NOT temp% EXIT FOR
NEXT
IF temp% THEN ENDPROC : REM Waiting to acquire semaphore
sem%(n%) = i%
delay%(n%) = 200
PRINT "Task "; n% " acquired semaphore"
ENDPROC
DEF PROCcleanup
LOCAL i%
FOR i% = 1 TO 6
PROC_killtimer(tID%(i%))
NEXT
ENDPROC</lang>
Output:
Task 1 acquired semaphore Task 2 acquired semaphore Semaphore count now 2 Task 1 released semaphore Task 3 acquired semaphore Task 2 released semaphore Task 4 acquired semaphore Task 3 released semaphore Task 5 acquired semaphore Task 4 released semaphore Task 6 acquired semaphore Task 5 released semaphore Semaphore count now 1 Task 6 released semaphore Semaphore count now 0
C
<lang c>#include <semaphore.h>
- include <pthread.h>
- include <stdlib.h>
- include <stdio.h>
- include <unistd.h>
sem_t sem; int count = 3;
/* the whole point of a semaphore is that you don't count it:
* p/v are atomic. Unless it's locked while you are doing * something with the count, the value is only informative */
- define getcount() count
void acquire() { sem_wait(&sem); count--; }
void release() { count++; sem_post(&sem); }
void* work(void * id) { int i = 10; while (i--) { acquire(); printf("#%d acquired sema at %d\n", *(int*)id, getcount()); usleep(rand() % 4000000); /* sleep 2 sec on average */ release(); usleep(0); /* effectively yield */ } return 0; }
int main() { pthread_t th[4]; int i, ids[] = {1, 2, 3, 4};
sem_init(&sem, 0, count);
for (i = 4; i--;) pthread_create(th + i, 0, work, ids + i); for (i = 4; i--;) pthread_join(th[i], 0); printf("all workers done\n");
return sem_destroy(&sem); }</lang>
C#
C# has built in semaphore system where acquire is called via Wait(), release with Release() and count with semaphore.CurrentCount. <lang csharp>using System; using System.Threading; using System.Threading.Tasks;
namespace RosettaCode {
internal sealed class Program
{
private static void Worker(object arg, int id)
{
var sem = arg as SemaphoreSlim;
sem.Wait();
Console.WriteLine("Thread {0} has a semaphore & is now working.", id);
Thread.Sleep(2*1000);
Console.WriteLine("#{0} done.", id);
sem.Release();
}
private static void Main()
{
var semaphore = new SemaphoreSlim(Environment.ProcessorCount*2, int.MaxValue);
Console.WriteLine("You have {0} processors availiabe", Environment.ProcessorCount);
Console.WriteLine("This program will use {0} semaphores.\n", semaphore.CurrentCount);
Parallel.For(0, Environment.ProcessorCount*3, y => Worker(semaphore, y)); } }
}</lang>
D
<lang d>module meteredconcurrency ; import std.stdio ; import std.thread ; import std.c.time ;
class Semaphore {
private int lockCnt, maxCnt ;
this(int count) { maxCnt = lockCnt = count ;}
void acquire() {
if(lockCnt < 0 || maxCnt <= 0)
throw new Exception("Negative Lock or Zero init. Lock") ;
while(lockCnt == 0)
Thread.getThis.yield ; // let other threads release lock
synchronized lockCnt-- ;
}
void release() {
synchronized
if (lockCnt < maxCnt)
lockCnt++ ;
else
throw new Exception("Release lock before acquire") ;
}
int getCnt() { synchronized return lockCnt ; }
}
class Worker : Thread {
private static int Id = 0 ;
private Semaphore lock ;
private int myId ;
this (Semaphore l) { super() ; lock = l ; myId = Id++ ; }
override int run() {
lock.acquire ;
writefln("Worker %d got a lock(%d left).", myId, lock.getCnt) ;
msleep(2000) ; // wait 2.0 sec
lock.release ;
writefln("Worker %d released a lock(%d left).", myId, lock.getCnt) ;
return 0 ;
}
}
void main() {
Worker[10] crew ; Semaphore lock = new Semaphore(4) ; foreach(inout c ; crew) (c = new Worker(lock)).start ; foreach(inout c ; crew) c.wait ;
}</lang>
Phobos with tools
Using the scrapple.tools extension library for Phobos .. <lang d>module metered;
import tools.threads, tools.log, tools.time, tools.threadpool;
void main() {
log_threads = false;
auto done = new Semaphore, lock = new Semaphore(4);
auto tp = new Threadpool(10);
for (int i = 0; i < 10; ++i) {
tp.addTask(i /apply/ (int i) {
scope(exit) done.release;
lock.acquire;
logln(i, ": lock acquired");
sleep(2.0);
lock.release;
logln(i, ": lock released");
});
}
for (int i = 0; i < 10; ++i)
done.acquire;
}</lang>
E
This semaphore slightly differs from the task description; the release operation is not on the semaphore itself but given out with each acquisition, and cannot be invoked too many times.
<lang e>def makeSemaphore(maximum :(int > 0)) {
var current := 0
def waiters := <elib:vat.makeQueue>()
def notify() {
while (current < maximum && waiters.hasMoreElements()) {
current += 1
waiters.optDequeue().resolve(def released)
when (released) -> {
current -= 1
notify()
}
}
}
def semaphore {
to acquire() {
waiters.enqueue(def response)
notify()
return response
}
to count() { return current }
}
return semaphore
}
def work(label, interval, semaphore, timer, println) {
when (def releaser := semaphore <- acquire()) -> {
println(`$label: I have acquired the lock.`)
releaser.resolve(
timer.whenPast(timer.now() + interval, fn {
println(`$label: I will have released the lock.`)
})
)
}
}
def semaphore := makeSemaphore(3) for i in 1..5 {
work(i, 2000, semaphore, timer, println)
}</lang>
EchoLisp
<lang scheme> (require 'tasks) ;; tasks library
(define (task id) (wait S) ;; acquire, p-op (printf "task %d acquires semaphore @ %a" id (date->time-string (current-date))) (sleep 2000) (signal S) ;; release, v-op id)
(define S (make-semaphore 4)) ;; semaphore with init count 4
- run 10 // tasks
(for ([i 10]) (task-run (make-task task i ) (random 500))) </lang>
- Output:
task 1 acquires semaphore @ 19:23:03 task 6 acquires semaphore @ 19:23:03 task 4 acquires semaphore @ 19:23:03 task 7 acquires semaphore @ 19:23:03 task 8 acquires semaphore @ 19:23:05 task 9 acquires semaphore @ 19:23:05 task 0 acquires semaphore @ 19:23:05 task 3 acquires semaphore @ 19:23:05 task 2 acquires semaphore @ 19:23:07 task 1 acquires semaphore @ 19:23:07 task 6 acquires semaphore @ 19:23:07 task 5 acquires semaphore @ 19:23:08 task 7 acquires semaphore @ 19:23:09 task 4 acquires semaphore @ 19:23:09 task 9 acquires semaphore @ 19:23:10 task 8 acquires semaphore @ 19:23:10 task 0 acquires semaphore @ 19:23:11 ;; etc.
Erlang
In this implementation the semaphore is handled as its own process. Taking advantage of erlang's receive queues, which act as a FIFO queue for 'acquire' requests. As workers come online and request the semaphore they will receive it in order. 'receive' has the effect of pausing the process until a message is matched, so there's no idle looping. <lang erlang> -module(metered). -compile(export_all).
create_semaphore(N) ->
spawn(?MODULE, sem_loop, [N,N]).
sem_loop(0,Max) ->
io:format("Resources exhausted~n"),
receive
{release, PID} ->
PID ! released,
sem_loop(1,Max);
{stop, _PID} ->
ok
end;
sem_loop(N,N) ->
receive
{acquire, PID} ->
PID ! acquired,
sem_loop(N-1,N);
{stop, _PID} ->
ok
end;
sem_loop(N,Max) ->
receive
{release, PID} ->
PID ! released,
sem_loop(N+1,Max);
{acquire, PID} ->
PID ! acquired,
sem_loop(N-1,Max);
{stop, _PID} ->
ok
end.
release(Sem) ->
Sem ! {release, self()},
receive
released ->
ok
end.
acquire(Sem) ->
Sem ! {acquire, self()},
receive
acquired ->
ok
end.
start() -> create_semaphore(10).
stop(Sem) -> Sem ! {stop, self()}.
worker(P,N,Sem) ->
acquire(Sem),
io:format("Worker ~b has the acquired semaphore~n",[N]),
timer:sleep(500 * random:uniform(4)),
release(Sem),
io:format("Worker ~b has released the semaphore~n",[N]),
P ! {done, self()}.
test() ->
Sem = start(),
Pids = lists:map(fun (N) ->
spawn(?MODULE, worker, [self(),N,Sem])
end, lists:seq(1,20)),
lists:foreach(fun (P) -> receive {done, P} -> ok end end, Pids),
stop(Sem).
</lang>
Euphoria
<lang euphoria>sequence sems sems = {} constant COUNTER = 1, QUEUE = 2
function semaphore(integer n)
if n > 0 then
sems = append(sems,{n,{}})
return length(sems)
else
return 0
end if
end function
procedure acquire(integer id)
if sems[id][COUNTER] = 0 then
task_suspend(task_self())
sems[id][QUEUE] &= task_self()
task_yield()
end if
sems[id][COUNTER] -= 1
end procedure
procedure release(integer id)
sems[id][COUNTER] += 1
if length(sems[id][QUEUE])>0 then
task_schedule(sems[id][QUEUE][1],1)
sems[id][QUEUE] = sems[id][QUEUE][2..$]
end if
end procedure
function count(integer id)
return sems[id][COUNTER]
end function
procedure delay(atom delaytime)
atom t
t = time()
while time() - t < delaytime do
task_yield()
end while
end procedure
integer sem
procedure worker()
acquire(sem)
printf(1,"- Task %d acquired semaphore.\n",task_self())
delay(2)
release(sem)
printf(1,"+ Task %d released semaphore.\n",task_self())
end procedure
integer task
sem = semaphore(4)
for i = 1 to 10 do
task = task_create(routine_id("worker"),{})
task_schedule(task,1)
task_yield()
end for
while length(task_list())>1 do
task_yield()
end while</lang>
Output:
- Task 1 acquired semaphore. - Task 2 acquired semaphore. - Task 3 acquired semaphore. - Task 4 acquired semaphore. + Task 1 released semaphore. - Task 5 acquired semaphore. + Task 4 released semaphore. - Task 6 acquired semaphore. + Task 3 released semaphore. - Task 7 acquired semaphore. + Task 2 released semaphore. - Task 8 acquired semaphore. + Task 7 released semaphore. - Task 9 acquired semaphore. + Task 6 released semaphore. - Task 10 acquired semaphore. + Task 5 released semaphore. + Task 8 released semaphore. + Task 10 released semaphore. + Task 9 released semaphore.
Go
Buffered channel
Recommended solution for simplicity. Acquire operation is channel send, release is channel receive, and count is provided with len(channel). Also WaitGroup used as a completion checkpoint. <lang go>package main
import (
"log" "os" "sync" "time"
)
// log package serializes output var fmt = log.New(os.Stdout, "", 0)
// library analogy per WP article const nRooms = 10 const nStudents = 20
func main() {
// buffered channel used as a counting semaphore
rooms := make(chan int, nRooms)
for i := 0; i < nRooms; i++ {
rooms <- 1
}
// WaitGroup used to wait for all students to have studied
// before terminating program
var studied sync.WaitGroup
studied.Add(nStudents)
// nStudents run concurrently
for i := 0; i < nStudents; i++ {
go student(rooms, &studied)
}
studied.Wait()
}
func student(rooms chan int, studied *sync.WaitGroup) {
<-rooms // acquire operation
// report per task descrption. also exercise count operation
fmt.Printf("Room entered. Count is %d. Studying...\n",
len(rooms)) // len function provides count operation
time.Sleep(2 * time.Second) // sleep per task description
rooms <- 1 // release operation
studied.Done() // signal that student is done
}</lang> Output for this and the other Go programs here shows 10 students studying immediately, about a 2 second pause, 10 more students studying, then another pause of about 2 seconds before returning to the command prompt.
Sync.Cond
A more traditional approach implementing a counting semaphore object with sync.Cond. It has a constructor and methods for the three operations requested by the task. <lang go>package main
import (
"log" "os" "sync" "sync/atomic" "time"
)
var fmt = log.New(os.Stdout, "", 0)
type countSem struct {
c int32 cond *sync.Cond
}
func newCount(n int) *countSem {
return &countSem{int32(n), sync.NewCond(new(sync.Mutex))}
}
func (cs *countSem) count() int {
return int(atomic.LoadInt32(&cs.c))
}
func (cs *countSem) acquire() {
if atomic.AddInt32(&cs.c, -1) < 0 {
atomic.AddInt32(&cs.c, 1)
cs.cond.L.Lock()
for atomic.AddInt32(&cs.c, -1) < 0 {
atomic.AddInt32(&cs.c, 1)
cs.cond.Wait()
}
cs.cond.L.Unlock()
}
}
func (cs *countSem) release() {
atomic.AddInt32(&cs.c, 1) cs.cond.Signal()
}
func main() {
librarian := newCount(10)
nStudents := 20
var studied sync.WaitGroup
studied.Add(nStudents)
for i := 0; i < nStudents; i++ {
go student(librarian, &studied)
}
studied.Wait()
}
func student(studyRoom *countSem, studied *sync.WaitGroup) {
studyRoom.acquire()
fmt.Printf("Room entered. Count is %d. Studying...\n", studyRoom.count())
time.Sleep(2 * time.Second)
studyRoom.release()
studied.Done()
}</lang>
Monitor
Monitor-style solution implements counting semaphore as a monitor goroutine encapsulating the count. It implements semaphore operations with separate Go channels. <lang go>package main
import (
"log" "os" "sync" "time"
)
var fmt = log.New(os.Stdout, "", 0)
func main() {
// three operations per task description acquire := make(chan int) release := make(chan int) count := make(chan chan int)
// library analogy per WP article
go librarian(acquire, release, count, 10)
nStudents := 20
var studied sync.WaitGroup
studied.Add(nStudents)
for i := 0; i < nStudents; i++ {
go student(acquire, release, count, &studied)
}
// wait until all students have studied before terminating program
studied.Wait()
}
func librarian(a, r chan int, c chan chan int, count int) {
p := a // acquire operation is served or not depending on count
for {
select {
case <-p: // acquire/p/wait operation
count--
if count == 0 {
p = nil
}
case <-r: // release/v operation
count++
p = a
case cc := <-c: // count operation
cc <- count
}
}
}
func student(a, r chan int, c chan chan int, studied *sync.WaitGroup) {
cc := make(chan int)
a <- 0 // acquire
c <- cc // request count
fmt.Printf("Room entered. Count is %d. Studying...\n", <-cc)
time.Sleep(2 * time.Second) // sleep per task description
r <- 0 // release
studied.Done() // signal done
}</lang>
Groovy
Solution: <lang groovy>class CountingSemaphore {
private int count = 0 private final int max
CountingSemaphore(int max) { this.max = max }
synchronized int acquire() {
while (count >= max) { wait() }
++count
}
synchronized int release() {
if (count) { count--; notifyAll() }
count
}
synchronized int getCount() { count }
}</lang>
Test: <lang groovy>def cs = new CountingSemaphore(4) (1..12).each { threadID ->
Thread.start {
def id = "Thread #${(threadID as String).padLeft(2,'0')}"
try {
def sCount = cs.acquire()
println("${id} has acquired Semaphore at count = ${sCount}")
sleep(2000)
} finally {
println("${id} is releasing Semaphore at count = ${cs.count}")
cs.release()
}
}
}</lang>
Output:
Thread #03 has acquired Semaphore at count = 4 Thread #07 has acquired Semaphore at count = 2 Thread #02 has acquired Semaphore at count = 1 Thread #09 has acquired Semaphore at count = 3 Thread #03 is releasing Semaphore at count = 4 Thread #02 is releasing Semaphore at count = 4 Thread #09 is releasing Semaphore at count = 4 Thread #07 is releasing Semaphore at count = 4 Thread #12 has acquired Semaphore at count = 4 Thread #05 has acquired Semaphore at count = 3 Thread #06 has acquired Semaphore at count = 4 Thread #08 has acquired Semaphore at count = 2 Thread #12 is releasing Semaphore at count = 4 Thread #06 is releasing Semaphore at count = 4 Thread #05 is releasing Semaphore at count = 4 Thread #10 has acquired Semaphore at count = 4 Thread #11 has acquired Semaphore at count = 4 Thread #08 is releasing Semaphore at count = 3 Thread #01 has acquired Semaphore at count = 4 Thread #04 has acquired Semaphore at count = 4 Thread #11 is releasing Semaphore at count = 4 Thread #10 is releasing Semaphore at count = 4 Thread #04 is releasing Semaphore at count = 2 Thread #01 is releasing Semaphore at count = 2
Haskell
The QSem (quantity semaphore) waitQSem and signalQSem functions are the Haskell acquire and release equivalents, and the MVar (synchronizing variable) functions are used to put the workers statuses on the main thread for printing. Note that this code is likely only compatible with GHC due to the use of "threadDelay" from Control.Concurrent.
<lang Haskell>import Control.Concurrent import Control.Monad
worker :: QSem -> MVar String -> Int -> IO () worker q m n = do
waitQSem q putMVar m $ "Worker " ++ show n ++ " has acquired the lock." threadDelay 2000000 -- microseconds! signalQSem q putMVar m $ "Worker " ++ show n ++ " has released the lock."
main :: IO () main = do
q <- newQSem 3
m <- newEmptyMVar
let workers = 5
prints = 2 * workers
mapM_ (forkIO . worker q m) [1..workers]
replicateM_ prints $ takeMVar m >>= print</lang>
Icon and Unicon
Icon doesn't support concurrency. A Unicon solution is: <lang unicon>procedure main(A)
n := integer(A[1] | 3) # Max. number of active tasks
m := integer(A[2] | 2) # Number of visits by each task
k := integer(A[3] | 5) # Number of tasks
sem := [: |mutex([])\n :]
every put(threads := [], (i := 1 to k, thread
every 1 to m do {
write("unit ",i," ready")
until flag := trylock(!sem)
write("unit ",i," running")
delay(2000)
write("unit ",i," done")
unlock(flag)
}))
every wait(!threads)
end</lang>
Sample run:
->mc unit 2 ready unit 2 running unit 1 ready unit 1 running unit 3 ready unit 3 running unit 4 ready unit 5 ready unit 2 done unit 2 ready unit 5 running unit 1 done unit 2 running unit 1 ready unit 3 done unit 3 ready unit 4 running unit 5 done unit 5 ready unit 1 running unit 2 done unit 5 running unit 4 done unit 3 running unit 4 ready unit 1 done unit 4 running unit 5 done unit 3 done unit 4 done ->
Java
<lang java>public class CountingSemaphore{
private int lockCount = 0; private int maxCount;
CountingSemaphore(int Max){
maxCount = Max;
}
public synchronized void acquire() throws InterruptedException{
while( lockCount >= maxCount){
wait();
}
lockCount++;
}
public synchronized void release(){
if (lockCount > 0)
{
lockCount--;
notifyAll();
}
}
public synchronized int getCount(){
return lockCount;
}
}
public class Worker extends Thread{
private CountingSemaphore lock; private int id;
Worker(CountingSemaphore coordinator, int num){
lock = coordinator;
id = num;
}
Worker(){
}
public void run(){
try{
lock.acquire();
System.out.println("Worker " + id + " has acquired the lock.");
sleep(2000);
}
catch (InterruptedException e){
}
finally{
lock.release();
}
}
public static void main(String[] args){
CountingSemaphore lock = new CountingSemaphore(3);
Worker crew[];
crew = new Worker[5];
for (int i = 0; i < 5; i++){
crew[i] = new Worker(lock, i);
crew[i].start();
}
}
}</lang>
Logtalk
Using Logtalk's multi-threading notifications, which use a per-object FIFO message queue, thus avoiding the need of idle-loops. Works when using SWI-Prolog, XSB, or YAP as the backend compiler. <lang logtalk>
- - object(metered_concurrency).
:- threaded.
:- public(run/2).
run(Workers, Max) :-
% start the semaphore and the workers
threaded_ignore(semaphore(Max, Max)),
forall(
integer::between(1, Workers, Worker),
threaded_call(worker(Worker))
),
% wait for the workers to finish
forall(
integer::between(1, Workers, Worker),
threaded_exit(worker(Worker))
),
% tell the semaphore thread to stop
threaded_notify(worker(stop, _)).
:- public(run/0).
run :-
% default values: 7 workers, 2 concurrent workers
run(7, 2).
semaphore(N, Max) :-
threaded_wait(worker(Action, Worker)),
( Action == acquire, N > 0 ->
M is N - 1,
threaded_notify(semaphore(acquired, Worker)),
semaphore(M, Max)
; Action == release ->
M is N + 1,
threaded_notify(semaphore(released, Worker)),
semaphore(M, Max)
; Action == stop ->
true
; % Action == acquire, N =:= 0,
threaded_wait(worker(release, OtherWorker)),
threaded_notify(semaphore(released, OtherWorker)),
threaded_notify(semaphore(acquired, Worker)),
semaphore(N, Max)
).
worker(Worker) :-
% use a random setup time for the worker
random::random(0.0, 2.0, Setup),
thread_sleep(Setup),
threaded_notify(worker(acquire, Worker)),
threaded_wait(semaphore(acquired, Worker)),
write('Worker '), write(Worker), write(' acquired semaphore\n'),
thread_sleep(2),
threaded_notify(worker(release, Worker)),
write('Worker '), write(Worker), write(' releasing semaphore\n'),
threaded_wait(semaphore(released, Worker)).
- - end_object.
</lang> Output: <lang text> | ?- metered_concurrency::run. Worker 1 acquired semaphore Worker 6 acquired semaphore Worker 1 releasing semaphore Worker 2 acquired semaphore Worker 6 releasing semaphore Worker 5 acquired semaphore Worker 2 releasing semaphore Worker 7 acquired semaphore Worker 5 releasing semaphore Worker 3 acquired semaphore Worker 7 releasing semaphore Worker 4 acquired semaphore Worker 3 releasing semaphore Worker 4 releasing semaphore yes </lang>
Oforth
A semaphore can be emulated with a channel starting with n objects. Acquiring the semaphore is receiving an object from the channel Releasing the semaphore is sending by the object into the channel.
If the channel is empty a task will wait until it is no more empty.
<lang oforth>import: parallel
Object Class new: Semaphore(ch)
Semaphore method: initialize(n)
Channel newSize(n) dup := ch #[ 1 over send drop ] times(n) drop ;
Semaphore method: acquire @ch receive drop ; Semaphore method: release 1 @ch send drop ;</lang>
Usage :
<lang oforth>: mytask(s)
while( true ) [
s acquire "Semaphore acquired" .cr
2000 sleep
s release "Semaphore released" .cr
] ;
- test(n)
| s i |
Semaphore new(n) ->s 10 loop: i [ #[ s mytask ] & ] ;</lang>
Oz
Counting semaphores can be implemented in terms of mutexes (called "locks" in Oz) and dataflow variables (used as condition variables here). The mutex protects both the counter and the mutable reference to the dataflow variable. <lang oz>declare
fun {NewSemaphore N}
sem(max:N count:{NewCell 0} 'lock':{NewLock} sync:{NewCell _})
end
proc {Acquire Sem=sem(max:N count:C 'lock':L sync:S)}
Sync
Acquired
in
lock L then
if @C < N then
C := @C + 1
Acquired = true
else
Sync = @S
Acquired = false
end
end
if {Not Acquired} then
{Wait Sync}
{Acquire Sem}
end
end
proc {Release sem(count:C 'lock':L sync:S ...)}
lock L then
C := @C - 1
@S = unit %% wake up waiting threads
S := _ %% prepare for new waiters
end
end
proc {WithSemaphore Sem Proc}
{Acquire Sem}
try
{Proc}
finally
{Release Sem}
end
end
S = {NewSemaphore 4}
proc {StartWorker Name}
thread
for do {WithSemaphore S proc {$} {System.showInfo Name#" acquired semaphore"} {Delay 2000} end } {Delay 100} end
end end
in
for I in 1..10 do
{StartWorker I}
end</lang>
Perl
See Coro::Semaphore.
Perl 6
Uses a buffered channel to hand out a limited number of tickets. <lang perl6>class Semaphore {
has $.tickets = Channel.new;
method new ($max) {
my $s = self.bless;
$s.tickets.send(True) xx $max;
$s;
}
method acquire { $.tickets.receive }
method release { $.tickets.send(True) }
}
sub MAIN ($units = 5, $max = 2) {
my $sem = Semaphore.new($max);
my @units = do for ^$units -> $u {
start {
$sem.acquire; say "unit $u acquired";
sleep 2;
$sem.release; say "unit $u released";
}
}
await @units;
}</lang>
- Output:
unit 0 acquired unit 1 acquired unit 0 released unit 1 released unit 3 acquired unit 2 acquired unit 3 released unit 2 released unit 4 acquired unit 4 released
PicoLisp
<lang PicoLisp>(let Sem (tmp "sem")
(for U 4 # Create 4 concurrent units
(unless (fork)
(ctl Sem
(prinl "Unit " U " aquired the semaphore")
(wait 2000)
(prinl "Unit " U " releasing the semaphore") )
(bye) ) ) )</lang>
PureBasic
This launches a few threads in parallel, but restricted by the counter. After a thread has completed it releases the Semaphore and a new thread will be able to start. <lang PureBasic>#Threads=10
- Parallels=3
Global Semaphore=CreateSemaphore(#Parallels)
Procedure Worker(*arg.i)
WaitSemaphore(Semaphore) Debug "Thread #"+Str(*arg)+" active." Delay(Random(2000)) SignalSemaphore(Semaphore)
EndProcedure
- Start a multi-thread based work
Dim thread(#Threads) For i=0 To #Threads
thread(i)=CreateThread(@Worker(),i)
Next Debug "Launcher done."
- Wait for all threads to finish before closing down
For i=0 To #Threads
If IsThread(i) WaitThread(i) EndIf
Next</lang> Sample output
Thread #0 active. Thread #2 active. Thread #4 active. Launcher done. Thread #1 active. Thread #3 active. Thread #5 active. Thread #7 active. Thread #9 active. Thread #6 active. Thread #8 active. Thread #10 active.
Python
Python threading module includes a semaphore implementation. This code show how to use it.
<lang python>import time import threading
- Only 4 workers can run in the same time
sem = threading.Semaphore(4)
workers = [] running = 1
def worker():
me = threading.currentThread()
while 1:
sem.acquire()
try:
if not running:
break
print '%s acquired semaphore' % me.getName()
time.sleep(2.0)
finally:
sem.release()
time.sleep(0.01) # Let others acquire
- Start 10 workers
for i in range(10):
t = threading.Thread(name=str(i), target=worker) workers.append(t) t.start()
- Main loop
try:
while 1:
time.sleep(0.1)
except KeyboardInterrupt:
running = 0
for t in workers:
t.join()</lang>
Racket
<lang racket>
- lang racket
(define sema (make-semaphore 4)) ; allow 4 concurrent jobs
- start 20 jobs and wait for all of them to end
(for-each
thread-wait
(for/list ([i 20])
(thread (λ() (semaphore-wait sema)
(printf "Job #~a acquired semaphore\n" i)
(sleep 2)
(printf "Job #~a done\n" i)
(semaphore-post sema)))))
</lang>
Raven
Counting semaphores are built in:
<lang raven># four workers may be concurrent 4 semaphore as sem
thread worker
5 each as i
sem acquire
# tid is thread id
tid "%d acquired semaphore\n" print
2000 ms
sem release
# let others acquire
100 ms
- start 10 threads
group
10 each drop worker
list as workers</lang>
Thread joining is automatic by default.
Tcl
Uses the Thread package, which is expected to form part of the overall Tcl 8.6 release. <lang tcl>package require Tcl 8.6 package require Thread
- Create the global shared state of the semaphore
set handle semaphore0 tsv::set $handle mutex [thread::mutex create] tsv::set $handle cv [thread::cond create] tsv::set $handle count 0 tsv::set $handle max 3
- Make five worker tasks
for {set i 0} {$i<5} {incr i} {
lappend threads [thread::create -preserved {
# Not bothering to wrap this in an object for demonstration proc init {handle} { global mutex cv count max set mutex [tsv::object $handle mutex] set cv [tsv::object $handle cv] set count [tsv::object $handle count] set max [tsv::get $handle max] } proc acquire {} { global mutex cv count max thread::mutex lock [$mutex get] while {[$count get] >= $max} { thread::cond wait [$cv get] [$mutex get] } $count incr thread::mutex unlock [$mutex get] } proc release {} { global mutex cv count max thread::mutex lock [$mutex get] if {[$count get] > 0} { $count incr -1 thread::cond notify [$cv get] } thread::mutex unlock [$mutex get] }
# The core task of the worker
proc run {handle id} { init $handle acquire puts "worker $id has acquired the lock" after 2000 release puts "worker $id is done" }
# Wait for further instructions from the main thread
thread::wait
}]
}
- Start the workers doing useful work, giving each a unique id for pretty printing
set i 0 foreach t $threads {
puts "starting thread [incr i]" thread::send -async $t [list run $handle $i]
}
- Wait for all the workers to finish
foreach t $threads {
thread::release -wait $t
}</lang>
UnixPipes
The number of concurrent jobs can be set by issuing that many echo '1s at the begining to sem.
<lang bash>rm -f sem ; mkfifo sem
acquire() {
x=;while test -z "$x"; do read x; done;
}
release() {
echo '1'
}
job() {
n=$1; echo "Job $n acquired Semaphore">&2 ; sleep 2; echo "Job $n released Semaphore">&2 ;
}
( acquire < sem ; job 1 ; release > sem ) & ( acquire < sem ; job 2 ; release > sem ) & ( acquire < sem ; job 3 ; release > sem ) &
echo 'Initialize Jobs' >&2 ; echo '1' > sem</lang>
Visual Basic .NET
This code shows using a local semaphore. Semaphores can also be named, in which case they will be shared system wide.
<lang vbnet>Dim sem As New Semaphore(5, 5) 'Indicates that up to 5 resources can be aquired sem.WaitOne() 'Blocks until a resouce can be aquired Dim oldCount = sem.Release() 'Returns a resource to the pool 'oldCount has the Semaphore's count before Release was called</lang>
zkl
Semaphores are built in. <lang zkl>fcn job(name,sem){
name.println(" wait"); sem.acquire();
name.println(" go"); Atomic.sleep(2);
sem.release(); name.println(" done")
}
// start 3 threads using the same semphore
s:=Thread.Semaphore(1); job.launch("1",s); job.launch("2",s); job.launch("3",s);</lang>
- Output:
2 wait 2 go 1 wait 3 wait 2 done 1 go 1 done 3 go 3 done