Dining philosophers
You are encouraged to solve this task according to the task description, using any language you may know.
The dining philosophers problem illustrates non-composability of low-level synchronization primitives like semaphores. It is a modification of a problem posed by Edsger Dijkstra.
Five philosophers, Aristotle, Kant, Spinoza, Marx, and Russell (the tasks) spend their time thinking and eating spaghetti. They eat at a round table with five individual seats. For eating each philosopher needs two forks (the resources). There are five forks on the table, one left and one right of each seat. When a philosopher cannot grab both forks it sits and waits. Eating takes random time, then the philosopher puts the forks down and leaves the dining room. After spending some random time thinking about the nature of the universe, he again becomes hungry, and the circle repeats itself.
It can be observed that a straightforward solution, when forks are implemented by semaphores, is exposed to deadlock. There exist two deadlock states when all five philosophers are sitting at the table holding one fork each. One deadlock state is when each philosopher has grabbed the fork left of him, and another is when each has the fork on his right.
There are many solutions of the problem, program at least one, and explain how the deadlock is prevented.
Ada
Array of mutexes
The following solution uses an array of mutexes in order to model the forks. The forks used by a philosopher compose a subset in the array. When the the philosopher seizes his forks from the subset the array object prevents deadlocking since it is an atomic operation. <lang ada>with Ada.Numerics.Float_Random; use Ada.Numerics.Float_Random; with Ada.Text_IO; use Ada.Text_IO;
with Synchronization.Generic_Mutexes_Array;
procedure Test_Dining_Philosophers is
type Philosopher is (Aristotle, Kant, Spinoza, Marx, Russel); package Fork_Arrays is new Synchronization.Generic_Mutexes_Array (Philosopher); use Fork_Arrays;
Life_Span : constant := 20; -- In his life a philosopher eats 20 times
Forks : aliased Mutexes_Array; -- Forks for hungry philosophers
function Left_Of (Fork : Philosopher) return Philosopher is
begin
if Fork = Philosopher'First then
return Philosopher'Last;
else
return Philosopher'Pred (Fork);
end if;
end Left_Of;
task type Person (ID : Philosopher);
task body Person is
Cutlery : aliased Mutexes_Set := ID or Left_Of (ID);
Dice : Generator;
begin
Reset (Dice);
for Life_Cycle in 1..Life_Span loop
Put_Line (Philosopher'Image (ID) & " is thinking");
delay Duration (Random (Dice) * 0.100);
Put_Line (Philosopher'Image (ID) & " is hungry");
declare
Lock : Set_Holder (Forks'Access, Cutlery'Access);
begin
Put_Line (Philosopher'Image (ID) & " is eating");
delay Duration (Random (Dice) * 0.100);
end;
end loop;
Put_Line (Philosopher'Image (ID) & " is leaving");
end Person;
Ph_1 : Person (Aristotle); -- Start philosophers Ph_2 : Person (Kant); Ph_3 : Person (Spinoza); Ph_4 : Person (Marx); Ph_5 : Person (Russel);
begin
null; -- Nothing to do in the main task, just sit and behold
end Test_Dining_Philosophers;</lang>
Ordered mutexes
In the following solution forks are implemented as plain mutexes. The deadlock is prevented by ordering mutexes. Philosopher tasks seize them in same order 1, 2, 3, 4, 5. <lang ada>with Ada.Numerics.Float_Random; use Ada.Numerics.Float_Random; with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Dining_Philosophers is
type Philosopher is (Aristotle, Kant, Spinoza, Marx, Russel);
protected type Fork is
entry Grab;
procedure Put_Down;
private
Seized : Boolean := False;
end Fork;
protected body Fork is
entry Grab when not Seized is
begin
Seized := True;
end Grab;
procedure Put_Down is
begin
Seized := False;
end Put_Down;
end Fork;
Life_Span : constant := 20; -- In his life a philosopher eats 20 times
task type Person (ID : Philosopher; First, Second : not null access Fork);
task body Person is
Dice : Generator;
begin
Reset (Dice);
for Life_Cycle in 1..Life_Span loop
Put_Line (Philosopher'Image (ID) & " is thinking");
delay Duration (Random (Dice) * 0.100);
Put_Line (Philosopher'Image (ID) & " is hungry");
First.Grab;
Second.Grab;
Put_Line (Philosopher'Image (ID) & " is eating");
delay Duration (Random (Dice) * 0.100);
Second.Put_Down;
First.Put_Down;
end loop;
Put_Line (Philosopher'Image (ID) & " is leaving");
end Person;
Forks : array (1..5) of aliased Fork; -- Forks for hungry philosophers
-- Start philosophers
Ph_1 : Person (Aristotle, Forks (1)'Access, Forks (2)'Access);
Ph_2 : Person (Kant, Forks (2)'Access, Forks (3)'Access);
Ph_3 : Person (Spinoza, Forks (3)'Access, Forks (4)'Access);
Ph_4 : Person (Marx, Forks (4)'Access, Forks (5)'Access);
Ph_5 : Person (Russel, Forks (1)'Access, Forks (5)'Access);
begin
null; -- Nothing to do in the main task, just sit and behold
end Test_Dining_Philosophers;</lang>
Host of the dining room
Both deadlocks happen when all philosophers are in the dining room. The following solution has a host of the room who chatters the last philosopher while four of them are in the room. So there are never more than four of them in there, which prevents deadlock. Now the forks can be picked up in a "wrong" order, i.e. the left one first. <lang ada>with Ada.Numerics.Float_Random; use Ada.Numerics.Float_Random; with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Dining_Philosophers is
type Philosopher is (Aristotle, Kant, Spinoza, Marx, Russel);
protected type Fork is
entry Grab;
procedure Put_Down;
private
Seized : Boolean := False;
end Fork;
protected Host is
entry Greet;
procedure Farewell;
private
Guests : Natural := 0;
end Host;
protected body Fork is
entry Grab when not Seized is
begin
Seized := True;
end Grab;
procedure Put_Down is
begin
Seized := False;
end Put_Down;
end Fork;
protected body Host is
entry Greet when Guests < 5 is
begin
Guests := Guests + 1;
end Greet;
procedure Farewell is
begin
Guests := Guests - 1;
end Farewell;
end Host;
Life_Span : constant := 20; -- In his life a philosopher eats 20 times
task type Person (ID : Philosopher; Left, Right : not null access Fork);
task body Person is
Dice : Generator;
begin
Reset (Dice);
for Life_Cycle in 1..Life_Span loop
Put_Line (Philosopher'Image (ID) & " is thinking");
delay Duration (Random (Dice) * 0.100);
Put_Line (Philosopher'Image (ID) & " is hungry");
Host.Greet;
Left.Grab;
Right.Grab;
Put_Line (Philosopher'Image (ID) & " is eating");
delay Duration (Random (Dice) * 0.100);
Left.Put_Down;
Right.Put_Down;
Host.Farewell;
end loop;
Put_Line (Philosopher'Image (ID) & " is leaving");
end Person;
Forks : array (1..5) of aliased Fork; -- Forks for hungry philosophers
-- Start philosophers
Ph_1 : Person (Aristotle, Forks (1)'Access, Forks (2)'Access);
Ph_2 : Person (Kant, Forks (2)'Access, Forks (3)'Access);
Ph_3 : Person (Spinoza, Forks (3)'Access, Forks (4)'Access);
Ph_4 : Person (Marx, Forks (4)'Access, Forks (5)'Access);
Ph_5 : Person (Russel, Forks (5)'Access, Forks (1)'Access);
begin
null; -- Nothing to do in the main task, just sit and behold
end Test_Dining_Philosophers;</lang>
AutoHotkey
AutoHotkey doesn't support concurrency, so we fake it with timers. Deadlock is prevented by releasing a single fork when the other is unobtainable. Livelock is prevented by randomly trying for the opposite fork first. Starvation will only occur if one (or more) of the philosophers never stops eating. Try changing EnoughForks to 4 and fork supply per philosopher to 2. <lang ahk>#Persistent SetWorkingDir, %A_ScriptDir% FileDelete, output.txt EnoughForks := 2 ; required forks to begin eating Fork1 := Fork2 := Fork3 := Fork4 := Fork5 := 1 ; fork supply per philosopher SetTimer, AristotleWaitForLeftFork SetTimer, KantWaitForLeftFork SetTimer, SpinozaWaitForLeftFork SetTimer, MarxWaitForLeftFork SetTimer, RussellWaitForLeftFork Return ;---------------------------------------------------------------
AristotleWaitForLeftFork: WaitForFork("Aristotle", "left", Fork1, Fork2, AristotleLeftForkCount, AristotleRightForkCount, AristotleWaitCount, EnoughForks) Return AristotleWaitForRightFork: WaitForFork("Aristotle", "right", Fork2, Fork1, AristotleRightForkCount, AristotleLeftForkCount, AristotleWaitCount, EnoughForks) Return AristotleFinishEating: ReturnForks("Aristotle", Fork1, Fork2, AristotleLeftForkCount, AristotleRightForkCount, EnoughForks) Return
KantWaitForLeftFork: WaitForFork("Kant", "left", Fork2, Fork3, KantLeftForkCount, KantRightForkCount, KantWaitCount, EnoughForks) Return KantWaitForRightFork: WaitForFork("Kant", "right", Fork3, Fork2, KantRightForkCount, KantLeftForkCount, KantWaitCount, EnoughForks) Return KantFinishEating: ReturnForks("Kant", Fork2, Fork3, KantLeftForkCount, KantRightForkCount, EnoughForks) Return
SpinozaWaitForLeftFork: WaitForFork("Spinoza", "left", Fork3, Fork4, SpinozaLeftForkCount, SpinozaRightForkCount, SpinozaWaitCount, EnoughForks) Return SpinozaWaitForRightFork: WaitForFork("Spinoza", "right", Fork4, Fork3, SpinozaRightForkCount, SpinozaLeftForkCount, SpinozaWaitCount, EnoughForks) Return SpinozaFinishEating: ReturnForks("Spinoza", Fork3, Fork4, SpinozaLeftForkCount, SpinozaRightForkCount, EnoughForks) Return
MarxWaitForLeftFork: WaitForFork("Marx", "left", Fork4, Fork5, MarxLeftForkCount, MarxRightForkCount, MarxWaitCount, EnoughForks) Return MarxWaitForRightFork: WaitForFork("Marx", "right", Fork5, Fork4, MarxRightForkCount, MarxLeftForkCount, MarxWaitCount, EnoughForks) Return MarxFinishEating: ReturnForks("Marx", Fork4, Fork5, MarxLeftForkCount, MarxRightForkCount, EnoughForks) Return
RussellWaitForLeftFork: WaitForFork("Russell", "left", Fork5, Fork1, RussellLeftForkCount, RussellRightForkCount, RussellWaitCount, EnoughForks) Return RussellWaitForRightFork: WaitForFork("Russell", "right", Fork1, Fork5, RussellRightForkCount, RussellLeftForkCount, RussellWaitCount, EnoughForks) Return RussellFinishEating: ReturnForks("Russell", Fork5, Fork1, RussellLeftForkCount, RussellRightForkCount, EnoughForks) Return
ReturnForks(Philosopher, ByRef ThisFork, ByRef OtherFork, ByRef CurrentThisForkCount, ByRef CurrentOtherForkCount, EnoughForks) { OutputDebug, %Philosopher% finishes eating. FileAppend, %Philosopher% finishes eating.`n,output.txt ThisFork += CurrentThisForkCount ; return this fork OtherFork += CurrentOtherForkCount ; return other fork CurrentThisForkCount := 0 ; release this fork CurrentOtherForkCount := 0 ; release other fork OutputDebug, %Philosopher% returns all forks. FileAppend, %Philosopher% returns all forks.`n,output.txt
; do something while resting
Random, Rand, 0, 1 Rand := Rand ? "Left" : "Right" SetTimer, %Philosopher%WaitFor%Rand%Fork }
WaitForFork(Philosopher, This, ByRef ThisFork, ByRef OtherFork, ByRef CurrentThisForkCount, ByRef CurrentOtherForkCount, ByRef CurrentWaitCount, EnoughForks) { If This not in Left,Right Return Error Other := (This="right") ? "left" : "right" OutputDebug, %Philosopher% is hungry. FileAppend, %Philosopher% is hungry.`n,output.txt If (ThisFork) ; if this fork available { SetTimer, %Philosopher%WaitFor%This%Fork, Off CurrentWaitCount := 0 ThisFork-- ; take this fork CurrentThisForkCount++ ; receive this fork OutputDebug, %Philosopher% grabs %This% fork. FileAppend, %Philosopher% grabs %This% fork.`n,output.txt If (CurrentThisForkCount + CurrentOtherForkCount = EnoughForks) ; if philosopher has enough forks { OutputDebug, %Philosopher% starts eating. FileAppend, %Philosopher% starts eating.`n,output.txt
; do something while eating
SetTimer, %Philosopher%FinishEating, -250 } Else If (EnoughForks=2) { SetTimer, %Philosopher%WaitFor%Other%Fork } Else { Random, Rand, 0, 1 Rand := Rand ? "Left" : "Right" SetTimer, %Philosopher%WaitFor%Rand%Fork } } Else If (CurrentOtherForkCount and CurrentWaitCount > 5) ; if we've been holding other fork too long { SetTimer, %Philosopher%WaitFor%This%Fork, Off CurrentWaitCount := 0 OtherFork++ ; return other fork CurrentOtherForkCount-- ; release other fork OutputDebug, %Philosopher% drops %Other% fork. FileAppend, %Philosopher% drops %Other% fork.`n,output.txt Random, Rand, 0, 1 Rand := Rand ? "Left" : "Right" SetTimer, %Philosopher%WaitFor%Rand%Fork } Else If (CurrentThisForkCount and CurrentWaitCount > 5) ; if we've been holding one of this fork too long { SetTimer, %Philosopher%WaitFor%This%Fork, Off CurrentWaitCount := 0 ThisFork++ ; return other fork CurrentThisForkCount-- ; release other fork OutputDebug, %Philosopher% drops %This% fork. FileAppend, %Philosopher% drops %This% fork.`n,output.txt Random, Rand, 0, 1 Rand := Rand ? "Left" : "Right" SetTimer, %Philosopher%WaitFor%Rand%Fork } Else { CurrentWaitCount++ } }</lang>
- Output:
Aristotle is hungry. Aristotle grabs left fork. Kant is hungry. Kant grabs left fork. Spinoza is hungry. Spinoza grabs left fork. Marx is hungry. Marx grabs left fork. Russell is hungry. Russell grabs left fork. Aristotle is hungry. Kant is hungry. Spinoza is hungry. Marx is hungry. Russell is hungry. Aristotle is hungry. Kant is hungry. Spinoza is hungry. Marx is hungry. Russell is hungry. Aristotle is hungry. Kant is hungry. Spinoza is hungry. Marx is hungry. Russell is hungry. Aristotle is hungry. Kant is hungry. Spinoza is hungry. Marx is hungry. Russell is hungry. Aristotle is hungry. Kant is hungry. Spinoza is hungry. Marx is hungry. Russell is hungry. Aristotle is hungry. Kant is hungry. Spinoza is hungry. Marx is hungry. Russell is hungry. Aristotle is hungry. Aristotle drops left fork. Kant is hungry. Kant drops left fork. Spinoza is hungry. Spinoza drops left fork. Marx is hungry. Marx drops left fork. Russell is hungry. Russell grabs right fork. Russell starts eating. Marx is hungry. Marx grabs left fork. Aristotle is hungry. Aristotle grabs right fork. Kant is hungry. Kant grabs right fork. Spinoza is hungry. Russell finishes eating. Russell returns all forks. Aristotle is hungry. Aristotle grabs left fork. Aristotle starts eating. Kant is hungry. Spinoza is hungry. Marx is hungry. Marx grabs right fork. Marx starts eating. Russell is hungry. Kant is hungry. Spinoza is hungry. Aristotle finishes eating. Aristotle returns all forks. Marx finishes eating. Marx returns all forks. Russell is hungry. Russell grabs left fork. Kant is hungry. Spinoza is hungry. Spinoza grabs right fork. Marx is hungry. Russell is hungry. Russell grabs right fork. Russell starts eating. Aristotle is hungry. Aristotle grabs right fork. Kant is hungry. Aristotle is hungry. Spinoza is hungry. Marx is hungry. Russell finishes eating. Russell returns all forks. Kant is hungry. Aristotle is hungry. Aristotle grabs left fork. Aristotle starts eating. Spinoza is hungry. Marx is hungry. Marx grabs right fork. Russell is hungry. ⋮
BBC BASIC
This avoids deadlocks using the same strategy as the C solution (one of the philosophers picks up the left fork first). <lang bbcbasic> INSTALL @lib$+"TIMERLIB"
nSeats% = 5
DIM Name$(nSeats%-1), Fork%(nSeats%-1), tID%(nSeats%-1), Leftie%(nSeats%-1)
Name$() = "Aristotle", "Kant", "Spinoza", "Marx", "Russell"
Fork%() = TRUE : REM All forks are initially on the table
Leftie%(RND(nSeats%)-1) = TRUE : REM One philosopher is lefthanded
tID%(0) = FN_ontimer(10, PROCphilosopher0, 1)
tID%(1) = FN_ontimer(10, PROCphilosopher1, 1)
tID%(2) = FN_ontimer(10, PROCphilosopher2, 1)
tID%(3) = FN_ontimer(10, PROCphilosopher3, 1)
tID%(4) = FN_ontimer(10, PROCphilosopher4, 1)
ON CLOSE PROCcleanup : QUIT
ON ERROR PRINT REPORT$ : PROCcleanup : END
DEF PROCphilosopher0 : PROCtask(0) : ENDPROC
DEF PROCphilosopher1 : PROCtask(1) : ENDPROC
DEF PROCphilosopher2 : PROCtask(2) : ENDPROC
DEF PROCphilosopher3 : PROCtask(3) : ENDPROC
DEF PROCphilosopher4 : PROCtask(4) : ENDPROC
REPEAT
WAIT 0
UNTIL FALSE
END
DEF PROCtask(n%)
PRIVATE state%(), lh%(), rh%()
DIM state%(nSeats%-1), lh%(nSeats%-1), rh%(nSeats%-1)
REM States: 0 = waiting for forks, > 0 = eating, < 0 = left the room
CASE TRUE OF
WHEN state%(n%) < 0:
state%(n%) += 1 : REM Waiting to get hungry again
IF state%(n%) = 0 PRINT Name$(n%) " is hungry again"
WHEN state%(n%) > 0:
state%(n%) -= 1 : REM Eating
IF state%(n%) = 0 THEN
SWAP Fork%((n%-1+nSeats%) MOD nSeats%), lh%(n%)
SWAP Fork%((n% + 1) MOD nSeats%), rh%(n%)
state%(n%) = -RND(100)
PRINT Name$(n%) " is leaving the room"
ENDIF
WHEN state%(n%) = 0:
IF Leftie%(n%) THEN
IF NOT lh%(n%) SWAP Fork%((n%-1+nSeats%) MOD nSeats%), lh%(n%)
IF lh%(n%) IF NOT rh%(n%) SWAP Fork%((n% + 1) MOD nSeats%), rh%(n%)
ELSE
IF NOT rh%(n%) SWAP Fork%((n% + 1) MOD nSeats%), rh%(n%)
IF rh%(n%) IF NOT lh%(n%) SWAP Fork%((n%-1+nSeats%) MOD nSeats%), lh%(n%)
ENDIF
IF lh%(n%) AND rh%(n%) THEN
state%(n%) = RND(100)
PRINT Name$(n%) " is eating (" ; state%(n%) " ticks)"
ENDIF
ENDCASE
ENDPROC
DEF PROCcleanup
LOCAL I%
FOR I% = 0 TO nSeats%-1
PROC_killtimer(tID%(I%))
NEXT
ENDPROC</lang>
Sample output:
Russell is eating (92 ticks) Marx is eating (94 ticks) Russell is leaving the room Spinoza is eating (96 ticks) Marx is leaving the room Kant is eating (40 ticks) Marx is hungry again Kant is leaving the room Kant is hungry again Russell is hungry again Spinoza is leaving the room Aristotle is eating (30 ticks) Aristotle is leaving the room Marx is eating (19 ticks) Spinoza is hungry again Marx is leaving the room Kant is eating (20 ticks) Marx is hungry again Aristotle is hungry again Kant is leaving the room Russell is eating (100 ticks) Marx is eating (7 ticks) Marx is leaving the room Kant is hungry again Marx is hungry again Russell is leaving the room Spinoza is eating (7 ticks) Kant is eating (82 ticks) Spinoza is leaving the room Aristotle is eating (74 ticks)
C
Avoid deadlocks by making each philosopher have a different order of picking up forks. As long as one person waits for left fork first and another waits for right first, cycles can't form. <lang c>#include <pthread.h>
- include <stdio.h>
- include <stdlib.h>
- include <unistd.h>
- include <stdarg.h>
- define N 5
const char *names[N] = { "Aristotle", "Kant", "Spinoza", "Marx", "Russell" }; pthread_mutex_t forks[N];
- define M 5 /* think bubbles */
const char *topic[M] = { "Spaghetti!", "Life", "Universe", "Everything", "Bathroom" };
- define lock pthread_mutex_lock
- define unlock pthread_mutex_unlock
- define xy(x, y) printf("\033[%d;%dH", x, y)
- define clear_eol(x) print(x, 12, "\033[K")
void print(int y, int x, const char *fmt, ...) { static pthread_mutex_t screen = PTHREAD_MUTEX_INITIALIZER; va_list ap; va_start(ap, fmt);
lock(&screen); xy(y + 1, x), vprintf(fmt, ap); xy(N + 1, 1), fflush(stdout); unlock(&screen); }
void eat(int id) { int f[2], ration, i; /* forks */ f[0] = f[1] = id;
/* make some (but not all) philosophers leftie. could have been f[!id] = (id + 1) %N; for example */ f[id & 1] = (id + 1) % N;
clear_eol(id); print(id, 12, "..oO (forks, need forks)");
for (i = 0; i < 2; i++) { lock(forks + f[i]); if (!i) clear_eol(id);
print(id, 12 + (f[i] != id) * 6, "fork%d", f[i]); /* delay 1 sec to clearly show the order of fork acquisition */ sleep(1); }
for (i = 0, ration = 3 + rand() % 8; i < ration; i++) print(id, 24 + i * 4, "nom"), sleep(1);
/* done nomming, give up forks (order doesn't matter) */ for (i = 0; i < 2; i++) unlock(forks + f[i]); }
void think(int id) { int i, t; char buf[64] = {0};
do { clear_eol(id); sprintf(buf, "..oO (%s)", topic[t = rand() % M]);
for (i = 0; buf[i]; i++) { print(id, i+12, "%c", buf[i]); if (i < 5) usleep(200000); } usleep(500000 + rand() % 1000000); } while (t); }
void* philosophize(void *a) { int id = *(int*)a; print(id, 1, "%10s", names[id]); while(1) think(id), eat(id); }
int main() { int i, id[N]; pthread_t tid[N];
for (i = 0; i < N; i++) pthread_mutex_init(forks + (id[i] = i), 0);
for (i = 0; i < N; i++) pthread_create(tid + i, 0, philosophize, id + i);
/* wait forever: the threads don't actually stop */ return pthread_join(tid[0], 0); }</lang>
This uses a modified version of the Python algorithm version below. Uses POSIX threads. <lang C>#include <pthread.h>
- include <stdio.h>
- include <stdlib.h>
- include <unistd.h>
typedef struct philData {
pthread_mutex_t *fork_lft, *fork_rgt; const char *name; pthread_t thread; int fail;
} Philosopher;
int running = 1;
void *PhilPhunction(void *p) {
Philosopher *phil = (Philosopher*)p; int failed; int tries_left; pthread_mutex_t *fork_lft, *fork_rgt, *fork_tmp;
while (running) {
printf("%s is sleeping --er thinking\n", phil->name);
sleep( 1+ rand()%8);
fork_lft = phil->fork_lft;
fork_rgt = phil->fork_rgt;
printf("%s is hungry\n", phil->name);
tries_left = 2; /* try twice before being forceful */
do {
failed = pthread_mutex_lock( fork_lft);
failed = (tries_left>0)? pthread_mutex_trylock( fork_rgt )
: pthread_mutex_lock(fork_rgt);
if (failed) {
pthread_mutex_unlock( fork_lft);
fork_tmp = fork_lft;
fork_lft = fork_rgt;
fork_rgt = fork_tmp;
tries_left -= 1;
}
} while(failed && running);
if (!failed) {
printf("%s is eating\n", phil->name);
sleep( 1+ rand() % 8);
pthread_mutex_unlock( fork_rgt);
pthread_mutex_unlock( fork_lft);
}
}
return NULL;
}
void Ponder() {
const char *nameList[] = { "Kant", "Guatma", "Russel", "Aristotle", "Bart" };
pthread_mutex_t forks[5];
Philosopher philosophers[5];
Philosopher *phil;
int i;
int failed;
for (i=0;i<5; i++) {
failed = pthread_mutex_init(&forks[i], NULL);
if (failed) {
printf("Failed to initialize mutexes.");
exit(1);
}
}
for (i=0;i<5; i++) {
phil = &philosophers[i];
phil->name = nameList[i];
phil->fork_lft = &forks[i];
phil->fork_rgt = &forks[(i+1)%5];
phil->fail = pthread_create( &phil->thread, NULL, PhilPhunction, phil);
}
sleep(40);
running = 0;
printf("cleanup time\n");
for(i=0; i<5; i++) {
phil = &philosophers[i];
if ( !phil->fail && pthread_join( phil->thread, NULL) ) {
printf("error joining thread for %s", phil->name);
exit(1);
}
}
}
int main() {
Ponder(); return 0;
}</lang>
This version uses C11 threads and the approach of making one of the philosophers left-handed to avoid deadlock. <lang C>#include <stdio.h>
- include <threads.h>
- include <stdlib.h>
- define NUM_THREADS 5
struct timespec time1; mtx_t forks[NUM_THREADS];
typedef struct { char *name; int left; int right; } Philosopher;
Philosopher *create(char *nam, int lef, int righ) { Philosopher *x = malloc(sizeof(Philosopher)); x->name = nam; x->left = lef; x->right = righ; return x; }
int eat(void *data) { time1.tv_sec = 1; Philosopher *foo = (Philosopher *) data; mtx_lock(&forks[foo->left]); mtx_lock(&forks[foo->right]); printf("%s is eating\n", foo->name); thrd_sleep(&time1, NULL); printf("%s is done eating\n", foo->name); mtx_unlock(&forks[foo->left]); mtx_unlock(&forks[foo->right]); return 0; }
int main(void) {
thrd_t threadId[NUM_THREADS];
Philosopher *all[NUM_THREADS] = {create("Teral", 0 ,1), create("Billy", 1, 2), create("Daniel", 2,3), create("Philip", 3, 4), create("Bennet", 0, 4)}; for (int i = 0; i < NUM_THREADS; i++){ if (mtx_init(&forks[i], mtx_plain) != thrd_success){ puts("FAILED IN MUTEX INIT!"); return 0; } }
for (int i=0; i < NUM_THREADS; ++i) {
if (thrd_create(threadId+i, eat, all[i]) != thrd_success) {
printf("%d-th thread create error\n", i);
return 0;
}
}
for (int i=0; i < NUM_THREADS; ++i)
thrd_join(threadId[i], NULL);
return 0;
} </lang>
C#
<lang C sharp> using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading; using System.Threading.Tasks;
namespace Dining_Philosophers {
class Program
{
private const int DinerCount = 5;
private static List<Diner> Diners = new List<Diner>();
private static List<Fork> Forks = new List<Fork>();
private static DateTime TimeToStop;
static void Main(string[] args)
{
Initialize();
WriteHeaderLine();
do
{
WriteStatusLine();
Thread.Sleep(1000);
}
while (DateTime.Now < TimeToStop);
TearDown();
}
private static void Initialize()
{
for (int i = 0; i < DinerCount; i++)
Forks.Add(new Fork());
for (int i = 0; i < DinerCount; i++)
Diners.Add(new Diner(i, Forks[i], Forks[(i + 1) % DinerCount]));
TimeToStop = DateTime.Now.AddSeconds(60);
}
private static void TearDown()
{
foreach (var diner in Diners)
diner.Dispose();
}
private static void WriteHeaderLine()
{
Console.Write("|");
foreach (Diner d in Diners)
Console.Write("D " + d.ID + "|");
Console.Write(" |");
for (int i = 0; i < DinerCount; i++)
Console.Write("F" + i + "|");
Console.WriteLine();
}
private static void WriteStatusLine()
{
Console.Write("|");
foreach (Diner d in Diners)
Console.Write(FormatDinerState(d) + "|");
Console.Write(" |");
foreach (Fork f in Forks)
Console.Write(FormatForkState(f) + "|");
Console.WriteLine();
}
private static string FormatDinerState(Diner diner)
{
switch (diner.State)
{
case Diner.DinerState.Eating:
return "Eat";
case Diner.DinerState.Pondering:
return "Pon";
case Diner.DinerState.TryingToGetForks:
return "Get";
default:
throw new Exception("Unknown diner state.");
}
}
private static string FormatForkState(Fork fork)
{
return (!ForkIsBeingUsed(fork) ? " " : "D" + GetForkHolder(fork));
}
private static bool ForkIsBeingUsed(Fork fork)
{
return Diners.Count(d => d.CurrentlyHeldForks.Contains(fork)) > 0;
}
private static int GetForkHolder(Fork fork)
{
return Diners.Single(d => d.CurrentlyHeldForks.Contains(fork)).ID;
}
}
class Diner : IDisposable
{
private bool IsCurrentlyHoldingLeftFork = false;
private bool IsCurrentlyHoldingRightFork = false;
private const int MaximumWaitTime = 100;
private static Random Randomizer = new Random();
private bool ShouldStopEating = false;
public int ID { get; private set; }
public Fork LeftFork { get; private set; }
public Fork RightFork { get; private set; }
public DinerState State { get; private set; }
public IEnumerable<Fork> CurrentlyHeldForks
{
get
{
var forks = new List<Fork>();
if (IsCurrentlyHoldingLeftFork)
forks.Add(LeftFork);
if (IsCurrentlyHoldingRightFork)
forks.Add(RightFork);
return forks;
}
}
public Diner(int id, Fork leftFork, Fork rightFork)
{
InitializeDinerState(id, leftFork, rightFork);
BeginDinerActivity();
}
private void KeepTryingToEat()
{
do
if (State == DinerState.TryingToGetForks)
{
TryToGetLeftFork();
if (IsCurrentlyHoldingLeftFork)
{
TryToGetRightFork();
if (IsCurrentlyHoldingRightFork)
{
Eat();
DropForks();
Ponder();
}
else
{
DropForks();
WaitForAMoment();
}
}
else
WaitForAMoment();
}
else
State = DinerState.TryingToGetForks;
while (!ShouldStopEating);
}
private void InitializeDinerState(int id, Fork leftFork, Fork rightFork)
{
ID = id;
LeftFork = leftFork;
RightFork = rightFork;
State = DinerState.TryingToGetForks;
}
private async void BeginDinerActivity()
{
await Task.Run(() => KeepTryingToEat());
}
private void TryToGetLeftFork()
{
Monitor.TryEnter(LeftFork, ref IsCurrentlyHoldingLeftFork);
}
private void TryToGetRightFork()
{
Monitor.TryEnter(RightFork, ref IsCurrentlyHoldingRightFork);
}
private void DropForks()
{
DropLeftFork();
DropRightFork();
}
private void DropLeftFork()
{
if (IsCurrentlyHoldingLeftFork)
{
IsCurrentlyHoldingLeftFork = false;
Monitor.Exit(LeftFork);
}
}
private void DropRightFork()
{
if (IsCurrentlyHoldingRightFork)
{
IsCurrentlyHoldingRightFork = false;
Monitor.Exit(RightFork);
}
}
private void Eat()
{
State = DinerState.Eating;
WaitForAMoment();
}
private void Ponder()
{
State = DinerState.Pondering;
WaitForAMoment();
}
private static void WaitForAMoment()
{
Thread.Sleep(Randomizer.Next(MaximumWaitTime));
}
public void Dispose()
{
ShouldStopEating = true;
}
public enum DinerState
{
Eating,
TryingToGetForks,
Pondering
}
}
class Fork { }
} </lang>
C++
Compile with (something like): g++ -Wall -lboost_thread dining_philosophers.cpp -o dining_philosophers <lang cpp>
- include <vector>
- include <string>
- include <iostream>
- include <boost/cstdint.hpp>
- include <boost/thread.hpp>
- include <boost/thread/locks.hpp>
- include <boost/format.hpp>
- include <boost/shared_ptr.hpp>
typedef boost::mutex Fork; typedef boost::shared_ptr< Fork > ForkPtr; typedef boost::lock_guard< Fork > ForkLock;
- define MIN_WAIT_TIME 100
- define NUM_MEALS 10
- define MAX_JITTER 50
template< typename Stream > class AtomicLogger { public:
AtomicLogger( Stream& stream ) :
m_mutex(),
m_stream( stream )
{
}
void log( const std::string& str ) {
boost::mutex::scoped_lock lock( m_mutex );
m_stream << str << std::endl;
}
private:
mutable boost::mutex m_mutex; Stream& m_stream;
}; typedef AtomicLogger< std::ostream > AtomicLoggerOstream; typedef boost::shared_ptr< AtomicLoggerOstream > AtomicLoggerOstreamPtr;
class Philosopher { public:
Philosopher(
const std::string& name, ForkPtr fork_left, ForkPtr fork_right, AtomicLoggerOstreamPtr p_logger ) :
m_name( name ), m_continue( true ), mp_fork_left( fork_left ), mp_fork_right( fork_right ), m_thread( boost::thread( boost::bind( &Philosopher::thread_func,
this, &m_continue, mp_fork_left, mp_fork_right ) ) ),
m_meals_left( NUM_MEALS ),
mp_logger( p_logger )
{
}
~Philosopher() {
done_dining();
wait_for_cmplt();
}
void done_dining() { m_continue = false; }
void wait_for_cmplt() { m_thread.join(); }
private:
inline bool can_grab_fork( ForkPtr& p_fork ) { return p_fork->try_lock(); }
void thread_func( volatile bool* p_continue, ForkPtr fork_left, ForkPtr fork_right ) {
bool failed_to_grab_fork = false;
while( p_continue && m_meals_left ) {
mp_logger->log( boost::str( boost::format( "%1% is thinking" ) % this->m_name ) );
wait();
mp_logger->log( boost::str( boost::format( "%1% is hungry" ) % this->m_name ) );
// attempt to grab forks
if( can_grab_fork( fork_left ) ) {
ForkLock lock_left( *fork_left, boost::adopt_lock ); if( can_grab_fork( fork_right ) ) { ForkLock lock_right( *fork_right, boost::adopt_lock ); // eating mp_logger->log( boost::str( boost::format( "%1% is eating (%2%)..." ) % m_name % m_meals_left ) ); wait(); // record the meal --m_meals_left; } else { failed_to_grab_fork = true; }
} else {
failed_to_grab_fork = true;
}
if( failed_to_grab_fork ) {
mp_logger->log( boost::str( boost::format( "%1% couldn't get forks; waiting..." ) % m_name ) ); failed_to_grab_fork = false; wait();
} }
mp_logger->log( boost::str( boost::format( "%1% is done dining" ) % m_name ) ); }
inline void wait() {
wait( MIN_WAIT_TIME + ( std::rand() % MAX_JITTER ) );
}
inline void wait( boost::uint32_t time_in_ms ) {
boost::this_thread::sleep( boost::posix_time::milliseconds( time_in_ms ) );
}
std::string m_name; volatile bool m_continue; ForkPtr mp_fork_left; // must be declared before the thread ForkPtr mp_fork_right; // must be declared before the thread boost::thread m_thread; boost::uint32_t m_meals_left; AtomicLoggerOstreamPtr mp_logger;
}; typedef boost::shared_ptr< Philosopher > PhilosopherPtr;
int main() {
const int N = 5;
std::string names[] = { "Aristotle", "Spinoza", "Russell", "Kant", "Plato" };
std::vector< PhilosopherPtr > philosophers; philosophers.reserve( N );
// create logger AtomicLoggerOstreamPtr p_logger( new AtomicLoggerOstream( std::cout ) );
// create forks
std::vector< ForkPtr > forks;
forks.reserve( N );
for( int i = 0; i < N; ++i ) {
forks.push_back( ForkPtr( new Fork() ) );
}
// create philosophers
for( int i = 0; i < N; ++i ) {
philosophers.push_back( PhilosopherPtr(
new Philosopher( names[ i ], forks[ i ], forks[ (i + 1) % N ], p_logger ) ) );
}
// wait for them to finish
for( int i = 0; i < N; ++i ) {
philosophers[ i ]->wait_for_cmplt();
}
p_logger->log( "Everyone is done dining." );
return 0;
}
</lang>
Standard C++ only
<lang cpp>#include <algorithm>
- include <array>
- include <atomic>
- include <chrono>
//We are using only standard library, so snprintf instead of Boost::Format
- include <cstdio>
- include <iostream>
- include <mutex>
- include <random>
- include <string>
- include <thread>
std::mutex cout_mutex;
struct Fork {
std::mutex mutex;
};
struct Dinner {
std::atomic<bool> ready {false};
std::array<Fork, 5> forks;
~Dinner() { std::cout << "Dinner is over"; }
};
class Philosopher {
std::mt19937 rng{std::random_device {}()};
const std::string name; const Dinner& dinner; Fork& left; Fork& right; std::thread worker;
void live();
void dine();
void ponder();
public:
Philosopher(std::string name_, const Dinner& dinn, Fork& l, Fork& r)
: name(std::move(name_)), dinner(dinn) , left(l), right(r), worker(&Philosopher::live, this)
{}
~Philosopher()
{
worker.join();
std::lock_guard<std::mutex> cout_lock(cout_mutex);
std::cout << name << " went to sleep." << std::endl;
}
};
void Philosopher::live() {
while (not dinner.ready)
; //You spin me right round, baby, right round...
do {//Aquire forks first
//lock uses deadlock prevention mechanism to acquire mutexes safely
std::lock(left.mutex, right.mutex);
dine(); //Dine adopts lock on forks and releases them
if(not dinner.ready) break;
ponder();
} while(dinner.ready);
}
void Philosopher::dine() {
std::lock_guard<std::mutex> left_lock( left.mutex, std::adopt_lock); std::lock_guard<std::mutex> right_lock(right.mutex, std::adopt_lock);
thread_local std::array<const char*, 3> foods Template:"chicken", "rice", "soda"; thread_local std::array<const char*, 3> reactions Template:"I like this %s!", "This %s is good.", "Mmm, %s..."; thread_local std::uniform_int_distribution<> dist(1, 6); std::shuffle( foods.begin(), foods.end(), rng); std::shuffle(reactions.begin(), reactions.end(), rng);
if(not dinner.ready) return;
{
std::lock_guard<std::mutex> cout_lock(cout_mutex);
std::cout << name << " started eating." << std::endl;
}
constexpr size_t buf_size = 64;
char buffer[buf_size];
for(int i = 0; i < 3; ++i) {
std::this_thread::sleep_for(std::chrono::milliseconds(dist(rng)*50));
snprintf(buffer, buf_size, reactions[i], foods[i]);
std::lock_guard<std::mutex> cout_lock(cout_mutex);
std::cout << name << ": " << buffer << std::endl;
}
std::this_thread::sleep_for(std::chrono::milliseconds(dist(rng))*50);
std::lock_guard<std::mutex> cout_lock(cout_mutex);
std::cout << name << " finished and left." << std::endl;
}
void Philosopher::ponder() {
static constexpr std::array<const char*, 5> topics Template:"politics", "art", "meaning of life", "source of morality", "how many straws makes a bale"; thread_local std::uniform_int_distribution<> wait(1, 6); thread_local std::uniform_int_distribution<> dist(0, topics.size() - 1); while(dist(rng) > 0) { std::this_thread::sleep_for(std::chrono::milliseconds(wait(rng)*150)); std::lock_guard<std::mutex> cout_lock(cout_mutex); std::cout << name << " is pondering about " << topics[dist(rng)] << '.' << std::endl; if(not dinner.ready) return; } std::this_thread::sleep_for(std::chrono::milliseconds(wait(rng)*150)); std::lock_guard<std::mutex> cout_lock(cout_mutex); std::cout << name << " is hungry again!" << std::endl;
}
int main() {
Dinner dinner;
std::array<Philosopher, 5> philosophers {{
{"Aristotle", dinner, dinner.forks[0], dinner.forks[1]},
{"Kant", dinner, dinner.forks[1], dinner.forks[2]},
{"Spinoza", dinner, dinner.forks[2], dinner.forks[3]},
{"Marx", dinner, dinner.forks[3], dinner.forks[4]},
{"Russell", dinner, dinner.forks[4], dinner.forks[0]},
}};
std::this_thread::sleep_for(std::chrono::seconds(1));
std::cout << "Dinner started!" << std::endl;
dinner.ready = true;
std::this_thread::sleep_for(std::chrono::seconds(5));
dinner.ready = false;
std::lock_guard<std::mutex> cout_lock(cout_mutex);
std::cout << "It is dark outside..." << std::endl;
}</lang> Output: http://coliru.stacked-crooked.com/a/1b34c1fc36f5a30c
Dinner started!
Spinoza started eating.
Aristotle started eating.
Spinoza: This soda is good.
Spinoza: Mmm, rice...
Aristotle: Mmm, rice...
Spinoza: I like this chicken!
Aristotle: I like this soda!
Spinoza finished and left.
Marx started eating.
Marx: This soda is good.
Aristotle: This chicken is good.
Aristotle finished and left.
Kant started eating.
Marx: I like this rice!
Kant: I like this chicken!
Marx: Mmm, chicken...
Kant: Mmm, rice...
Marx finished and left.
Russell started eating.
Kant: This soda is good.
Spinoza is pondering about source of morality.
Kant finished and left.
Russell: I like this soda!
Aristotle is hungry again!
Russell: This rice is good.
Marx is pondering about art.
Russell: Mmm, chicken...
Russell finished and left.
Aristotle started eating.
Aristotle: This rice is good.
Kant is pondering about meaning of life.
Spinoza is pondering about source of morality.
Aristotle: Mmm, soda...
Spinoza is pondering about source of morality.
Aristotle: I like this chicken!
Aristotle finished and left.
Marx is hungry again!
Marx started eating.
Russell is pondering about how many straws makes a bale.
Kant is hungry again!
Kant started eating.
Aristotle is pondering about meaning of life.
Russell is pondering about how many straws makes a bale.
Marx: This soda is good.
Kant: Mmm, rice...
Russell is pondering about art.
Spinoza is pondering about politics.
Marx: Mmm, rice...
Aristotle is pondering about how many straws makes a bale.
Kant: I like this soda!
Marx: I like this chicken!
Marx finished and left.
Kant: This chicken is good.
Aristotle is pondering about how many straws makes a bale.
Aristotle is pondering about how many straws makes a bale.
Kant finished and left.
Russell is pondering about how many straws makes a bale.
Aristotle is hungry again!
Aristotle started eating.
Spinoza is pondering about source of morality.
Marx is hungry again!
Marx started eating.
Kant is pondering about source of morality.
Marx: This soda is good.
Russell is hungry again!
Aristotle: Mmm, soda...
Spinoza is pondering about source of morality.
Marx: I like this chicken!
Aristotle: This chicken is good.
Kant is pondering about source of morality.
Spinoza is pondering about source of morality.
Marx: Mmm, rice...
Aristotle: I like this rice!
Aristotle finished and left.
It is dark outside...
Marx finished and left.
Russell went to sleep.
Marx went to sleep.
Aristotle is hungry again!
Spinoza is pondering about meaning of life.
Spinoza went to sleep.
Kant is pondering about politics.
Kant went to sleep.
Aristotle went to sleep.
Dinner is over
Clojure
Clojure's STM allows us to avoid low-level synchronization primitives like semaphores. In order to simulate the Dining Philosophers scenario, the forks are references to a boolean indicating whether or not it is available for use. Each philosopher (also held in a ref) has a fixed amount of food he will try to eat, first by trying to acquire both forks, eating for some period of time, releasing both forks, then thinking for some period of time; if the forks cannot be acquired, the philosopher waits for a fixed amount of time and tries again. <lang clojure>(defn make-fork []
(ref true))
(defn make-philosopher [name forks food-amt]
(ref {:name name :forks forks :eating? false :food food-amt}))
(defn start-eating [phil]
(dosync
(if (every? true? (map ensure (:forks @phil))) ; <-- the essential solution
(do
(doseq [f (:forks @phil)] (alter f not))
(alter phil assoc :eating? true)
(alter phil update-in [:food] dec)
true)
false)))
(defn stop-eating [phil]
(dosync
(when (:eating? @phil)
(alter phil assoc :eating? false)
(doseq [f (:forks @phil)] (alter f not)))))
(defn dine [phil retry-interval max-eat-duration max-think-duration]
(while (pos? (:food @phil))
(if (start-eating phil)
(do
(Thread/sleep (rand-int max-eat-duration))
(stop-eating phil)
(Thread/sleep (rand-int max-think-duration)))
(Thread/sleep retry-interval))))</lang>
The second line of the start-eating function contains the essential solution: by invoking ensure on every fork reference, we are guaranteed that the state of the forks will not be modified by other transactions, thus we can rely on those values for the rest of the transaction. Now we just need to run it: <lang clojure>(def *forks* (cycle (take 5 (repeatedly #(make-fork)))))
(def *philosophers*
(doall (map #(make-philosopher %1 [(nth *forks* %2) (nth *forks* (inc %2))] 1000)
["Aristotle" "Kant" "Spinoza" "Marx" "Russell"]
(range 5))))
(defn start []
(doseq [phil *philosophers*] (.start (Thread. #(dine phil 5 100 100)))))
(defn status []
(dosync
(doseq [i (range 5)]
(let [f @(nth *forks* i)
p @(nth *philosophers* i)]
(println (str "fork: available=" f))
(println (str (:name p)
": eating=" (:eating? p)
" food=" (:food p)))))))</lang>
The status function inspects the data structures within a transaction so as to give consistent results (e.g., every unavailable fork has exactly one "eating" philosopher adjacent).
Common Lisp
This is a translation of the Python solution with small improvements.
Random times are calculated based upon a normal distribution; the main loop doesn't sleep to wait for all philosophers to end dining, it uses a condition variable instead.
<lang lisp>(in-package :common-lisp-user)
- FLAG -- if using quicklisp, you can get bordeaux-threads loaded up
- with
- (ql:quickload :bordeaux-threads)
(defvar *philosophers* '(Aristotle Kant Spinoza Marx Russell))
(defclass philosopher ()
((name :initarg :name :reader name-of) (left-fork :initarg :left-fork :accessor left-fork-of) (right-fork :initarg :right-fork :accessor right-fork-of) (meals-left :initarg :meals-left :accessor meals-left-of)))
(defclass fork ()
((lock :initform (bt:make-lock "fork") :reader lock-of)))
(defun random-normal (&optional (mean 0.0) (sd 1.0))
(do* ((x1 #1=(1- (* 2.0d0 (random 1d0))) #1#)
(x2 #2=(1- (* 2.0d0 (random 1d0))) #2#)
(w #3=(+ (* x1 x1) (* x2 x2)) #3#))
((< w 1d0) (+ (* (* x1 (sqrt (/ (* -2d0 (log w)) w))) sd) mean))))
(defun sleep* (time) (sleep (max time (/ (expt 10 7)))))
(defun dining-philosophers (&key (philosopher-names *philosophers*)
(meals 30)
(dining-time'(1 2))
(thinking-time '(1 2))
((stream e) *error-output*))
(let* ((count (length philosopher-names))
(forks (loop repeat count collect (make-instance 'fork)))
(philosophers (loop for i from 0
for name in philosopher-names collect
(make-instance 'philosopher
:left-fork (nth (mod i count) forks)
:right-fork (nth (mod (1+ i) count) forks)
:name name
:meals-left meals)))
(condition (bt:make-condition-variable))
(lock (bt:make-lock "main loop"))
(output-lock (bt:make-lock "output lock")))
(dolist (p philosophers)
(labels ((think ()
(/me "is now thinking")
(sleep* (apply #'random-normal thinking-time))
(/me "is now hungry")
(dine))
(dine ()
(bt:with-lock-held ((lock-of (left-fork-of p)))
(or (bt:acquire-lock (lock-of (right-fork-of p)) nil)
(progn (/me "couldn't get a fork and ~
returns to thinking")
(bt:release-lock (lock-of (left-fork-of p)))
(return-from dine (think))))
(/me "is eating")
(sleep* (apply #'random-normal dining-time))
(bt:release-lock (lock-of (right-fork-of p)))
(/me "is done eating (~A meals left)"
(decf (meals-left-of p))))
(cond ((<= (meals-left-of p) 0)
(/me "leaves the dining room")
(bt:with-lock-held (lock)
(setq philosophers (delete p philosophers))
(bt:condition-notify condition)))
(t (think))))
(/me (control &rest args)
(bt:with-lock-held (output-lock)
(write-sequence (string (name-of p)) e)
(write-char #\Space e)
(apply #'format e (concatenate 'string control "~%")
args))))
(bt:make-thread #'think)))
(loop (bt:with-lock-held (lock)
(when (endp philosophers)
(format e "all philosophers are done dining~%")
(return)))
(bt:with-lock-held (lock)
(bt:condition-wait condition lock)))))
</lang> Alternative solution using library STMX which provides Software Transactional Memory, as well as BORDEAUX-THREADS above. Depends on Quicklisp. TAKE will wait until something is available in a TCELL, then remove it. PUT will wait for a TCELL to become empty, then add it. ATOMIC ensures STM operations in its body happen atomically. <lang lisp>(ql:quickload '(:stmx :bordeaux-threads))
(defpackage :dining-philosophers
(:use :cl))
(in-package :dining-philosophers)
(defstruct philosopher
name left-fork right-fork)
(defparameter *philosophers* '("Aristotle" "Kant" "Spinoza" "Marx" "Russell")) (defparameter *eating-max* 5.0) (defparameter *thinking-max* 5.0) (defvar *log-lock* (bt:make-lock)) (defvar *running* nil)
(defun print-log (name status)
(bt:with-lock-held (*log-lock*) (format t "~a is ~a~%" name status)))
(defun philosopher-cycle (philosopher)
"Continously atomically grab and return the left and right forks of the given PHILOSOPHER."
(with-slots (name left-fork right-fork) philosopher
(loop while *running*
do
(print-log name "hungry")
(stmx:atomic
(stmx.util:take left-fork)
(stmx.util:take right-fork))
(print-log name "eating")
(sleep (random *eating-max*))
(stmx:atomic
(stmx.util:put left-fork t)
(stmx.util:put right-fork t))
(print-log name "thinking")
(sleep (random *thinking-max*)))))
(defun scenario ()
(let ((forks (loop repeat (length *philosophers*) collect (stmx.util:tcell t))))
(setf *running* t)
(loop for name in *philosophers*
for left-fork in forks
for right-fork in (append (cdr forks) (list (car forks)))
do (let ((philosopher (make-philosopher :name name :left-fork left-fork :right-fork right-fork)))
(bt:make-thread (lambda () (philosopher-cycle philosopher))
:initial-bindings (cons (cons '*standard-output* *standard-output*)
bt:*default-special-bindings*))))))
</lang>
- Output:
<lang lisp>DINING-PHILOSOPHERS> (scenario) Aristotle is hungry Aristotle is eating Kant is hungry Spinoza is hungry Spinoza is eating Marx is hungry NIL Russell is hungry Aristotle is thinking Russell is eating Spinoza is thinking Kant is eating Spinoza is hungry Russell is thinking Marx is eating Kant is thinking Aristotle is hungry Aristotle is eating Marx is thinking Spinoza is eating Spinoza is thinking Marx is hungry Marx is eating Russell is hungry Marx is thinking Kant is hungry Aristotle is thinking Russell is eating Kant is eating Marx is hungry Spinoza is hungry Kant is thinking Spinoza is eating Kant is hungry Aristotle is hungry Russell is thinking Aristotle is eating Aristotle is thinking Aristotle is hungry Aristotle is eating Spinoza is thinking Marx is eating ...</lang>
D
This code is using a strict order for the forks/mutexes to prevent a deadlock.
<lang d>import std.stdio, std.algorithm, std.string, std.parallelism,
core.sync.mutex;
void eat(in size_t i, in string name, Mutex[] forks) {
writeln(name, " is hungry."); immutable j = (i + 1) % forks.length;
// Take forks i and j. The lower one first to prevent deadlock. auto fork1 = forks[min(i, j)]; auto fork2 = forks[max(i, j)];
fork1.lock; scope(exit) fork1.unlock;
fork2.lock; scope(exit) fork2.unlock;
writeln(name, " is eating."); writeln(name, " is full.");
}
void think(in string name) {
writeln(name, " is thinking.");
}
void main() {
const philosophers = "Aristotle Kant Spinoza Marx Russell".split;
Mutex[philosophers.length] forks;
foreach (ref fork; forks)
fork = new Mutex;
defaultPoolThreads = forks.length;
foreach (i, philo; taskPool.parallel(philosophers)) {
foreach (immutable _; 0 .. 100) {
eat(i, philo, forks);
philo.think;
}
}
}</lang>
- Sample output:
Spinoza is full. Spinoza is thinking. Russel is eating. Russel is full. Russel is thinking. Russel is hungry. Kant is eating. Kant is full. Kant is thinking. Kant is hungry. Spinoza is hungry. Aristotle is eating. Aristotle is full.
E
A classic article on solving a version of this problem in E is Satan Comes to Dinner in E.
EchoLisp
We introduce a laquais who checks that no more than 4 philosophers are sitting at the same time. This prevents deadlocks. Reference : The little book of semaphores.
<lang scheme> (lib 'tasks)
(define names #(Aristotle Kant Spinoza Marx Russell)) (define abouts #("Wittgenstein" "the nature of the World" "Kant" "starving"
"spaghettis" "the essence of things" "Ω" "📞" "⚽️" "🍅" "🌿" "philosophy" "💔" "👠" "rosetta code" "his to-do list" ))
(define (about) (format "thinking about %a." (vector-ref abouts (random (vector-length abouts)))))
- statistics
(define rounds (make-vector 5 0)) (define (eat i) (vector-set! rounds i (1+ (vector-ref rounds i))))
- forks are resources = semaphores
(define (left i) i) (define (right i) (modulo (1+ i) 5)) (define forks (for/vector ((i 5)) (make-semaphore 1))) (define (fork i) (vector-ref forks i))
(define laquais (make-semaphore 4))
- philosophers tasks
(define (philo i)
- thinking
(writeln (vector-ref names i) (about)) (sleep (+ 2000 (random 1000))) (wait laquais)
- get forks
(writeln (vector-ref names i) 'sitting)
(wait (fork (left i)))
(wait (fork (right i)))
(writeln (vector-ref names i) 'eating)
(eat i)
(sleep (+ 6000 (random 1000)))
- put-forks
(signal (fork (left i))) (signal (fork (right i))) (signal laquais) i)
(define tasks (for/vector ((i 5)) (make-task philo i))) </lang>
- Output:
<lang scheme> (define (observe dummmy) (writeln 'observer 'rounds= rounds) #t) (define observer (make-task observe #t ))
(define (dinner) (task-run observer 5000) (for ((t tasks)) (task-run t)))
(dinner)
</lang>
Marx thinking about philosophy.
Russell thinking about Kant.
Aristotle thinking about 🌿.
Spinoza thinking about Ω.
Kant thinking about 🍅.
Marx sitting
Marx eating
Russell sitting
Aristotle sitting
Aristotle eating
Spinoza sitting
observer rounds= #( 1 0 0 1 0)
observer rounds= #( 1 0 0 1 0)
Spinoza eating
Marx thinking about 🍅.
Kant sitting
Russell eating
Aristotle thinking about 💔.
observer rounds= #( 1 0 1 1 1)
Marx sitting
Kant eating
Aristotle sitting
Spinoza thinking about Ω.
observer rounds= #( 1 1 1 1 1)
Marx eating
Russell thinking about 🌿.
Spinoza sitting
observer rounds= #( 1 1 1 2 1)
Russell sitting
Aristotle eating
Kant thinking about 💔.
Spinoza eating
Marx thinking about 📞.
Kant sitting
observer rounds= #( 2 1 2 2 1)
Russell eating
Marx sitting
Aristotle thinking about Kant.
Kant eating
Spinoza thinking about spaghettis.
observer rounds= #( 2 2 2 2 2)
Aristotle sitting
observer rounds= #( 2 2 2 2 2)
Spinoza sitting
Marx eating
Russell thinking about 📞.
Aristotle eating
Kant thinking about the essence of things.
Russell sitting
[...] CTRL-C to stop.
Eiffel
This solution for the dining philosophers is programmed in Eiffel using Simple Concurrent Object-Oriented Programming (SCOOP). In SCOOP for Eiffel, the keyword separate in a declaration designates that the associated object may be handled by a SCOOP processor other than (separate from) the one handling the current object. So, in this example, philosophers and forks are all declared as separate types.
The synchronization of access to the resources (the forks) occurs when the routine eat is called. The two arguments are the two separate forks adjacent to the philosopher. The eat routine will not proceed until exclusive access to all separate arguments is assured. The resources are released when the routine terminates.
The example uses numbers (versus names) to identify the philosophers in order to allow the user to vary the number of philosophers.
<lang eiffel >class
DINING_PHILOSOPHERS
create
make
feature -- Initialization
make
-- Create philosophers and forks.
local
first_fork: separate FORK
left_fork: separate FORK
right_fork: separate FORK
philosopher: separate PHILOSOPHER
i: INTEGER
do
print ("Dining Philosophers%N" + philosopher_count.out + " philosophers, " + round_count.out + " rounds%N%N")
create philosophers.make
from
i := 1
create first_fork.make (philosopher_count, 1)
left_fork := first_fork
until
i > philosopher_count
loop
if i < philosopher_count then
create right_fork.make (i, i + 1)
else
right_fork := first_fork
end
create philosopher.make (i, left_fork, right_fork, round_count)
philosophers.extend (philosopher)
left_fork := right_fork
i := i + 1
end
philosophers.do_all (agent launch_philosopher)
print ("Make Done!%N")
end
feature {NONE} -- Implementation
philosopher_count: INTEGER = 5
-- Number of philosophers.
round_count: INTEGER = 30
-- Number of times each philosopher should eat.
philosophers: LINKED_LIST [separate PHILOSOPHER]
-- List of philosophers.
launch_philosopher (a_philosopher: separate PHILOSOPHER)
-- Launch a_philosopher.
do
a_philosopher.live
end
end -- class DINING_PHILOSOPHERS</lang> <lang eiffel >class
PHILOSOPHER
create
make
feature -- Initialization
make (philosopher: INTEGER; left, right: separate FORK; rounds: INTEGER)
-- Initialize with ID of `philosopher', forks `left' and `right', and for `rounds' times to eat.
require
valid_id: philosopher >= 1
valid_times_to_eat: rounds >= 1
do
id := philosopher
left_fork := left
right_fork := right
round_count := rounds
report ("announced")
ensure
id_set: id = philosopher
left_fork_set: left_fork = left
right_fork_set: right_fork = right
rounds_set: round_count = rounds
end
feature -- Access
id: INTEGER
-- Philosopher's id.
feature -- Basic operations
live
-- Model philosopher's life.
do
from
report ("joined")
has_eaten_count := 0
until
has_eaten_count >= round_count
loop
think
eat (left_fork, right_fork)
end
report ("done")
end
eat (left, right: separate FORK)
-- Eat, having acquired `left' and `right' forks.
do
-- Take forks.
report ("taking forks")
left.pick (Current)
right.pick (Current)
-- Eat.
report ("eating")
delay (200)
-- Put forks back.
report ("putting forks back")
left.put (Current)
right.put (Current)
-- Report statistics.
has_eaten_count := has_eaten_count + 1
report ("has eaten " + has_eaten_count.out + " times")
end
think
-- Think ... for a short time.
do
report ("thinking")
delay (400)
end
feature {NONE} -- Output
report (task: STRING)
-- Report about execution of the specified `task'.
do
print ("Philosopher " + id.out + ": " + task + ".%N")
end
feature {NONE} -- Timing
delay (milliseconds: INTEGER_64)
-- Delay execution by `milliseconds'.
do
(create {EXECUTION_ENVIRONMENT}).sleep (milliseconds * 1_000_000)
end
feature {NONE} -- Status
round_count: INTEGER
-- Number of times philosopher should eat.
has_eaten_count: INTEGER
-- Number of times philosopher has eaten so far.
left_fork: separate FORK
-- Left fork used for eating.
right_fork: separate FORK
-- Right fork used for eating.
invariant
valid_id: id >= 1 valid_round_count: round_count >= 1 valid_has_eaten_count: has_eaten_count <= round_count
end -- class PHILOSOPHER</lang> <lang eiffel >class
FORK
create
make
feature -- Initialization
make (left, right: INTEGER)
-- Initialize between philosophers `left' and `right'.
do
id := left.out + "F" + right.out
end
feature -- Access
id: STRING
-- Identification: `F' enclosed by adjacent philosopher id's.
feature -- Basic operations
pick (philosopher: separate PHILOSOPHER)
-- Report fork picked up.
do
print ("Fork " + id + " picked up by Philosopher " + philosopher.id.out + ".%N")
end
put (philosopher: separate PHILOSOPHER)
-- Report fork put back.
do
print ("Fork " + id + " put back by Philosopher " + philosopher.id.out + ".%N")
end
end -- class FORK</lang>
Elixir
Implements the Chandy-Misra algorithm.
<lang Elixir> defmodule Philosopher do
defstruct missing: [], clean: [], promised: []
def run_demo do
pid1 = spawn(__MODULE__, :init, ["Russell"])
pid2 = spawn(__MODULE__, :init, ["Marx"])
pid3 = spawn(__MODULE__, :init, ["Spinoza"])
pid4 = spawn(__MODULE__, :init, ["Kant"])
pid5 = spawn(__MODULE__, :init, ["Aristotle"])
# a chopstick is simply represented by the pid of the neighbour that shares it.
send(pid1, {:run, %Philosopher{}})
send(pid2, {:run, %Philosopher{missing: [pid1]}})
send(pid3, {:run, %Philosopher{missing: [pid2]}})
send(pid4, {:run, %Philosopher{missing: [pid3]}})
send(pid5, {:run, %Philosopher{missing: [pid1, pid4]}})
end
def init(philosopher_name) do
receive do
{:run, state} ->
spawn(__MODULE__, :change_state, [self()])
case flip_coin() do
:heads -> thinking(philosopher_name, state)
:tails -> hungry(philosopher_name, state)
end
end
end
defp thinking(philosopher_name, state) do
receive do
{:change_state} ->
hungry(philosopher_name, state)
{:chopstick_request, pid} ->
if clean?(pid, state) do
thinking(philosopher_name, promise_chopstick(philosopher_name, pid, state))
else
give_chopstick(philosopher_name, self(), pid)
%{missing: missing} = state
thinking(philosopher_name, %{state | missing: [pid | missing]})
end
end
end
defp hungry(philosopher_name, state) do
IO.puts "#{philosopher_name} is hungry."
%{missing: missing} = state
for pid <- missing, do: request_chopstick(philosopher_name, self(), pid)
wait_for_chopsticks(philosopher_name, state)
end
defp wait_for_chopsticks(philosopher_name, state) do
if has_chopsticks?(state) do
eating(philosopher_name, state)
end
receive do
{:chopstick_request, pid} ->
if clean?(pid, state) do
wait_for_chopsticks(philosopher_name, promise_chopstick(philosopher_name, pid, state))
else
give_chopstick(philosopher_name, self(), pid)
request_chopstick(philosopher_name, self(), pid)
%{missing: missing} = state
wait_for_chopsticks(philosopher_name, %{state | missing: [pid | missing]})
end
{:chopstick_response, pid} ->
%{missing: missing, clean: clean} = state
wait_for_chopsticks(philosopher_name, %{state | missing: List.delete(missing, pid), clean: [pid | clean]})
end
end
defp eating(philosopher_name, state) do
IO.puts "*** #{philosopher_name} is eating."
receive do
{:change_state} ->
%{promised: promised} = state
for pid <- promised, do: give_chopstick(philosopher_name, self(), pid)
thinking(philosopher_name, %Philosopher{missing: promised})
end
end
defp clean?(pid, state) do
%{clean: clean} = state
Enum.member?(clean, pid)
end
defp has_chopsticks?(state) do
%{missing: missing} = state
Enum.empty?(missing)
end
defp promise_chopstick(philosopher_name, pid, state) do
IO.puts "#{philosopher_name} promises a chopstick."
%{promised: promised} = state
%{state | promised: [pid | promised]}
end
defp request_chopstick(philosopher_name, snd_pid, recv_pid) do
IO.puts "#{philosopher_name} requests a chopstick."
send(recv_pid, {:chopstick_request, snd_pid})
end
defp give_chopstick(philosopher_name, snd_pid, recv_pid) do
IO.puts "#{philosopher_name} gives a chopstick."
send(recv_pid, {:chopstick_response, snd_pid})
end
defp flip_coin do
case Enum.random(0..1) do
0 -> :heads
1 -> :tails
end
end
def change_state(pid) do
Process.sleep(Enum.random(1..10) * 1000)
send(pid, {:change_state})
change_state(pid)
end
</lang>
Erlang
This solution avoids deadlock by employing a waiter that only serves a plate of spaghetti when a philosopher has access to two forks. The philosophers wait until a plate is put in front of them before grabbing the forks.
<lang erlang>-module(philosophers). -export([dining/0]).
sleep(T) ->
receive
after T -> true end.
doForks(ForkList) ->
receive
{grabforks, {Left, Right}} -> doForks(ForkList -- [Left, Right]); {releaseforks, {Left, Right}} -> doForks([Left, Right| ForkList]); {available, {Left, Right}, Sender} ->
Sender ! {areAvailable, lists:member(Left, ForkList) andalso lists:member(Right, ForkList)},
doForks(ForkList);
{die} -> io:format("Forks put away.~n")
end.
areAvailable(Forks) -> forks ! {available, Forks, self()}, receive {areAvailable, false} -> false; {areAvailable, true} -> true end.
processWaitList([]) -> false;
processWaitList([H|T]) ->
{Client, Forks} = H,
case areAvailable(Forks) of
true -> Client ! {served},
true;
false -> processWaitList(T)
end.
doWaiter([], 0, 0, false) -> forks ! {die}, io:format("Waiter is leaving.~n"), diningRoom ! {allgone}; doWaiter(WaitList, ClientCount, EatingCount, Busy) -> receive {waiting, Client} -> WaitList1 = [Client|WaitList], %% add to waiting list case (not Busy) and (EatingCount<2) of true -> Busy1 = processWaitList(WaitList1); false -> Busy1 = Busy end, doWaiter(WaitList1, ClientCount, EatingCount, Busy1);
{eating, Client} -> doWaiter(WaitList -- [Client], ClientCount, EatingCount+1, false);
{finished} -> doWaiter(WaitList, ClientCount, EatingCount-1, processWaitList(WaitList)); {leaving} -> doWaiter(WaitList, ClientCount-1, EatingCount, Busy) end.
philosopher(Name, Forks, 0) ->
io:format("~s is leaving.~n", [Name]),
waiter ! {leaving};
philosopher(Name, Forks, Cycle) ->
io:format("~s is thinking.~n", [Name]),
sleep(random:uniform(1000)),
io:format("~s is hungry.~n", [Name]), waiter ! {waiting, {self(), Forks}}, %%sit at table
receive {served}-> forks ! {grabforks, Forks}, %%grab forks waiter ! {eating, {self(), Forks}}, %%start eating io:format("~s is eating.~n", [Name]) end,
sleep(random:uniform(1000)), forks ! {releaseforks, Forks}, %% put forks down waiter ! {finished},
philosopher(Name, Forks, Cycle-1).
dining() -> AllForks = [1, 2, 3, 4, 5],
Clients = 5,
register(diningRoom, self()),
register(forks, spawn(fun()-> doForks(AllForks) end)), register(waiter, spawn(fun()-> doWaiter([], Clients, 0, false) end)), Life_span = 20, spawn(fun()-> philosopher('Aristotle', {5, 1}, Life_span) end), spawn(fun()-> philosopher('Kant', {1, 2}, Life_span) end), spawn(fun()-> philosopher('Spinoza', {2, 3}, Life_span) end), spawn(fun()-> philosopher('Marx', {3, 4}, Life_span) end), spawn(fun()-> philosopher('Russel', {4, 5}, Life_span) end),
receive
{allgone} -> io:format("Dining room closed.~n")
end, unregister(diningRoom).</lang>
Euphoria
<lang Euphoria>constant FREE = 0, LOCKED = 1 sequence forks forks = repeat(FREE,5)
procedure person(sequence name, integer left_fork, integer right_fork)
while 1 do
while forks[left_fork] = LOCKED or forks[right_fork] = LOCKED do
if forks[left_fork] = FREE then
puts(1, name & " hasn't right fork.\n")
elsif forks[right_fork] = FREE then
puts(1, name & " hasn't left fork.\n")
else
puts(1, name & " hasn't both forks.\n")
end if
puts(1, name & " is waiting.\n")
task_yield()
end while
puts(1, name & " grabs forks.\n")
forks[left_fork] = LOCKED
forks[right_fork] = LOCKED
for i = 1 to rand(10) do
puts(1, name & " is eating.\n")
task_yield()
end for
puts(1, name & " puts forks down and leaves the dinning room.\n")
forks[left_fork] = FREE
forks[right_fork] = FREE
for i = 1 to rand(10) do
puts(1, name & " is thinking.\n")
task_yield()
end for
puts(1, name & " becomes hungry.\n")
end while
end procedure
integer rid atom taskid rid = routine_id("person") taskid = task_create(rid,{"Aristotle",1,2}) task_schedule(taskid,{1,2}) taskid = task_create(rid,{"Kant",2,3}) task_schedule(taskid,{1,2}) taskid = task_create(rid,{"Spinoza",3,4}) task_schedule(taskid,{1,2}) taskid = task_create(rid,{"Marx",4,5}) task_schedule(taskid,{1,2}) taskid = task_create(rid,{"Russell",5,1}) task_schedule(taskid,{1,2})
while get_key() = -1 do
task_yield()
end while</lang>
Sample output:
Russell grabs forks. Russell is eating. Marx hasn't right fork. Marx is waiting. Spinoza grabs forks. Spinoza is eating. Kant hasn't right fork. Kant is waiting. Aristotle hasn't left fork. Aristotle is waiting. Russell is eating. Marx hasn't both forks. Marx is waiting. Spinoza is eating. Kant hasn't right fork. Kant is waiting. Aristotle hasn't left fork. Aristotle is waiting. Russell is eating. Marx hasn't both forks. Marx is waiting. Spinoza is eating. Kant hasn't right fork. Kant is waiting. Aristotle hasn't left fork. Aristotle is waiting. Russell puts forks down and leaves the dinning room. Russell is thinking. Marx hasn't left fork. Marx is waiting. Spinoza puts forks down and leaves the dinning room. Spinoza is thinking. Kant grabs forks. Kant is eating. Aristotle hasn't right fork. Aristotle is waiting. Russell becomes hungry. Russell grabs forks. Russell is eating. Marx hasn't right fork. Marx is waiting. Spinoza is thinking. Kant is eating. Aristotle hasn't both forks. Aristotle is waiting.
F#
This solution avoids deadlock by employing a waiter.
<lang fsharp> open System
let flip f x y = f y x
let rec cycle s = seq { yield! s; yield! cycle s }
type Agent<'T> = MailboxProcessor<'T>
type Message = Waiting of (Set<int> * AsyncReplyChannel<unit>) | Done of Set<int>
let reply (c: AsyncReplyChannel<_>) = c.Reply()
let strategy forks waiting =
let aux, waiting = List.partition (fst >> flip Set.isSubset forks) waiting let forks = aux |> List.map fst |> List.fold (-) forks List.iter (snd >> reply) aux forks, waiting
let waiter strategy forkCount =
Agent<_>.Start(fun inbox ->
let rec loop forks waiting =
async { let forks, waiting = strategy forks waiting
let! msg = inbox.Receive()
match msg with
| Waiting r -> return! loop forks (waiting @ [r])
| Done f -> return! loop (forks + f) waiting }
loop (Set.ofList (List.init forkCount id)) [])
let philosopher (waiter: Agent<_>) name forks =
let rng = new Random()
let forks = Set.ofArray forks
Agent<_>.Start(fun inbox ->
let rec loop () =
async { printfn "%s is thinking" name
do! Async.Sleep(rng.Next(100, 500))
printfn "%s is hungry" name
do! waiter.PostAndAsyncReply(fun c -> Waiting (forks, c))
printfn "%s is eating" name
do! Async.Sleep(rng.Next(100, 500))
printfn "%s is done eating" name
waiter.Post(Done (forks))
return! loop () }
loop ())
[<EntryPoint>] let main args =
let forks = Seq.init 5 id |> cycle |> Seq.windowed 2 |> Seq.take 5 |> Seq.toList let names = ["plato"; "aristotel"; "kant"; "nietzsche"; "russel"] let waiter = waiter strategy 5 List.map2 (philosopher waiter) names forks |> ignore Console.ReadLine() |> ignore 0
</lang>
Go
Channels
Goroutine synchronization done with Go channels. Deadlock prevented by making one philosopher "left handed." <lang go>package main
import (
"hash/fnv" "log" "math/rand" "os" "time"
)
// Number of philosophers is simply the length of this list. // It is not otherwise fixed in the program. var ph = []string{"Aristotle", "Kant", "Spinoza", "Marx", "Russell"}
const hunger = 3 // number of times each philosopher eats const think = time.Second / 100 // mean think time const eat = time.Second / 100 // mean eat time
var fmt = log.New(os.Stdout, "", 0) // for thread-safe output
var done = make(chan bool)
// This solution uses channels to implement synchronization. // Sent over channels are "forks." type fork byte
// A fork object in the program models a physical fork in the simulation. // A separate channel represents each fork place. Two philosophers // have access to each fork. The channels are buffered with capacity = 1, // representing a place for a single fork.
// Goroutine for philosopher actions. An instance is run for each // philosopher. Instances run concurrently. func philosopher(phName string,
dominantHand, otherHand chan fork, done chan bool) {
fmt.Println(phName, "seated")
// each philosopher goroutine has a random number generator,
// seeded with a hash of the philosopher's name.
h := fnv.New64a()
h.Write([]byte(phName))
rg := rand.New(rand.NewSource(int64(h.Sum64())))
// utility function to sleep for a randomized nominal time
rSleep := func(t time.Duration) {
time.Sleep(t/2 + time.Duration(rg.Int63n(int64(t))))
}
for h := hunger; h > 0; h-- {
fmt.Println(phName, "hungry")
<-dominantHand // pick up forks
<-otherHand
fmt.Println(phName, "eating")
rSleep(eat)
dominantHand <- 'f' // put down forks
otherHand <- 'f'
fmt.Println(phName, "thinking")
rSleep(think)
}
fmt.Println(phName, "satisfied")
done <- true
fmt.Println(phName, "left the table")
}
func main() {
fmt.Println("table empty")
// Create fork channels and start philosopher goroutines,
// supplying each goroutine with the appropriate channels
place0 := make(chan fork, 1)
place0 <- 'f' // byte in channel represents a fork on the table.
placeLeft := place0
for i := 1; i < len(ph); i++ {
placeRight := make(chan fork, 1)
placeRight <- 'f'
go philosopher(ph[i], placeLeft, placeRight, done)
placeLeft = placeRight
}
// Make one philosopher left handed by reversing fork place
// supplied to philosopher's dominant hand.
// This makes precedence acyclic, preventing deadlock.
go philosopher(ph[0], place0, placeLeft, done)
// they are all now busy eating
for range ph {
<-done // wait for philosphers to finish
}
fmt.Println("table empty")
}</lang> Output:
table empty Kant seated Marx seated Spinoza seated Aristotle seated Kant hungry Russell seated Marx hungry Russell hungry Kant eating Marx eating Aristotle hungry Spinoza hungry Kant thinking Marx thinking Spinoza eating Russell eating Kant hungry Russell thinking Aristotle eating Marx hungry Spinoza thinking Marx eating Russell hungry Marx thinking Aristotle thinking Russell eating Kant eating Russell thinking Aristotle hungry Kant thinking Aristotle eating Spinoza hungry Spinoza eating Marx hungry Aristotle thinking Russell hungry Aristotle hungry Kant hungry Spinoza thinking Kant eating Marx eating Marx thinking Russell eating Kant thinking Marx satisfied Marx left the table Russell thinking Aristotle eating Spinoza hungry Spinoza eating Russell satisfied Russell left the table Kant satisfied Kant left the table Spinoza thinking Aristotle thinking Aristotle satisfied Aristotle left the table Spinoza satisfied Spinoza left the table table empty
Mutexes and WaitGroup
The first solution just uses channels for synchronization. Channels can solve lots of problems but the sync library has a few other functions to more directly model common operations. In Dining Philosophers, fork use is mutually exclusive so it's very clear to model forks with sync.Mutex objects. Also waiting for a number of concurrent tasks to finish is a common pattern directly implemented with sync.WaitGroup.
One more concurrency technique actually used in both solutions is to use the log package for output rather than the fmt package. Output from concurrent goroutines can get accidentally interleaved in some cases. While neither package makes claims about this problem, the log package historically has been coded to avoid interleaved output. <lang go>package main
import (
"hash/fnv" "log" "math/rand" "os" "sync" "time"
)
var ph = []string{"Aristotle", "Kant", "Spinoza", "Marx", "Russell"}
const hunger = 3 const think = time.Second / 100 const eat = time.Second / 100
var fmt = log.New(os.Stdout, "", 0)
var dining sync.WaitGroup
func philosopher(phName string, dominantHand, otherHand *sync.Mutex) {
fmt.Println(phName, "seated")
h := fnv.New64a()
h.Write([]byte(phName))
rg := rand.New(rand.NewSource(int64(h.Sum64())))
rSleep := func(t time.Duration) {
time.Sleep(t/2 + time.Duration(rg.Int63n(int64(t))))
}
for h := hunger; h > 0; h-- {
fmt.Println(phName, "hungry")
dominantHand.Lock() // pick up forks
otherHand.Lock()
fmt.Println(phName, "eating")
rSleep(eat)
dominantHand.Unlock() // put down forks
otherHand.Unlock()
fmt.Println(phName, "thinking")
rSleep(think)
}
fmt.Println(phName, "satisfied")
dining.Done()
fmt.Println(phName, "left the table")
}
func main() {
fmt.Println("table empty")
dining.Add(5)
fork0 := &sync.Mutex{}
forkLeft := fork0
for i := 1; i < len(ph); i++ {
forkRight := &sync.Mutex{}
go philosopher(ph[i], forkLeft, forkRight)
forkLeft = forkRight
}
go philosopher(ph[0], fork0, forkLeft)
dining.Wait() // wait for philosphers to finish
fmt.Println("table empty")
}</lang>
Groovy
Deadlocks are avoided by always getting locks on forks with lower numbers first. <lang groovy>import groovy.transform.Canonical
import java.util.concurrent.locks.Lock import java.util.concurrent.locks.ReentrantLock
@Canonical class Fork {
String name Lock lock = new ReentrantLock()
void pickUp(String philosopher) {
lock.lock()
println " $philosopher picked up $name"
}
void putDown(String philosopher) {
lock.unlock()
println " $philosopher put down $name"
}
}
@Canonical class Philosopher extends Thread {
Fork f1 Fork f2
@Override
void run() {
def random = new Random()
(1..20).each { bite ->
println "$name is hungry"
f1.pickUp name
f2.pickUp name
println "$name is eating bite $bite"
Thread.sleep random.nextInt(300) + 100
f2.putDown name
f1.putDown name
}
}
}
void diningPhilosophers(names) {
def forks = (1..names.size()).collect { new Fork(name: "Fork $it") }
def philosophers = []
names.eachWithIndex{ n, i ->
def (i1, i2) = [i, (i + 1) % 5]
if (i2 < i1) (i1, i2) = [i2, i]
def p = new Philosopher(name: n, f1: forks[i1], f2: forks[i2])
p.start()
philosophers << p
}
philosophers.each { it.join() }
}
diningPhilosophers(['Aristotle', 'Kant', 'Spinoza', 'Marx', 'Russell'])</lang>
Haskell
Using the built-in Software Transactional Memory in GHC. <lang haskell>module Philosophers where
import Control.Monad import Control.Concurrent import Control.Concurrent.STM import System.Random
-- TMVars are transactional references. They can only be used in transactional actions. -- They are either empty or contain one value. Taking an empty reference fails and -- putting a value in a full reference fails. A transactional action only succeeds -- when all the component actions succeed, else it rolls back and retries until it -- succeeds. -- The Int is just for display purposes. type Fork = TMVar Int
newFork :: Int -> IO Fork newFork i = newTMVarIO i
-- The basic transactional operations on forks takeFork :: Fork -> STM Int takeFork fork = takeTMVar fork
releaseFork :: Int -> Fork -> STM () releaseFork i fork = putTMVar fork i
type Name = String
runPhilosopher :: Name -> (Fork, Fork) -> IO () runPhilosopher name (left, right) = forever $ do
putStrLn (name ++ " is hungry.") -- Run the transactional action atomically. -- The type system ensures this is the only way to run transactional actions. (leftNum, rightNum) <- atomically $ do leftNum <- takeFork left rightNum <- takeFork right return (leftNum, rightNum) putStrLn (name ++ " got forks " ++ show leftNum ++ " and " ++ show rightNum ++ " and is now eating.") delay <- randomRIO (1,10) threadDelay (delay * 1000000) -- 1, 10 seconds. threadDelay uses nanoseconds. putStrLn (name ++ " is done eating. Going back to thinking.")
atomically $ do releaseFork leftNum left releaseFork rightNum right delay <- randomRIO (1, 10) threadDelay (delay * 1000000)
philosophers :: [String] philosophers = ["Aristotle", "Kant", "Spinoza", "Marx", "Russel"]
main = do
forks <- mapM newFork [1..5]
let namedPhilosophers = map runPhilosopher philosophers
forkPairs = zip forks (tail . cycle $ forks)
philosophersWithForks = zipWith ($) namedPhilosophers forkPairs
putStrLn "Running the philosophers. Press enter to quit."
mapM_ forkIO philosophersWithForks
-- All threads exit when the main thread exits.
getLine
</lang>
Icon and Unicon
Icon doesn't support concurrency. This Unicon solution avoids deadlock and livelock (but not starvation) by not allowing philosophers to hold onto one fork if they can't get the other, and by having each philosopher pick up their lowest numbered fork first. The code would be slightly simpler if the philosophers wouldn't waste time waiting when they can't get both forks and went back to thinking instead. (Take away their grant money.)
<lang unicon>global forks, names
procedure main(A)
names := ["Aristotle","Kant","Spinoza","Marks","Russell"]
write("^C to terminate")
nP := *names
forks := [: |mutex([])\nP :]
every p := !nP do thread philosopher(p)
delay(-1)
end
procedure philosopher(n)
f1 := forks[min(n, n%*forks+1)]
f2 := forks[max(n, n%*forks+1)]
repeat {
write(names[n]," thinking")
delay(1000*?5)
write(names[n]," hungry")
repeat {
fork1 := lock(f1)
if fork2 := trylock(f2) then {
write(names[n]," eating")
delay(1000*?5)
break (unlock(fork2), unlock(fork1)) # full
}
unlock(fork1) # Free first fork and go back to waiting
}
}
end</lang>
A sample run, terminated after some time.
->dp ^C to terminate Kant thinking Spinoza thinking Aristotle thinking Russell thinking Marks thinking Kant hungry Russell hungry Kant eating Spinoza hungry Russell eating Aristotle hungry Marks hungry Kant thinking Spinoza eating Russell thinking Aristotle eating Kant hungry Spinoza thinking Marks eating Aristotle thinking Kant eating Russell hungry Spinoza hungry Russell eating Marks thinking Aristotle hungry Kant thinking Spinoza eating Russell thinking Marks hungry Aristotle eating Kant hungry Spinoza thinking Marks eating Russell hungry Aristotle thinking Spinoza hungry Kant eating Russell eating Marks thinking Kant thinking Marks hungry Spinoza eating Aristotle hungry Russell thinking Aristotle eating ^C
J
These philosophers are very smart and polite: they figured out immediately that at most two of them can eat simultaneously (take the floor of n divided by 2 for n philosophers); so, when they are hungry and it is necessary, they wait in line. (In general, for n > 1, because they are very smart and polite, when a philosopher seats he leaves exactly one empty seat between himself and one of the philosophers which are already eating if any.)
J does not support concurrency; so, this is a discrete-event simulation (DES). The time spent thinking and eating is assumed to be exponentially distributed, respectively, at the rates of 1 and 0.5 per time unit.
The simulation code
The simulation is defined in terms of fixed tacit (stateless point-free) code (a Turing complete dialect of J; see, https://rosettacode.org/wiki/Universal_Turing_machine#J),
<lang j>". noun define -. CRLF NB. Fixed tacit simulation code...
simulate=. "_@:((<@:(1 -~ 1&({::)) 1} ])@:(([ 0 0&$@(1!:2&2)@:(((6j3 ": 9&({::)) , ': '"_) , ' starts waiting and thinking about hunger.' ,~ 8&({::) {:: 0&({::)))@
- (<@:(6&({::) , 8&({::)) 6} ])@:((<@:((0 (0 {:: ])`(<@:(1 {:: ]))`(2 {:: ])}
])@:(3 8 2&{)) 2} ])@:(<@:2: 3} ]))@:((<@:((0 (0 {:: ])`(<@:(1 {:: ]))`(2 {::
])} ])@:(5 8 4&{)) 4} ])@:(<@:_: 5} ]))`(([ 0 0&$@(1!:2&2)@:(((6j3 ": 9&({::
)) , ': '"_) , ' starts eating.' ,~ 8&({::) {:: 0&({::)))@:((<@:((0 (0 {:: ]) `(<@:(1 {:: ]))`(2 {:: ])} ])@:(3 8 2&{)) 2} ])@:(<@:1: 3} ]))@:((<@:((0 (0 {
- ])`(<@:(1 {:: ]))`(2 {:: ])} ])@:(5 8 4&{)) 4} ])@:(<@:(_2 * ^.@:?@:0:) 5}
])))@.(7&({::) > 1 +/@:= 2&({::))`((<@:(}.@:(6&({::))) 6} ])@:(([ 0 0&$@(1!:
2&2)@:(((6j3 ": 9&({::)) , ': '"_) , ' starts eating.' ,~ 8&({::) {:: 0&({::) ))@:((<@:((0 (0 {:: ])`(<@:(1 {:: ]))`(2 {:: ])} ])@:(3 8 2&{)) 2} ])@:(<@:1:
3} ]))@:((<@:((0 (0 {:: ])`(<@:(1 {:: ]))`(2 {:: ])} ])@:(5 8 4&{)) 4} ])@:(
<@:(_2 * ^.@:?@:0:) 5} ])))@:(<@:({.@:(6&({::))) 8} ])^:(1 <: #@:(6&({::)))@: ([ 0 0&$@(1!:2&2)@:(((6j3 ": 9&({::)) , ': '"_) , ' starts thinking.' ,~ 8&({
- ) {:: 0&({::)))@:((<@:((0 (0 {:: ])`(<@:(1 {:: ]))`(2 {:: ])} ])@:(3 8 2&{)
) 2} ])@:(<@:0: 3} ]))@:((<@:((0 (0 {:: ])`(<@:(1 {:: ]))`(2 {:: ])} ])@:(5 8
4&{)) 4} ])@:(<@:(_1 * ^.@:?@:0:) 5} ])))@.( ($ ,) 8&({::) { 2&({::)))@:(<
@:(0 I.@:= 4&({::)) 8} ])@:(<@:((- <./)@:(4&({::))) 4} ])@:(<@:(9&({::) + <./ @:(4&({::))) 9} ])^:(0 < 1&({::))^:_)@:(([ 0 0&$@(1!:2&2)@:(((6j3 ": 9&({::))
, ': '"_) , 'All of them start thinking.'"_))@:((0 ; <.@:(2 %~ #@:(0&({::)))
) 9 7} ])@:((0:"_1 ,&< (_1 * ^.@:?@:0:)&>)@:(0&({::)) 2 4} ])@:((;:@:(0&({::) ) ,&< "_) 0 6} ]))@:(,&(;:8$','))@:;
)</lang>
Simulation of 11 chronological events for five philosophers
<lang j> 'Aristotle Kant Spinoza Marx Russell' simulate 11
0.000: All of them start thinking. 0.097: Spinoza starts eating. 0.474: Aristotle starts eating. 0.950: Russell starts waiting and thinking about hunger. 1.125: Kant starts waiting and thinking about hunger. 2.263: Spinoza starts thinking. 2.263: Russell starts eating. 2.762: Marx starts waiting and thinking about hunger. 2.771: Spinoza starts waiting and thinking about hunger. 4.769: Russell starts thinking. 4.769: Kant starts eating. 4.845: Russell starts waiting and thinking about hunger. 5.166: Aristotle starts thinking. 5.166: Marx starts eating. 5.915: Marx starts thinking. 5.915: Spinoza starts eating.</lang>
Simulation of 22 chronological events for eight philosophers
<lang j> 'Aristotle Kant Spinoza Marx Russell Laozi Nezahualcoyotl Averroes' simulate 22
0.000: All of them start thinking. 0.077: Nezahualcoyotl starts eating. 0.312: Marx starts eating. 0.424: Laozi starts eating. 0.502: Kant starts eating. 0.541: Marx starts thinking. 0.545: Marx starts eating. 0.660: Laozi starts thinking. 0.715: Laozi starts eating. 0.766: Aristotle starts waiting and thinking about hunger. 0.871: Laozi starts thinking. 0.871: Aristotle starts eating. 0.893: Averroes starts waiting and thinking about hunger. 1.035: Nezahualcoyotl starts thinking. 1.035: Averroes starts eating. 1.071: Laozi starts waiting and thinking about hunger. 1.168: Kant starts thinking. 1.168: Laozi starts eating. 1.614: Russell starts waiting and thinking about hunger. 1.660: Spinoza starts waiting and thinking about hunger. 1.813: Aristotle starts thinking. 1.813: Russell starts eating. 2.022: Marx starts thinking. 2.022: Spinoza starts eating. 2.164: Russell starts thinking. 2.182: Aristotle starts eating. 2.339: Marx starts waiting and thinking about hunger. 2.446: Aristotle starts thinking. 2.446: Marx starts eating.</lang>
The structured derivation of the verb (function)
The fixed tacit code of the verb (simulate) was produced by means of an unorthodox tacit toolkit; however, the verb produced is orthodox (compliant):
<lang j>NB. Quick and dirty tacit toolkit...
o=. @: c=."_
ver=. (0:`)([:^:)
d=. (fix=. (;:'f.')ver) (train=.(;:'`:')ver&6) (an=. <@:((,'0') (,&<) ])) ver=. (an f. o fix'ver')ver o an f. z=. ((an)`($ ,)`) (`:6) d=. (a0=. `) (a1=. (@[) ((<'&')`) (`:6)) (a2=. (`(<(":0);_)) (`:6)) av=. ((an o fix'a0')`) (`(an o fix'a1')) (`(an o fix'a2') ) (`:6)
Fetch=. (ver o train ;:'&{::')&.> o i. f.av tie=. ver o train ;:'`'
indices=. (, $~ 1 -.~ $) o (train"0 o ((1 -: L.)S:1 # <S:1) o (tie&) o fix :: ]) f=. ((ver o train ;:'&{')) o indices o train f.av
'A B'=. 2 Fetch head=. (;:'<@:') {.~ 2 * 1 = #@[ h=. train o (indices o train o (A f) (head , (B f)@] , < o an@[ , (;:'}]')c) ]) f.av
DropIfNB=. < o ('('"_ , ] , ')'"_) o ((}: ^: ('NB.' -: 3&{. o > o {:)) &. ;:) pipe=. ([ , ' o ' , ])&:>/ o |.
is=. ". o (, o ": o > , '=. ' , pipe o (DropIfNB;._2) o ". o ('0 ( : 0)'c)) f.av
NB.--------------------------------------------------------------------------------------
NB. Producing the verb simulate...
Note 0
NB. X and Y...
N - Philosophers names C - Number of chronological events to simulate
NB. Local...
A - Activity (0 - Thinking, 1 -eating, 2 - Thinking while queuing,) B - New activity T - Residual time left for the activity S - Starting time for the new activity Q - Queue U - Upper bound for the number of philosophers which can eat simultaneously P - Active philosopher E - Elapsed Time (only for information purposes)
)
amend=. 0 (0 {:: ])`(<@:(1 {:: ]))`(2 {:: ])} ]
'N C A B T S Q U P E'=. 10 Fetch NB. 10 Boxes
thinktime=. _1 * ^. o ? o 0: NB. Exponentially distributed at a rate of one eattime =. _2 * ^. o ? o 0: NB. Exponentially distributed at a rate of one-half j=. ,&<
time=. (6j3 ": E) , ': 'c
start is
(N Q)`((;: o N) j (c)) h NB. Boxing the names, empty queue (A T)`((0:items j thinktime&>) o N) h NB. All start thinking (E U)`(0 ; <. o (2 %~ # o N)) h NB. elapsed time 0, Upper bound [ echo o (time , 'All of them start thinking.'c)
)
CanEat=. U > 1 +/ o = A NB. Can eat if there is a suitable place at the table
eat is
T`(amend o ((S P T)f))h o (S`eattime h) NB. Eating time
A`(amend o ((B P A)f))h o (B`1: h) NB. Activity: eating
[ echo o (time , ' starts eating.' ,~ P {:: N)
)
enqueue is
T`(amend o ((S P T)f))h o (S`_:h) NB. Inactive until someone else ends eating
A`(amend o ((B P A)f))h o (B`2:h) NB. Activity: thinking while queuing
Q`(Q , P)h NB. Enqueuing
[ echo o (time , ' starts waiting and thinking about hunger.' ,~ P {:: N)
)
thinking=. enqueue`eat@.CanEat NB. Either enqueues or eats after thinking
dequeue is
P`({. o Q)h NB. Activating the one in front of the queue
eat NB. and starts eating
Q`(}. o Q)h NB. dequeuing
)
eating is NB. Thinks after eating
T`(amend o ((S P T)f))h o (S`thinktime h) NB. Thinking time
A`(amend o ((B P A)f))h o (B`0: h) NB. Activity: thinking
[ echo o ( time , ' starts thinking.' ,~ P {:: N)
dequeue ^: (1 <: # o Q) NB. dequeuing a philosopher (if possible)
)
update is
E`(E + <./ o T)h NB. Updating the elapsed time
T`((- <./)@:T) h NB. Updating the residual times
P`(0 I. o = T) h NB. Setting the active philosopher
thinking`eating@.((P { A)z) NB. Was thinking or eating?
C`(1 -~ C) h NB. One chronological event completed
)
simulate is NB. Discrete event simulation (dyadic verb)
; NB. Linking the arguments (N C) ,&(;:8$',') NB. Appending 8 local boxes (A B T S Q U P E) start update ^: (0 < C) ^: _ NB. Updating while events are less than C c
)
simulate=. simulate f.
NB. The simulation code is produced by the sentence, NB. 77 (-@:[ ]\ 5!:5@<@:]) 'simulate'</lang>
Java
This Java implementation uses a token system. If a philosopher's number is on the token, they pick up their left and right forks. Passing the token to their immediate neighbor would be pointless, so they increment the token by 2, passing it to the philosopher after their neighbor. The +2 works well for odd numbers of philosophers. With wait down at 1 millisecond I get about 1.5M eats/sec running 5 philosophers, down to about 0.5M eats/sec running 25. The single token generates good availability for 80% of 5 forks, but a much lower availability % of 25 forks. <lang Java> package diningphilosophers;
import java.util.ArrayList; import java.util.Random; import java.util.concurrent.atomic.AtomicBoolean; import java.util.concurrent.atomic.AtomicInteger;
enum PhilosopherState { Get, Eat, Pon }
class Fork {
public static final int ON_TABLE = -1; static int instances = 0; public int id; public AtomicInteger holder = new AtomicInteger(ON_TABLE);
Fork() { id = instances++; }
}
class Philosopher implements Runnable {
static final int maxWaitMs = 100; // must be > 0 static AtomicInteger token = new AtomicInteger(0); static int instances = 0; static Random rand = new Random(); AtomicBoolean end = new AtomicBoolean(false); int id; PhilosopherState state = PhilosopherState.Get; Fork left; Fork right; int timesEaten = 0;
Philosopher() {
id = instances++;
left = Main.forks.get(id);
right = Main.forks.get((id+1)%Main.philosopherCount);
}
void sleep() { try { Thread.sleep(rand.nextInt(maxWaitMs)); }
catch (InterruptedException ex) {} }
void waitForFork(Fork fork) {
do {
if (fork.holder.get() == Fork.ON_TABLE) {
fork.holder.set(id); // my id shows I hold it
return;
} else { // someone still holds it
sleep(); // check again later
}
} while (true);
}
public void run() {
do {
if (state == PhilosopherState.Pon) { // all that pondering
state = PhilosopherState.Get; // made me hungry
} else { // ==PhilosopherState.Get
if (token.get() == id) { // my turn now
waitForFork(left);
waitForFork(right); // Ah needs me some foahks!
token.set((id+2)% Main.philosopherCount);
state = PhilosopherState.Eat;
timesEaten++;
sleep(); // eat for a while
left.holder.set(Fork.ON_TABLE);
right.holder.set(Fork.ON_TABLE);
state = PhilosopherState.Pon; // ponder for a while
sleep();
} else { // token.get() != id, so not my turn
sleep();
}
}
} while (!end.get());
}
}
public class Main {
static final int philosopherCount = 5; // token +2 behavior good for odd #s static final int runSeconds = 15; static ArrayList<Fork> forks = new ArrayList<Fork>(); static ArrayList<Philosopher> philosophers = new ArrayList<Philosopher>();
public static void main(String[] args) {
for (int i = 0 ; i < philosopherCount ; i++) forks.add(new Fork());
for (int i = 0 ; i < philosopherCount ; i++)
philosophers.add(new Philosopher());
for (Philosopher p : philosophers) new Thread(p).start();
long endTime = System.currentTimeMillis() + (runSeconds * 1000);
do { // print status
StringBuilder sb = new StringBuilder("|");
for (Philosopher p : philosophers) {
sb.append(p.state.toString());
sb.append("|"); // This is a snapshot at a particular
} // instant. Plenty happens between.
sb.append(" |");
for (Fork f : forks) {
int holder = f.holder.get();
sb.append(holder==-1?" ":String.format("P%02d",holder));
sb.append("|");
}
System.out.println(sb.toString());
try {Thread.sleep(1000);} catch (Exception ex) {}
} while (System.currentTimeMillis() < endTime);
for (Philosopher p : philosophers) p.end.set(true);
for (Philosopher p : philosophers)
System.out.printf("P%02d: ate %,d times, %,d/sec\n",
p.id, p.timesEaten, p.timesEaten/runSeconds);
}
} </lang>
JoCaml
Minimal simple solution
This solution allows a philosopher to take only two forks at once, or none at all. This is achieved by making each fork into a channel, and guarding the eating process by two forks. There are two channels for each philosopher: a thinking philosopher and a hungry philosopher.
What this simple solution achieves:
- no deadlock (waiting for forks forever)
- no "livelock" (trying to pick up and put down forks forever)
- philosophers can eat at any time (no fixed order is imposed)
Deficiencies of this solution:
- Supports only a fixed set of philosophers, since all channels are declared statically. More philosophers needs more lines of code.
- The mean time of waiting while hungry is not bounded and grows very slowly (logarithmically) with time.
<lang jocaml>let random_wait n = Unix.sleep (Random.int n);; let print s m = Printf.printf "philosopher %s is %s\n" s m; flush(stdout);; let will_eat s = print s "eating"; random_wait 10;; let will_think s = print s "thinking"; random_wait 20; print s "hungry";;
(* a,b,c,d,e are thinking philosophers; ah,bh,ch,dh,eh are the same philosophers when hungry;
fab is the fork located between philosophers a and b; similarly for fbc, fcd, ... *)
def ah() & fab() & fea() = will_eat "Aristotle"; a() & fab() & fea()
or bh() & fab() & fbc() = will_eat "Kant"; b() & fab() & fbc() or ch() & fbc() & fcd() = will_eat "Spinoza"; c() & fbc() & fcd() or dh() & fcd() & fde() = will_eat "Marx"; d() & fcd() & fde() or eh() & fde() & fea() = will_eat "Russell"; e() & fde() & fea()
and a() = will_think "Aristotle"; ah() and b() = will_think "Kant"; bh() and c() = will_think "Spinoza"; ch() and d() = will_think "Marx"; dh() and e() = will_think "Russell"; eh()
spawn fab() & fbc() & fcd() & fde() & fea() & a() & b() & c() & d() & e();;</lang> Sample output:
philosopher Aristotle is thinking philosopher Russell is thinking philosopher Marx is thinking philosopher Kant is thinking philosopher Spinoza is thinking philosopher Kant is hungry philosopher Kant is eating philosopher Kant is thinking philosopher Russell is hungry philosopher Russell is eating philosopher Russell is thinking philosopher Spinoza is hungry philosopher Spinoza is eating philosopher Spinoza is thinking philosopher Spinoza is hungry philosopher Spinoza is eating philosopher Spinoza is thinking philosopher Aristotle is hungry philosopher Aristotle is eating philosopher Marx is hungry philosopher Marx is eating philosopher Russell is hungry philosopher Aristotle is thinking philosopher Russell is eating philosopher Marx is thinking philosopher Kant is hungry philosopher Kant is eating philosopher Russell is thinking philosopher Kant is thinking
Simple solution with statistics
This solution is logically the same as the "minimal simple" solution above, but now the timing information is printed. Statistical information is also printed on hungry waiting time before eating: average among all instances of eating, and maximum time ever waited by anyone.
<lang jocaml>let print s t m = Printf.printf "t=%d: philosopher %s is %s\n" t s m; flush(stdout);; let random_wait n = Unix.sleep (Random.int n);;
(* auxiliary function to keep track of time ticks, using integer seconds *) def ts () & counter(n) = counter(n) & reply n to ts or update_counter() & counter(n) = counter(n+1) & reply to update_counter and counter_sentinel() = Unix.sleep 1; update_counter(); counter_sentinel()
spawn counter(0) & counter_sentinel();;
def stats(n, waited, maxwaited) & report_wait_time(m) =
let (n', waited', maxwaited') = (n+1, waited+m, max maxwaited m) in Printf.printf "waiting average %f, max waited %d\n" (float_of_int waited' /. float_of_int n') maxwaited'; flush(stdout); stats(n',waited',maxwaited') & reply () to report_wait_time
spawn stats(0,0,0);;
let eat s t = print s t "eating"; random_wait 10;; let think s = print s (ts()) "thinking"; random_wait 20;;
(* "p" will be a philosopher channel, to be defined later
the messages ah, bh, ... do not need to be injected now. *)
let will_eat s t = let t' = ts() in report_wait_time(t'-t); eat s t';;
def ah(t,p) & fab() & fea() = will_eat "Aristotle" t; p() & fab() & fea() or bh(t,p) & fab() & fbc() = will_eat "Kant" t; p() & fab() & fbc() or ch(t,p) & fbc() & fcd() = will_eat "Spinoza" t; p() & fbc() & fcd() or dh(t,p) & fcd() & fde() = will_eat "Marx" t; p() & fcd() & fde() or eh(t,p) & fde() & fea() = will_eat "Russell" t; p() & fde() & fea()
spawn fab() & fbc() & fcd() & fde() & fea();;
(* define the thinking -> hungry transitions using local philosophers, and inject the philosophers *) List.map
(fun (h,s) -> def p() = think s; let t = ts() in print s t "hungry"; h(t,p) in spawn p()) [(ah,"Aristotle"); (bh,"Kant"); (ch,"Spinoza"); (dh,"Marx"); (eh,"Russell")]
(* this replaces repetitive code such as that shown in the previous solution *)
(* now we need to wait and do nothing; nobody will be able to inject godot() *) def wait_forever() & godot() = reply () to wait_forever in wait_forever();;</lang> Sample output (excerpt):
t=2: philosopher Aristotle is thinking t=3: philosopher Aristotle is hungry waiting average 0.000000, max waited 0 t=3: philosopher Aristotle is eating t=3: philosopher Aristotle is thinking t=4: philosopher Russell is hungry waiting average 0.000000, max waited 0 t=4: philosopher Russell is eating t=5: philosopher Marx is hungry t=5: philosopher Kant is hungry waiting average 0.000000, max waited 0 t=5: philosopher Kant is eating waiting average 0.666667, max waited 4 t=9: philosopher Marx is eating t=9: philosopher Russell is thinking t=14: philosopher Kant is thinking t=17: philosopher Marx is thinking t=18: philosopher Marx is hungry waiting average 0.571429, max waited 4 t=18: philosopher Marx is eating t=19: philosopher Spinoza is hungry t=20: philosopher Aristotle is hungry waiting average 0.500000, max waited 4 t=20: philosopher Aristotle is eating t=24: philosopher Russell is hungry waiting average 1.000000, max waited 5 t=24: philosopher Marx is thinking t=24: philosopher Spinoza is eating t=26: philosopher Kant is hungry waiting average 1.300000, max waited 5 t=28: philosopher Russell is eating t=28: philosopher Aristotle is thinking t=31: philosopher Russell is thinking t=33: philosopher Marx is hungry waiting average 1.181818, max waited 5 t=33: philosopher Marx is eating
Fair solution
This solution implements "fairness" -- if two neighbors are hungry, the one who waited more will eat first. The waiting time for each philosopher is bounded by twice the maximum eating time.
<lang jocaml>#!/usr/bin/jocamlrun jocaml
(* eating and thinking between 0 and this-1 *) let eating_max_interval = 10;; let thinking_max_interval = 10;; let number_of_philosophers = 5;; let random_wait n = Unix.sleep (Random.int n);;
(* counter for unique timestamp, not related to time in seconds *) def get_current_time () & unique_ts_counter(n) = unique_ts_counter(n+1) & reply n to get_current_time;; spawn unique_ts_counter(0);;
(* functions that wait and print diagnostics *) let name i = List.nth ["Aristotle"; "Kant"; "Spinoza"; "Marx"; "Russell"] i;; let message i m = Printf.printf "philosopher %s is %s\n" (name i) m; flush(stdout);; let eat i = message i "eating"; random_wait eating_max_interval;; let think i = message i "thinking"; random_wait thinking_max_interval;;
type philosopher_state_t = Eating | Hungry of int | Thinking;;
(* initial states *) let states = Array.make number_of_philosophers Thinking;; (* one philosopher's processes *) let make_philosopher i got_hungry done_eating =
def hungry() & forks() = eat i ; done_eating(i) & thinking() and thinking() = think i; got_hungry(i) & hungry() in spawn thinking(); forks
(* deciding who will eat first *) let next_phil i = (i+1) mod number_of_philosophers;; let prev_phil i = (number_of_philosophers+i-1) mod number_of_philosophers;; let is_hungry p = match p with
| Hungry h -> true | _ -> false;;
let not_eating p = match p with
| Eating -> false | _ -> true;;
let is_more_hungry p q = match q with
| Hungry hj -> ( match p with
| Hungry hi -> hi <= hj | _ -> false
) | _ -> true
let may_eat_first i =
is_hungry states.(i) && not_eating states.(next_phil i) && not_eating states.(prev_phil i) && is_more_hungry states.(i) states.(next_phil i) && is_more_hungry states.(i) states.(prev_phil i);;
let decide_eating i =
if (may_eat_first i) then (states.(i) <- Eating; true) else false;;
def waiter(all_forks) & got_hungry(i) =
states.(i) <- Hungry (get_current_time()); let will_eat = decide_eating i in ( waiter(all_forks) & (if will_eat then all_forks.(i)() else 0)
) or waiter(all_forks) & done_eating(i) =
states.(i) <- Thinking; let next_will_eat = decide_eating (next_phil i) in let prev_will_eat = decide_eating (prev_phil i) in ( waiter(all_forks) & (if next_will_eat then all_forks.(next_phil i)() else 0) & (if prev_will_eat then all_forks.(prev_phil i)() else 0) );;
let all_forks = Array.init number_of_philosophers (fun i -> make_philosopher i got_hungry done_eating) in spawn waiter(all_forks);;
(* now we need to wait and do nothing; nobody will be able to inject godot() *)
def wait_forever() & godot() = reply () to wait_forever in wait_forever();;</lang>
Sample output:
philosopher Aristotle is thinking philosopher Kant is thinking philosopher Marx is thinking philosopher philosopher Spinoza is thinking Russell is thinking philosopher Spinoza is eating philosopher Spinoza is thinking philosopher Marx is eating philosopher Marx is thinking philosopher Marx is eating philosopher Marx is thinking philosopher Aristotle is eating philosopher Aristotle is thinking philosopher Kant iphilosopher s eating Russell is eating philosopher Russell is thinking philosopher Kant is thinking philosopher Spinoza is eating philosopher Spinoza is thinking philosopher Marx is eating
Julia
Pentagonal table with assigned seats. Aristotle, seated on the north side, takes his left fork first since he was left-handed, see historical note in http://time.com/3107557/top-10-lefties/ and the others take the right fork first. The forks are represented by 5 channels. One lefty's taking left fork before right prevents deadlocks (see C solution). <lang julia> mutable struct Philosopher
name::String
hungry::Bool
righthanded::Bool
rightforkheld::Channel
leftforkheld::Channel
function Philosopher(name, leftfork, rightfork)
this = new()
this.name = name
this.hungry = rand([false, true]) # not specified so start as either
this.righthanded = (name == "Aristotle") ? false : true
this.leftforkheld = leftfork
this.rightforkheld = rightfork
this
end
end
mutable struct FiveForkTable
fork51::Channel
fork12::Channel
fork23::Channel
fork34::Channel
fork45::Channel
function FiveForkTable()
this = new()
this.fork51 = Channel(1); put!(this.fork51, "fork") # start with one fork per channel
this.fork12 = Channel(1); put!(this.fork12, "fork")
this.fork23 = Channel(1); put!(this.fork23, "fork")
this.fork34 = Channel(1); put!(this.fork34, "fork")
this.fork45 = Channel(1); put!(this.fork45, "fork")
this
end
end
table = FiveForkTable();
tasks = [Philosopher("Aristotle", table.fork12, table.fork51),
Philosopher("Kant", table.fork23, table.fork12),
Philosopher("Spinoza", table.fork34, table.fork23),
Philosopher("Marx", table.fork45, table.fork34),
Philosopher("Russell", table.fork51, table.fork45)]
function dine(t,p)
if p.righthanded
take!(p.rightforkheld); println("$(p.name) takes right fork")
take!(p.leftforkheld); println("$(p.name) takes left fork")
else
take!(p.leftforkheld); println("$(p.name) takes left fork")
take!(p.rightforkheld); println("$(p.name) takes right fork")
end
end
function leavetothink(t, p)
put!(p.rightforkheld, "fork"); println("$(p.name) puts down right fork")
put!(p.leftforkheld, "fork"); println("$(p.name) puts down left fork")
end
contemplate(t) = sleep(t)
function dophil(p, t, fullaftersecs=2.0, hungryaftersecs=10.0)
while true
if p.hungry
println("$(p.name) is hungry")
dine(table, p)
sleep(fullaftersecs)
p.hungry = false
leavetothink(t, p)
else
println("$(p.name) is out of the dining room for now.")
contemplate(hungryaftersecs)
p.hungry = true
end
end
end
function runall(tasklist)
for p in tasklist
@async dophil(p, table)
end
while true begin sleep(5) end end
end
runall(tasks) </lang>
- Output:
Aristotle is out of the dining room for now. Kant is out of the dining room for now. Spinoza is hungry Spinoza takes right fork Spinoza takes left fork Marx is hungry Russell is hungry Russell takes right fork Russell takes left fork Spinoza puts down right fork Spinoza puts down left fork Spinoza is out of the dining room for now. Marx takes right fork Marx takes left fork Russell puts down right fork Russell puts down left fork Russell is out of the dining room for now. Marx puts down right fork Marx puts down left fork Marx is out of the dining room for now. Aristotle is hungry Aristotle takes left fork Aristotle takes right fork Kant is hungry Aristotle puts down right fork Aristotle puts down left fork Aristotle is out of the dining room for now. Kant takes right fork Kant takes left fork Spinoza is hungry Russell is hungry Russell takes right fork Russell takes left fork Kant puts down right fork Kant puts down left fork Kant is out of the dining room for now. Spinoza takes right fork Spinoza takes left fork Marx is hungry Russell puts down right fork Russell puts down left fork Russell is out of the dining room for now. Spinoza puts down right fork Spinoza puts down left fork Spinoza is out of the dining room for now. Marx takes right fork Marx takes left fork Marx puts down right fork Marx puts down left fork Marx is out of the dining room for now. Aristotle is hungry Aristotle takes left fork Aristotle takes right fork Aristotle puts down right fork Aristotle puts down left fork Aristotle is out of the dining room for now. Kant is hungry Kant takes right fork Kant takes left fork Russell is hungry Russell takes right fork Russell takes left fork Kant puts down right fork Kant puts down left fork Kant is out of the dining room for now. Spinoza is hungry Spinoza takes right fork Spinoza takes left fork Russell puts down right fork Russell puts down left fork Russell is out of the dining room for now.
Kotlin
As noted in the Groovy entry, deadlocks are avoided by always getting locks on forks with lower numbers first. <lang scala>// Version 1.2.31
import java.util.Random import java.util.concurrent.locks.Lock import java.util.concurrent.locks.ReentrantLock
val rand = Random()
class Fork(val name: String) {
val lock = ReentrantLock()
fun pickUp(philosopher: String) {
lock.lock()
println(" $philosopher picked up $name")
}
fun putDown(philosopher: String) {
lock.unlock()
println(" $philosopher put down $name")
}
}
class Philosopher(val pname: String, val f1: Fork, val f2: Fork) : Thread() {
override fun run() {
(1..20).forEach {
println("$pname is hungry")
f1.pickUp(pname)
f2.pickUp(pname)
println("$pname is eating bite $it")
Thread.sleep(rand.nextInt(300) + 100L)
f2.putDown(pname)
f1.putDown(pname)
}
}
}
fun diningPhilosophers(names: List<String>) {
val size = names.size
val forks = List(size) { Fork("Fork ${it + 1}") }
val philosophers = mutableListOf<Philosopher>()
names.forEachIndexed { i, n ->
var i1 = i
var i2 = (i + 1) % size
if (i2 < i1) {
i1 = i2
i2 = i
}
val p = Philosopher(n, forks[i1], forks[i2])
p.start()
philosophers.add(p)
}
philosophers.forEach { it.join() }
}
fun main(args: Array<String>) {
val names = listOf("Aristotle", "Kant", "Spinoza", "Marx", "Russell")
diningPhilosophers(names)
}</lang>
Logtalk
Works when using SWI-Prolog, XSB, or YAP as the backend compiler: <lang logtalk>:- category(chopstick).
% chopstick actions (picking up and putting down) are synchronized using a notification % such that a chopstick can only be handled by a single philosopher at a time:
:- public(pick_up/0).
pick_up :-
threaded_wait(available).
:- public(put_down/0).
put_down :-
threaded_notify(available).
- - end_category.
- - object(cs1,
imports(chopstick)).
:- threaded. :- initialization(threaded_notify(available)).
- - end_object.
- - object(cs2,
imports(chopstick)).
:- threaded. :- initialization(threaded_notify(available)).
- - end_object.
- - object(cs3,
imports(chopstick)).
:- threaded. :- initialization(threaded_notify(available)).
- - end_object.
- - object(cs4,
imports(chopstick)).
:- threaded. :- initialization(threaded_notify(available)).
- - end_object.
- - object(cs5,
imports(chopstick)).
:- threaded. :- initialization(threaded_notify(available)).
- - end_object.
- - category(philosopher).
:- public(left_chopstick/1). :- public(right_chopstick/1). :- public(run/2).
:- private(message/1). :- synchronized(message/1).
:- uses(random, [random/3]).
run(0, _) :-
this(Philosopher),
message([Philosopher, ' terminated.']).
run(Count, MaxTime) :-
Count > 0,
think(MaxTime),
eat(MaxTime),
Count2 is Count - 1,
run(Count2, MaxTime).
think(MaxTime):-
this(Philosopher),
random(1, MaxTime, ThinkTime),
message(['Philosopher ', Philosopher, ' thinking for ', ThinkTime, ' seconds.']),
thread_sleep(ThinkTime).
eat(MaxTime):-
this(Philosopher),
random(1, MaxTime, EatTime),
::left_chopstick(LeftStick),
::right_chopstick(RightStick),
LeftStick::pick_up,
RightStick::pick_up,
message(['Philosopher ', Philosopher, ' eating for ', EatTime, ' seconds with chopsticks ', LeftStick, ' and ', RightStick, '.']),
thread_sleep(EatTime),
::LeftStick::put_down,
::RightStick::put_down.
% writing a message needs to be synchronized as it's accomplished
% using a combination of individual write/1 and nl/0 calls:
message([]) :-
nl,
flush_output.
message([Atom| Atoms]) :-
write(Atom),
message(Atoms).
- - end_category.
- - object(aristotle,
imports(philosopher)).
left_chopstick(cs1). right_chopstick(cs2).
- - end_object.
- - object(kant,
imports(philosopher)).
left_chopstick(cs2). right_chopstick(cs3).
- - end_object.
- - object(spinoza,
imports(philosopher)).
left_chopstick(cs3). right_chopstick(cs4).
- - end_object.
- - object(marx,
imports(philosopher)).
left_chopstick(cs4). right_chopstick(cs5).
- - end_object.
- - object(russell,
imports(philosopher)).
left_chopstick(cs1). % change order so that the chopsticks are picked right_chopstick(cs5). % in different order from the other philosophers
- - end_object.</lang>
M2000 Interpreter
Version 2 Improved program. Choose random Sequential or Concurrent plan. In Concurrent statemnts in blocks { } are executed sequential (without other part of other thread executed).
At the begining one of philosopher get a longer trigger period for entry. Also ransomly the program choose if philosophers can change the order of picking. Each philosopher has an energy, starting from 50. If thinking then energy drop, and can die if energy<1. When philosopher thinking and energy is above 70 then he leave back any fork and drop to 60 Each time he get two forks get a counter on a randome value of 4 to 8. So for 4 to 8 "cycles" hold two forks and ggain 4 to 8 energy units. With this we have varius eating time.
To avoid deadlock we have to put back a fork in thining phase as the rule bellow: If a philosopher thinking and has energy>70 or the common critical>5 then he place any fork back if energy>70 then energu drop to 60 (so we make a dead zone of 10 units for this automation)
The critical varianble get new value from another thread the Main.Task. Some times critical may get a value of 7 and then the thinking philospher found it and place his fork back (if has any).
<lang M2000 Interpreter> Module Dining_philosophers (whichplan) { Form 80, 32 if random(1,2)=1 then Const MayChangePick as boolean=False else Const MayChangePick as boolean=True end if dim energy(1 to 5)=50 Document Doc$ const nl$={ } Print $(,12), ' set column width to 12 Pen 14 Pen 15 { Doc$="Dining Philosophers"+nl$ \\ we can change thread plan only if no threads defined if whichplan=1 then Doc$="Sequential threads - to execute exclusive one threads code"+nl$ thread.plan sequential \\ need time_to_think>time_to_eat, but time_to_appear maybe the same for all time_to_think=150 ' one or more intervals time_to_eat=100 ' one interval to eat only time_to_appear=(150,150,150,150,150) Return time_to_appear, random(0,3):=300 else Doc$="Concurrent threads - to execute a statement or a block of code"+nl$ thread.plan concurrent time_to_think=100 ' one or more intervals time_to_eat=50 ' one interval to eat only time_to_appear=(100,100,100,100,100) Return time_to_appear, random(1,4):=200 end if Print #-2,Doc$ Print @(0,2),"Press left mouse button to exit" Print Part $(1), time_to_appear Print under } Pen 13 {Print "Aristotle", "Kant", "Spinoza", "Marx", "Russell"} enum philosopher { Aristotle, Kant, Spinoza, Marx, Russell } global enum forks {NoFork, Fork} RoundTable =(Fork, Fork, Fork, Fork, Fork) Getleft=lambda RoundTable (ph as philosopher) -> { where=(ph+4) mod 5 = RoundTable#val(where) Return RoundTable, where:=NoFork } GetRight=lambda RoundTable (ph as philosopher) -> { where=ph mod 5 =RoundTable#val(where) Return RoundTable, where:=NoFork } PlaceForks=lambda RoundTable (ph as philosopher) -> { Return RoundTable, (ph+4) mod 5:=Fork,ph mod 5:=Fork } PlaceAnyFork=lambda RoundTable (ph as philosopher, forkL, forkR) -> { If forkL then Return RoundTable, (ph+4) mod 5:=Fork If forkR Then Return RoundTable, ph mod 5:=Fork } ShowTable=lambda RoundTable -> { m=each(RoundTable) while m print if$(array(m)=NoFork->"No Fork", "Fork"), end while Print } noforks=lambda RoundTable -> { k=0 m=each(RoundTable) while m if array(m)=NoFork then k++ end while =k=5 }
def critical as long, basetick Document page$ m=each(philosopher) while m { \\ we make 5 threads \\ a thread has module scope (except for own static variables, and stack of values) thread { if energy(f)<1 then { call PlaceAnyFork(f, forkL, forkR) energy(f)=0 Page$=format$("{0::-12} - ",tick-basetick)+eval$(f)+" - Die"+nl$ thread this erase } else { Page$=format$("{0::-12} - ",tick-basetick)+eval$(f) Page$=if$(forkL=Nofork or forkR=Nofork->" thinking", " eating"+str$(eatcount)) Page$=if$(R->"- R", " - L")+nl$ } if not think then { \\ a block always run blocking all other threads energy(f)++ eatcount-- if eatcount>0 then exit Call PlaceForks(f) : forkL=Nofork:forkR=NoFork eatcount=random(4,8) if MayChangePick then R=random(-1,0) think=true :thread this interval time_to_think*random(1,5) } else.if energy(f)>70 or critical>5 then { call PlaceAnyFork(f, forkL, forkR) if energy(f)>70 then energy(f)=60 } else.if R then if forkR=Nofork then forkR=GetRight(f) if forkR=fork and forkL=Nofork then forkL=GetLeft(f) if forkL=fork then think=false:thread this interval time_to_eat else energy(f)-- else if forkL=Nofork then forkL=GetLeft(f) if forkL=fork and forkR=Nofork then forkR=GetRight(f) if forkR=fork then think=false:thread this interval time_to_eat else energy(f)-- end if
} as a interval time_to_appear#val(m^) \\ a is a variable which hold the number of thread (as returned from task manager) \\ so we can get 5 times a new number. \\ for each thread we make some static variables (only for each thread) \\ this statement execute a line of code in thread a thread a execute { \\ this executed on thread execution object static f=eval(m), think=true, forkL=NoFork static forkR=NoFork, eatcount=random(2,5) static R=-1 if MayChangePick then R=Random(-1,0) } } cls ,5 ' set split screen from fifth row \\ Main.Task is a thread also. Normaly exit if no other threads running in background \\ also serve a the wait loop for task manager (we can use Every 200 {} but isn't a thread, is a kind of a wait statement) \\ tick return the counter from task manager which used to triger threads basetick=tick \\ 4hz display results MaxCritical=0 Main.Task 1000/4 { { \\ a block always run blocking all other threads cls Print $(1),energy() Print $(0) Print "Table:" Call ShowTable() if noforks() then critical++ else critical=0 MaxCritical=if(MaxCritical<critical->critical,MaxCritical) Print "noforks on table counter:";critical, "Max:";MaxCritical Print #-2,Page$ Doc$=Page$ Clear Page$ } if critical>40 or keypress(1) then exit } threads erase Clipboard Doc$ } Dining_philosophers Random(1,2) </lang>
- Output:
Dining Philosophers
Concurrent threads - to execute a statement or a block of code
3 - Kant thinking - L
4 - Spinoza thinking- R
5 - Marx thinking- R
8 - Aristotle thinking - L
60 - Russell thinking- R
77 - Marx thinking- R
514 - Spinoza eating 4- R
729 - Kant thinking - L
734 - Aristotle thinking - L
736 - Spinoza eating 3- R
737 - Marx thinking- R
738 - Russell thinking- R
1058 - Aristotle thinking - L
1059 - Kant thinking - L
1060 - Spinoza eating 2- R
..................................
6182 - Aristotle thinking- R
6195 - Kant thinking - L
6196 - Spinoza eating 8 - L
6227 - Marx thinking- R
6228 - Russell eating 3- R
6238 - Spinoza eating 7 - L
6260 - Aristotle thinking- R
6303 - Kant thinking - L
6306 - Russell eating 2- R
6340 - Spinoza eating 6 - L
6342 - Russell eating 1- R
Modula-3
From this implementation's point of view, a "resource" is not a fork, but rather a place at the table. Rather than use one MUTEX for each fork, it uses one MUTEX for the entire table.
- Each philosopher starts on his feet, waits until the
MUTEXallows him to look for an available seat, and looks to see if two forks are available at one place. - After determining whether a place is available, the philosopher does one of two things before releasing the
MUTEX.- If a place is available, he sits down, take both forks at the place (which must be free), and releases the
MUTEX. - If a place is not available, the philosopher does the following.
- He notifies a "condition variable", comparable to what some restaurants call a "host", that he will wait on a place. He simultaneously releases the
MUTEX. - He receives the
MUTEXagain once the condition variable informs him that someone has risen from the table. (This brings us back to the first step.)
- He notifies a "condition variable", comparable to what some restaurants call a "host", that he will wait on a place. He simultaneously releases the
- If a place is available, he sits down, take both forks at the place (which must be free), and releases the
- When a philosopher has finished eating, he puts down both forks and rises to think a while.
Note. These philosophers actually follow the directions and spend a random amount of time eating and thinking.
It is easy to modify this so that each philosopher does not rise from the table, but eats and thinks only at his assigned place. In fact, the original implementation did precisely that, but when I saw that some implementations allowed the philosophers to sit at any place with two available forks, I opted for that.
While this implementation is not a translation of the Eiffel solution, it still owes it a heads-up for the basic principle. Bertrand Meyer's ACM Webinar on SCOOP directed my attention to this problem, and probably influenced the solution. <lang modula3>MODULE DiningPhilosophers EXPORTS Main;
IMPORT IO, Random, Thread;
CONST
PartySize = 5; (* modify for more/fewer philosophers *)
TYPE
Closure = Thread.Closure OBJECT (* thread information *) which: [1..PartySize]; (* identifies the thread *) OVERRIDES apply := Live; (* procedure to execute *) END;
VAR
(* how long to eat/think *) random: Random.T;
(* controls access to resources *)
test := NEW(MUTEX);
forks := NEW(Thread.Condition); (* condition variable, used for signaling *)
forkAvailable := ARRAY[1..PartySize] OF BOOLEAN {
TRUE, TRUE, TRUE, TRUE, TRUE
};
(* the philosophers/tasks *)
thread: ARRAY[1..PartySize] OF Thread.T;
name := ARRAY[1..PartySize] OF TEXT {
"Aristotle", "Kant", "Spinoza", "Marx", "Russell"
};
PROCEDURE PlaceAvailable(): CARDINAL = (*
Determines whether a place is available at the table. If so, returns the place number. Otherwise, returns 0. We consider a place available if and only if *both* forks are free.
- )
BEGIN
FOR i := 1 TO PartySize DO
IF forkAvailable[i] AND forkAvailable[((i+1) MOD PartySize) + 1] THEN
RETURN i;
END;
END;
RETURN 0;
END PlaceAvailable;
PROCEDURE Live(philosopher: Closure): REFANY = (* philosophers eat, sleep, ... and that's about it *) VAR
place: CARDINAL;
BEGIN
WITH which = philosopher.which DO
WHILE TRUE DO
(* first make sure a place is available: both forks must be free! *)
LOCK test DO
place := PlaceAvailable();
(* if not, release mutex and use condition variable to wait for one *)
WHILE place = 0 DO
IO.Put(name[which]); IO.Put(" starving!\n");
Thread.Wait(test, forks);
(* in Modula-3 we arrive here only if we have the lock again *)
place := PlaceAvailable();
END;
(* a place has come available! seize the forks while mutex is locked *)
forkAvailable[place] := FALSE;
forkAvailable[(place MOD PartySize) + 1] := FALSE;
IO.Put(name[which]); IO.Put(" eating at place "); IO.PutInt(place);
IO.PutChar('\n');
END;
Thread.Pause(FLOAT(random.integer(1,3), LONGREAL));
(* put down the forks *)
forkAvailable[place] := TRUE;
forkAvailable[(place MOD PartySize) + 1] := TRUE;
Thread.Signal(forks); (* signal the condition variable *)
LOCK test DO
IO.Put(name[which]); IO.Put(" thinking\n");
END;
Thread.Pause(FLOAT(random.integer(1,3), LONGREAL));
END; (* WHILE *)
END; (* WITH *)
RETURN NIL;
END Live;
BEGIN
random := NEW(Random.Default).init(); (* bring philosophers to life *) FOR i := 1 TO PartySize DO thread[i] := Thread.Fork(NEW(Closure, apply := Live, which := i)); END; (* We need to wait, otherwise the program will terminate, and the philosophers with it. Technically we could wait for just one philosopher, but in the interest of symmetry... *) FOR i := 1 TO PartySize DO EVAL Thread.Join(thread[i]); END;
END DiningPhilosophers.</lang>
- Output:
Aristotle eating at place 1 Kant eating at place 3 Spinoza starving! Marx starving! Russell starving! Aristotle thinking Spinoza eating at place 5 Aristotle eating at place 2 Kant thinking Marx starving! Kant eating at place 4 Spinoza thinking Russell starving! ...
Nim
Prevents deadlocks by ordering the forks. Compile with nim --threads:on c diningphilosophers.nim
<lang nim>import threadpool, locks, math, os, random
- to call randomize() as a seed, need to import random module
randomize()
type Philosopher = ref object
name: string food: string forkLeft, forkRight: int
const
n = 5 names = ["Aristotle", "Kant", "Spinoza", "Marx", "Russell"] foods = [" rat poison", " cockroaches", " dog food", " lemon-curd toast", " baked worms"]
var
forks: array[n, Lock] phils: array[n, Philosopher] threads: array[n, Thread[Philosopher]]
proc run(p: Philosopher) {.thread.} =
# random deprecated, use rand(x .. y) sleep rand(1..10) * 500 echo p.name, " is hungry." acquire forks[min(p.forkLeft, p.forkRight)] sleep rand(1..5) * 500 acquire forks[max(p.forkLeft, p.forkRight)] echo p.name, " starts eating", p.food, "." sleep rand(1..10) * 500 echo p.name, " finishes eating", p.food, " and leaves to think." release forks[p.forkLeft] release forks[p.forkRight]
for i in 0..<n:
initLock forks[i] phils[i] = Philosopher( name: names[i], food: foods[rand(0 .. n) mod n], forkLeft: i, forkRight: (i + 1) mod n ) createThread(threads[i], run, phils[i])
joinThreads(threads)</lang>
OxygenBasic
<lang oxygenbasic> '========================= class RoundTableWith5Seats '=========================
% hungry 0 % beingUsed 1 % putDown 0 % empty 0
sys fork[5], plate[5],chair[5],philosopher[5] sys first
method AddPasta() as sys
function rand() as sys
static seed=0x12345678
mov eax,seed
rol eax,7
mul seed
xor eax,0x5335ABD9
mov seed,eax
return seed
end function
return 4+(rand() and 15)
end method
method dine()
first++ 'PRIORITY DINER
if first>5 then first-=5
for i=1 to 5
kl=first+i-1
kr=first+i
if kl>5 then kl-=5
if kr>5 then kr-=5
if philosopher(kl) = hungry then
if not fork(kl) or fork(kr) = beingUsed then
plate(kl) = AddPasta()
fork(kl)=beingUsed
fork(kr)=beingUsed
end if
end if
'
next
'
for kl=1 to 5
kr=kl+1 : if kr>5 then kr-=5
if plate(kl)
philosopher(kl)+=1 'PHILOSOPHER DINING
--plate(kl)
if plate(kl)=empty
fork(kl)=PutDown
fork(kr)=PutDown
end if
else
if philosopher(kl)>0
--philosopher(kl) 'PHILOSOPHER THINKING
end if
end if
next
'
end method
method show() as string
cr=chr(13)+chr(10) : tab=chr(9)
pr="philos" tab "activity" tab "plate" tab "fork L" tab "fork R" cr cr
for i=1 to 5
j=i+1 : if j>5 then j-=5
if plate(i)=0 then
if philosopher(i)=0 then
act="waiting"
else
act="thinks"
end if
else
act="dining"
end if
'
pr+=i tab act tab plate(i) tab fork(i) tab fork(j) cr
next
return pr
end method
end class
'TEST '====
RoundTableWith5Seats Sopho for i=1 to 100
Sopho.dine
next
print Sopho.show 'putfile "s.txt",Sopho.show
'philos action plate fork L fork R ' '1 waiting 0 0 1 '2 dining 8 1 1 '3 thinks 0 1 1 '4 dining 8 1 1 '5 thinks 0 1 0 </lang>
Oz
Using first-class computation spaces as transactions on dataflow variables. Computations spaces are normally used to implement constraint search engines. But here we use them to bind two variables atomically.
<lang oz>declare
Philosophers = [aristotle kant spinoza marx russell]
proc {Start}
Forks = {MakeList {Length Philosophers}}
in
{ForAll Forks NewFork}
for
Name in Philosophers
LeftFork in Forks
RightFork in {RightShift Forks}
do
thread
{Philosopher Name LeftFork RightFork}
end
end
end
proc {Philosopher Name LeftFork RightFork}
for do
{ShowInfo Name#" is hungry."}
{TakeForks [LeftFork RightFork]}
{ShowInfo Name#" got forks."}
{WaitRandom}
{ReleaseFork LeftFork}
{ReleaseFork RightFork}
{ShowInfo Name#" is thinking."}
{WaitRandom}
end
end
proc {WaitRandom}
{Delay 1000 + {OS.rand} mod 4000} %% 1-5 seconds
end
proc {TakeForks Forks}
{ForAll Forks WaitForFork}
case {TryAtomically proc {$}
{ForAll Forks TakeFork}
end}
of true then
{ForAll Forks InitForkNotifier}
[] false then
{TakeForks Forks}
end
end
%% %% Fork type %%
%% A fork is a mutable reference to a pair
fun {NewFork}
{NewCell
unit(taken:_ %% a fork is taken by setting this value to a unique value
notify:unit %% to wait for a taken fork
)}
end
proc {TakeFork F}
(@F).taken = {NewName}
end
proc {InitForkNotifier F}
%% we cannot do this in TakeFork
%% because side effect are not allowed in subordinate spaces
New Old
in
{Exchange F Old New}
New = unit(taken:Old.taken notify:_)
end
proc {ReleaseFork F}
New Old
in
{Exchange F Old New}
New = unit(taken:_ notify:unit)
Old.notify = unit %% notify waiters
end
proc {WaitForFork F}
{Wait (@F).notify} %% returns immediatly if fork is free, otherwise blocks
end
%% %% Helpers %%
%% Implements transactions on data flow variables
%% with computation spaces. Returns success.
fun {TryAtomically P}
try
S = {Space.new proc {$ Sync} {P} Sync = unit end}
in
{Space.askVerbose S} \= failed = true {Wait {Space.merge S}} true
catch _ then
false
end end
fun {RightShift Xs} %% circular
case Xs of nil then nil
else {Append Xs.2 [Xs.1]}
end
end
ShowInfo = System.showInfo
in
{Start}</lang>
Pascal
This FreePascal implementation uses the idea of ordered resourses, each fork is numbered by index in the array.
In order to avoid deadlocks each member first takes fork with lower number. <lang Pascal> program dining_philosophers; {$mode objfpc}{$H+} uses
{$IFDEF UNIX}
cthreads,
{$ENDIF}
Classes, SysUtils, SyncObjs;
const
PHIL_COUNT = 5;
LIFESPAN = 7;
DELAY_RANGE = 950;
DELAY_LOW = 50;
PHIL_NAMES: array[1..PHIL_COUNT] of string = ('Aristotle', 'Kant', 'Spinoza', 'Marx', 'Russell');
type
TFork = TCriticalSection; TPhilosopher = class;
var
Forks: array[1..PHIL_COUNT] of TFork; Philosophers: array[1..PHIL_COUNT] of TPhilosopher;
type
TPhilosopher = class(TThread) private FName: string; FFirstFork, FSecondFork: TFork; protected procedure Execute; override; public constructor Create(const aName: string; aForkIdx1, aForkIdx2: Integer); end;
procedure TPhilosopher.Execute; var
LfSpan: Integer = LIFESPAN;
begin
while LfSpan > 0 do
begin
Dec(LfSpan);
WriteLn(FName, ' sits down at the table');
FFirstFork.Acquire;
FSecondFork.Acquire;
WriteLn(FName, ' eating');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
FSecondFork.Release;
FFirstFork.Release;
WriteLn(FName, ' is full and leaves the table');
if LfSpan = 0 then
continue;
WriteLn(FName, ' thinking');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
WriteLn(FName, ' is hungry');
end;
end;
constructor TPhilosopher.Create(const aName: string; aForkIdx1, aForkIdx2: Integer); begin
inherited Create(True);
FName := aName;
if aForkIdx1 < aForkIdx2 then
begin
FFirstFork := Forks[aForkIdx1];
FSecondFork := Forks[aForkIdx2];
end
else
begin
FFirstFork := Forks[aForkIdx2];
FSecondFork := Forks[aForkIdx1];
end;
end;
procedure DinnerBegin; var
I: Integer; Phil: TPhilosopher;
begin
for I := 1 to PHIL_COUNT do Forks[I] := TFork.Create; for I := 1 to PHIL_COUNT do Philosophers[I] := TPhilosopher.Create(PHIL_NAMES[I], I, Succ(I mod PHIL_COUNT)); for Phil in Philosophers do Phil.Start;
end;
procedure WaitForDinnerOver; var
Phil: TPhilosopher; Fork: TFork;
begin
for Phil in Philosophers do
begin
Phil.WaitFor;
Phil.Free;
end;
for Fork in Forks do
Fork.Free;
end;
begin
Randomize; DinnerBegin; WaitForDinnerOver;
end. </lang> The next variant exploits the idea of a single left-handed philosopher. <lang Pascal> program dining_philosophers2; {$mode objfpc}{$H+} uses
{$IFDEF UNIX}
cthreads,
{$ENDIF}
Classes, SysUtils, SyncObjs;
const
PHIL_COUNT = 5;
LIFESPAN = 7;
DELAY_RANGE = 950;
DELAY_LOW = 50;
PHIL_NAMES: array[1..PHIL_COUNT] of string = ('Aristotle', 'Kant', 'Spinoza', 'Marx', 'Russell');
type
TFork = TCriticalSection; TPhilosopher = class;
var
Forks: array[1..PHIL_COUNT] of TFork; Philosophers: array[1..PHIL_COUNT] of TPhilosopher;
type
TPhilosopher = class(TThread) private FName: string; FLeftFork, FRightFork: TFork; FLefty: Boolean; procedure SetLefty(aValue: Boolean); procedure SwapForks; protected procedure Execute; override; public constructor Create(const aName: string; aForkIdx1, aForkIdx2: Integer); property Lefty: Boolean read FLefty write SetLefty; end;
procedure TPhilosopher.SetLefty(aValue: Boolean); begin
if Lefty = aValue then exit; FLefty := aValue; SwapForks;
end;
procedure TPhilosopher.SwapForks; var
Fork: TFork;
begin
Fork := FLeftFork; FLeftFork := FRightFork; FRightFork := Fork;
end;
procedure TPhilosopher.Execute; var
LfSpan: Integer = LIFESPAN;
begin
while LfSpan > 0 do
begin
Dec(LfSpan);
WriteLn(FName, ' sits down at the table');
FLeftFork.Acquire;
FRightFork.Acquire;
WriteLn(FName, ' eating');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
FRightFork.Release;
FLeftFork.Release;
WriteLn(FName, ' is full and leaves the table');
if LfSpan = 0 then
continue;
WriteLn(FName, ' thinking');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
WriteLn(FName, ' is hungry');
end;
end;
constructor TPhilosopher.Create(const aName: string; aForkIdx1, aForkIdx2: Integer); begin
inherited Create(True); FName := aName; FLeftFork := Forks[aForkIdx1]; FRightFork := Forks[aForkIdx2];
end;
procedure DinnerBegin; var
I: Integer; Phil: TPhilosopher;
begin
for I := 1 to PHIL_COUNT do Forks[I] := TFork.Create; for I := 1 to PHIL_COUNT do Philosophers[I] := TPhilosopher.Create(PHIL_NAMES[I], I, Succ(I mod PHIL_COUNT)); Philosophers[Succ(Random(5))].Lefty := True; for Phil in Philosophers do Phil.Start;
end;
procedure WaitForDinnerOver; var
Phil: TPhilosopher; Fork: TFork;
begin
for Phil in Philosophers do
begin
Phil.WaitFor;
Phil.Free;
end;
for Fork in Forks do
Fork.Free;
end;
begin
Randomize; DinnerBegin; WaitForDinnerOver;
end. </lang> Another way to avoid deadlock: if a philosopher takes left fork but cannot take the right fork, he returns the left fork. <lang Pascal> program dining_philosophers3; {$mode objfpc}{$H+} uses
{$IFDEF UNIX}
cthreads,
{$ENDIF}
Classes, SysUtils, SyncObjs;
const
PHIL_COUNT = 5;
LIFESPAN = 7;
DELAY_RANGE = 950;
DELAY_LOW = 50;
PHIL_NAMES: array[1..PHIL_COUNT] of string = ('Aristotle', 'Kant', 'Spinoza', 'Marx', 'Russell');
type
TFork = TCriticalSection; TPhilosopher = class;
var
Forks: array[1..PHIL_COUNT] of TFork; Philosophers: array[1..PHIL_COUNT] of TPhilosopher;
type
TPhilosopher = class(TThread) private FName: string; FLeftFork, FRightFork: TFork; protected procedure Execute; override; public constructor Create(const aName: string; aForkIdx1, aForkIdx2: Integer); end;
procedure TPhilosopher.Execute; var
LfSpan: Integer = LIFESPAN;
begin
while LfSpan > 0 do
begin
Dec(LfSpan);
WriteLn(FName, ' sits down at the table');
repeat
FLeftFork.Acquire;
if not FRightFork.TryEnter then
begin
FLeftFork.Release;
Sleep(Random(DELAY_RANGE));
continue;
end;
break;
until False;
WriteLn(FName, ' eating');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
FRightFork.Release;
FLeftFork.Release;
WriteLn(FName, ' is full and leaves the table');
if LfSpan = 0 then
continue;
WriteLn(FName, ' thinking');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
WriteLn(FName, ' is hungry');
end;
end;
constructor TPhilosopher.Create(const aName: string; aForkIdx1, aForkIdx2: Integer); begin
inherited Create(True); FName := aName; FLeftFork := Forks[aForkIdx1]; FRightFork := Forks[aForkIdx2];
end;
procedure DinnerBegin; var
I: Integer; Phil: TPhilosopher;
begin
for I := 1 to PHIL_COUNT do Forks[I] := TFork.Create; for I := 1 to PHIL_COUNT do Philosophers[I] := TPhilosopher.Create(PHIL_NAMES[I], I, Succ(I mod PHIL_COUNT)); for Phil in Philosophers do Phil.Start;
end;
procedure WaitForDinnerOver; var
Phil: TPhilosopher; Fork: TFork;
begin
for Phil in Philosophers do
begin
Phil.WaitFor;
Phil.Free;
end;
for Fork in Forks do
Fork.Free;
end;
begin
Randomize; DinnerBegin; WaitForDinnerOver;
end. </lang> And the last: deadlock can only happen if all the members are seated at the table.
This variant tries to avoid this situation. <lang Pascal> program dining_philosophers4; {$mode objfpc}{$H+} uses
{$IFDEF UNIX}
cthreads,
{$ENDIF}
Classes, SysUtils, SyncObjs;
const
PHIL_COUNT = 5;
LIFESPAN = 7;
DELAY_RANGE = 950;
DELAY_LOW = 50;
PHIL_NAMES: array[1..PHIL_COUNT] of string = ('Aristotle', 'Kant', 'Spinoza', 'Marx', 'Russell');
type
TFork = TCriticalSection; TPhilosopher = class;
var
Forks: array[1..PHIL_COUNT] of TFork; Philosophers: array[1..PHIL_COUNT] of TPhilosopher; StilDining: Integer = 0;
procedure WaitForPlaceFree; begin
repeat
if InterlockedIncrement(StilDining) > Pred(PHIL_COUNT) then
begin
InterlockedDecrement(StilDining);
Sleep(Random(DELAY_LOW));
continue;
end;
exit;
until False;
end;
procedure FreePlace; begin
InterLockedDecrement(StilDining);
end;
type
TPhilosopher = class(TThread) private FName: string; FLeftFork, FRightFork: TFork; protected procedure Execute; override; public constructor Create(const aName: string; aForkIdx1, aForkIdx2: Integer); end;
procedure TPhilosopher.Execute; var
LfSpan: Integer = LIFESPAN;
begin
while LfSpan > 0 do
begin
Dec(LfSpan);
WaitForPlaceFree;
WriteLn(FName, ' sits down at the table');
FLeftFork.Acquire;
FRightFork.Acquire;
WriteLn(FName, ' eating');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
FRightFork.Release;
FLeftFork.Release;
FreePlace;
WriteLn(FName, ' is full and leaves the table');
if LfSpan = 0 then
continue;
WriteLn(FName, ' thinking');
Sleep(Random(DELAY_RANGE) + DELAY_LOW);
WriteLn(FName, ' is hungry');
end;
end;
constructor TPhilosopher.Create(const aName: string; aForkIdx1, aForkIdx2: Integer); begin
inherited Create(True); FName := aName; FLeftFork := Forks[aForkIdx1]; FRightFork := Forks[aForkIdx2];
end;
procedure DinnerBegin; var
I: Integer; Phil: TPhilosopher;
begin
for I := 1 to PHIL_COUNT do Forks[I] := TFork.Create; for I := 1 to PHIL_COUNT do Philosophers[I] := TPhilosopher.Create(PHIL_NAMES[I], I, Succ(I mod PHIL_COUNT)); for Phil in Philosophers do Phil.Start;
end;
procedure WaitForDinnerOver; var
Phil: TPhilosopher; Fork: TFork;
begin
for Phil in Philosophers do
begin
Phil.WaitFor;
Phil.Free;
end;
for Fork in Forks do
Fork.Free;
end;
begin
Randomize; DinnerBegin; WaitForDinnerOver;
end. </lang>
Perl
This solution requires that perl have been compiled with threads enabled.
Deadlock is prevented by having even numbered and odd numbered philosophers grab their forks in opposite orders.
<lang Perl> use threads; use threads::shared; my @names = qw(Aristotle Kant Spinoza Marx Russell);
my @forks = ('On Table') x @names; share $forks[$_] for 0 .. $#forks;
sub pick_up_forks {
my $philosopher = shift;
my ($first, $second) = ($philosopher, $philosopher-1);
($first, $second) = ($second, $first) if $philosopher % 2;
for my $fork ( @forks[ $first, $second ] ) {
lock $fork;
cond_wait($fork) while $fork ne 'On Table';
$fork = 'In Hand';
}
}
sub drop_forks {
my $philosopher = shift;
for my $fork ( @forks[$philosopher, $philosopher-1] ) {
lock $fork;
die unless $fork eq 'In Hand';
$fork = 'On Table';
cond_signal($fork);
}
}
sub philosopher {
my $philosopher = shift;
my $name = $names[$philosopher];
for my $meal ( 1..5 ) {
print $name, " is pondering\n";
sleep 1 + rand 8;
print $name, " is hungry\n";
pick_up_forks( $philosopher );
print $name, " is eating\n";
sleep 1 + rand 8;
drop_forks( $philosopher );
}
print $name, " is done\n";
}
my @t = map { threads->new(\&philosopher, $_) } 0 .. $#names; for my $thread ( @t ) {
$thread->join;
}
print "Done\n"; __END__ </lang>
One solution based on Coro and AnyEvent
To prevent deadlock the philosophers must not start eating at the same time and the time between getting the first fork and getting second one must be shorter as possible.
<lang Perl>
- !/usr/bin/perl
use common::sense; use Coro; use AnyEvent; use Coro::AnyEvent; use EV;
my @philosophers = qw(Aristotle Kant Spinoza Marx Russell); my @forks = (1..@philosophers); my @fork_sem;
$fork_sem[$_] = new Coro::Semaphore for (0..$#philosophers);
for(my $i = $#philosophers; $i >= 0; $i--){
say $philosophers[$i] . " has fork #" . $forks[$i] . " and fork #" . $forks[$i-1];
async {
my ($name, ,$no, $forks_got) = (@_);
$Coro::current->{desc} = $name; Coro::AnyEvent::sleep(rand 4);
while(1){ say $name . " is hungry."; $$forks_got[$no]->down(); Coro::AnyEvent::sleep(rand 1); #Let's make deadlock! $$forks_got[$no-1]->down(); say $name . " is eating."; Coro::AnyEvent::sleep(1 + rand 8);
$$forks_got[$no]->up(); $$forks_got[$no-1]->up();
say $name . " is thinking."; Coro::AnyEvent::sleep(1 + rand 8); } }($philosophers[$i], $i, \@fork_sem); }
EV::loop; </lang>
Phix
Deadlocks are avoided by always getting the lowest numbered fork first.
You can substitute the indicated line for Russell to prove that it does indeed deadlock when the program fails to follow that rule.
If you uncomment the sleep(1)s you will probably want do the same to the terminate checks, otherwise after keying 'Q' or Escape it could take 20 seconds per diner to finish. <lang Phix>integer fork1 = init_cs(),
fork2 = init_cs(),
fork3 = init_cs(),
fork4 = init_cs(),
fork5 = init_cs()
integer terminate = 0 -- control flag
procedure person(sequence name, atom left_fork, atom right_fork) -- (except Russell, who gets left and right the other way round)
while terminate=0 do
enter_cs(left_fork)
enter_cs(right_fork)
puts(1, name & " grabs forks.\n")
for i=1 to rand(10) do
-- if terminate then exit end if
puts(1, name & " is eating.\n")
-- sleep(1)
end for
puts(1, name & " puts forks down and leaves the dinning room.\n")
leave_cs(left_fork)
leave_cs(right_fork)
for i=1 to rand(10) do
-- if terminate then exit end if
puts(1, name & " is thinking.\n")
-- sleep(1)
end for
puts(1, name & " becomes hungry.\n")
end while
end procedure constant r_person = routine_id("person")
constant threads = {create_thread(r_person,{"Aristotle",fork1,fork2}),
create_thread(r_person,{"Kant",fork2,fork3}),
create_thread(r_person,{"Spinoza",fork3,fork4}),
create_thread(r_person,{"Marx",fork4,fork5}),
-- create_thread(r_person,{"Russell",fork5,fork1})} -- this will deadlock!
create_thread(r_person,{"Russell",fork1,fork5})}
constant ESC = #1B while not find(get_key(),{ESC,'q','Q'}) do
sleep(1)
end while terminate = 1 wait_thread(threads) -- (not strictly necessary) delete_cs(fork1) -- "" delete_cs(fork2) delete_cs(fork3) delete_cs(fork4) delete_cs(fork5)</lang>
PicoLisp
This following solution uses the built-in fininte state machine function 'state'. Deadlocks are avoided, as each philosopher releases the first fork if he doesn't succeed to obtain the second fork, and waits for a random time.
Another solution, using the Chandy/Misra method, can be found here. <lang PicoLisp>(de dining (Name State)
(loop
(prinl Name ": " State)
(state 'State # Dispatch according to state
(thinking 'hungry) # If thinking, get hungry
(hungry # If hungry, grab random fork
(if (rand T)
(and (acquire leftFork) 'leftFork)
(and (acquire rightFork) 'rightFork) ) )
(hungry 'hungry # Failed, stay hungry for a while
(wait (rand 1000 3000)) )
(leftFork # If holding left fork, try right one
(and (acquire rightFork) 'eating)
(wait 2000) ) # then eat for 2 seconds
(rightFork # If holding right fork, try left one
(and (acquire leftFork) 'eating)
(wait 2000) ) # then eat for 2 seconds
((leftFork rightFork) 'hungry # Otherwise, go back to hungry,
(release (val State)) # release left or right fork
(wait (rand 1000 3000)) ) # and stay hungry
(eating 'thinking # After eating, resume thinking
(release leftFork)
(release rightFork)
(wait 6000) ) ) ) ) # for 6 seconds
(setq *Philosophers
(maplist
'((Phils Forks)
(let (leftFork (tmp (car Forks)) rightFork (tmp (cadr Forks)))
(or
(fork) # Parent: Collect child process IDs
(dining (car Phils) 'hungry) ) ) ) # Initially hungry
'("Aristotle" "Kant" "Spinoza" "Marx" "Russell")
'("ForkA" "ForkB" "ForkC" "ForkD" "ForkE" .) ) )
(push '*Bye '(mapc kill *Philosophers)) # Terminate all upon exit</lang> Output:
Aristotle: hungry Aristotle: rightFork Kant: hungry Kant: rightFork Spinoza: hungry Spinoza: rightFork Marx: hungry Marx: rightFork Russell: hungry Marx: hungry Spinoza: hungry Kant: hungry Russell: hungry Aristotle: eating ...
Pike
using Pike Backend call_out(), this solution avoids deadlocks by adding a 20% chance that a philosopher drops the fork if he can't pick up both.
<lang Pike>class Philosopher {
string name; object left; object right;
void create(string _name, object _left, object _right)
{
name = _name;
left = _left;
right = _right;
}
void take_forks()
{
if (left->take(this) && right->take(this))
{
write("%s is EATING\n", name);
call_out(drop_forks, random(30));
}
else
{
write("%s is WAITING\n", name);
if (random(10) >= 8)
drop_forks();
call_out(take_forks, random(10));
}
}
void drop_forks()
{
left->drop(this);
right->drop(this);
write("%s is THINKING\n", name);
call_out(take_forks, random(30));
}
}
class Fork {
int number; Philosopher user;
void create(int _number)
{
number = _number;
}
int take(object new_user)
{
if (!user)
{
write("%s takes fork %d\n", new_user->name, number);
user = new_user;
return 1;
}
else if (new_user == user)
{
write("%s has fork %d\n", new_user->name, number);
return 1;
}
else
write("%s tries to take fork %d from %s\n", new_user->name, number, user->name);
}
void drop(object old_user)
{
if (old_user == user)
{
write("%s drops fork %d\n", old_user->name, number);
user = 0;
}
}
}
int main(int argc, array argv) {
array forks = ({ Fork(1), Fork(2), Fork(3), Fork(4), Fork(5) });
array philosophers = ({
Philosopher("einstein", forks[0], forks[1]),
Philosopher("plato", forks[1], forks[2]),
Philosopher("sokrates", forks[2], forks[3]),
Philosopher("chomsky", forks[3], forks[4]),
Philosopher("archimedes", forks[4], forks[0]),
});
call_out(philosophers[0]->take_forks, random(5));
call_out(philosophers[1]->take_forks, random(5));
call_out(philosophers[2]->take_forks, random(5));
call_out(philosophers[3]->take_forks, random(5));
call_out(philosophers[4]->take_forks, random(5));
return -1;
}</lang>
Prolog
Works with SWI-Prolog and XPCE.
Use the same solution as in Erlang (a waiter gives the forks to philosophers).
Bonus : the code of an animation in XPCE is given, and statistics are displayed at the end of the process.
<lang Prolog>dining_philosophers :-
new(D, window('Dining philosophers')),
new(S, window('Dining philosophers : statistics')),
send(D, size, new(_, size(800,800))),
new(E, ellipse(400,400)), send(E, center, point(400,400)), send(D, display, E),
new(F1, fork(0)), new(F2, fork(1)), new(F3, fork(2)), new(F4, fork(3)), new(F5, fork(4)),
send_list(D, display, [F1,F2,F3,F4,F5]),
new(Waiter, waiter(F1, F2, F3, F4, F5)),
create_plate(P1, 0), create_plate(P2, 1), create_plate(P3, 2), create_plate(P4, 3), create_plate(P5, 4),
create_point(0, Pt1), create_point(1, Pt2), create_point(2, Pt3), create_point(3, Pt4), create_point(4, Pt5),
new(Ph1, philosopher('Aristotle', Waiter, P1, D, S, 0, Pt1, left)),
new(Ph2, philosopher('Kant', Waiter, P2, D, S, 1, Pt2, left)),
new(Ph3, philosopher('Spinoza', Waiter, P3, D, S, 2, Pt3, right)),
new(Ph4, philosopher('Marx', Waiter, P4, D, S, 3, Pt4, right)),
new(Ph5, philosopher('Russell', Waiter, P5, D, S, 4, Pt5, left)),
send(Waiter, init_phi, Ph1, Ph2, Ph3, Ph4, Ph5),
send_list([Ph1, Ph2, Ph3, Ph4, Ph5], start),
send(D, done_message, and(message(Waiter, free), message(Ph1, free), message(Ph2, free), message(Ph3, free), message(Ph4, free), message(Ph5, free), message(S, open), message(D, destroy))),
send(D, open).
create_plate(P, N) :-
new(P, ellipse(80,80)),
X is 400 + 140 * cos(N * pi / 2.5),
Y is 400 + 140 * sin(N * pi / 2.5),
send(P, center, point(X, Y)).
create_point(N, point(X, Y)) :- X is 400 + 220 * cos(N * pi / 2.5), Y is 400 + 220 * sin(N * pi / 2.5) - 20.
- - pce_begin_class(waiter , object, "gives the forks to the philosophers").
variable(f1, fork, both, "free or used"). variable(f2, fork, both, "free or used"). variable(f3, fork, both, "free or used"). variable(f4, fork, both, "free or used"). variable(f5, fork, both, "free or used"). variable(phi1, philosopher, both, "philosopher"). variable(phi2, philosopher, both, "philosopher"). variable(phi3, philosopher, both, "philosopher"). variable(phi4, philosopher, both, "philosopher"). variable(phi5, philosopher, both, "philosopher").
initialise(P, F1, F2, F3, F4, F5) :-> send(P, slot, f1, F1), send(P, slot, f2, F2), send(P, slot, f3, F3), send(P, slot, f4, F4), send(P, slot, f5, F5).
init_phi(P, Phi1,Phi2, Phi3, Phi4, Phi5) :-> send(P, slot, phi1, Phi1), send(P, slot, phi2, Phi2), send(P, slot, phi3, Phi3), send(P, slot, phi4, Phi4), send(P, slot, phi5, Phi5).
want_forks(P, Phi) :->
( get(P, slot, phi1, Phi) ,!, check_forks(P, Phi, f5, f1);
get(P, slot, phi2, Phi),!, check_forks(P, Phi, f1, f2);
get(P, slot, phi3, Phi),!, check_forks(P, Phi, f2, f3);
get(P, slot, phi4, Phi),!, check_forks(P, Phi, f3, f4);
get(P, slot, phi5, Phi),!, check_forks(P, Phi, f4, f5)).
give_back_forks(P, Phi) :->
( get(P, slot, phi1, Phi) ,!, release_forks(P, phi1);
get(P, slot, phi2, Phi),!, release_forks(P, phi2);
get(P, slot, phi3, Phi),!, release_forks(P, phi3);
get(P, slot, phi4, Phi),!, release_forks(P, phi4);
get(P, slot, phi5, Phi),!, release_forks(P, phi5)),
get(P, slot, phi1, Phi1), check_forks(P, Phi1, f5, f1), get(P, slot, phi2, Phi2), check_forks(P, Phi2, f1, f2), get(P, slot, phi3, Phi3), check_forks(P, Phi3, f2, f3), get(P, slot, phi4, Phi4), check_forks(P, Phi4, f3, f4), get(P, slot, phi5, Phi5), check_forks(P, Phi5, f4, f5).
release_forks(P, phi1) :- get(P, slot, f5, F5), send(F5, free), get(P, slot, f1, F1), send(F1, free).
release_forks(P, phi2) :- get(P, slot, f1, F1), send(F1, free), get(P, slot, f2, F2), send(F2, free).
release_forks(P, phi3) :- get(P, slot, f2, F2), send(F2, free), get(P, slot, f3, F3), send(F3, free).
release_forks(P, phi4) :- get(P, slot, f3, F3), send(F3, free), get(P, slot, f4, F4), send(F4, free).
release_forks(P, phi5) :- get(P, slot, f4, F4), send(F4, free), get(P, slot, f5, F5), send(F5, free).
check_forks(P, Phi, F1, F2) :- get(P, slot, F1, FF1), get(P, slot, F2, FF2), ( (get(Phi, slot, status, waiting), get(FF1, slot, status, free), get(FF2, slot, status, free)) -> send(Phi, receive_forks), send(FF1, used, right), send(FF2, used, left) ; true).
- - pce_end_class.
- - pce_begin_class(philosopher , object, "eat, think or wait !").
variable(name, string, both). variable(window, object, both). variable(status, object, both, "eating/thinking/waiting"). variable(waiter, object, both). variable(plate, object, both). variable(mytimer, timer, both). variable(pos, point, both). variable(side, object, both). variable(old_text, object, both). variable(window_stat, object, both). variable(line_stat, number, both). variable(stat_wait, my_stat, both). variable(stat_eat, my_stat, both). variable(stat_think, my_stat, both).
% méthode appelée lors de la destruction de l'objet % On arrête d'abord le timer pour poursuivre ensuite % sans problème (appel par le timer de ressources libérées) unlink(P) :-> send(P?mytimer, stop),
get(P, status, Sta), stop_timer(P, Sta), get(P, slot, window_stat, WS), get(P, slot, line_stat, LS), get(LS, value, VLS), get(P, slot, name, Name), get(Name, value, V), sformat(A, 'Statistics of philosopher : ~w', [V]), new(Text, text(A)), send(Text, font, font(times, bold, 16)), Y is VLS * 30, send(WS, display, Text, point(30, Y)),
VLS1 is VLS+1, get(P, slot, stat_think, ST), send(ST, statistics, WS, VLS1),
VLS2 is VLS+2, get(P, slot, stat_eat, SE), send(SE, statistics, WS, VLS2),
VLS3 is VLS+3, get(P, slot, stat_wait, SW), send(SW, statistics, WS, VLS3),
send(P, send_super, unlink).
initialise(P, Name, Waiter, Plate, Window, Window_stat, Line_stat, Point, Side) :-> % gtrace, send(P, slot, name, Name), send(P, slot, window, Window), send(P, slot, window_stat, Window_stat), Line is Line_stat * 5, send(P, slot, line_stat, Line), send(P, slot, waiter,Waiter), send(P, slot, plate,Plate), send(P, slot, status, thinking), send(P, slot, pos, Point), send(P, slot, side, Side), send(Window, display, Plate), send(P, slot, old_text, new(_, text(' '))), send(P, display_status), send(P, slot, stat_wait, new(_, my_stat('Waiting'))), send(P, slot, stat_eat, new(_, my_stat('Eating'))), send(P, slot, stat_think, new(_, my_stat('Thinking'))).
stop_timer(P, eating) :-
get(P, slot, stat_eat, SE),
send(SE, stop).
stop_timer(P, waiting) :- get(P, slot, stat_wait, SW), send(SW, stop).
stop_timer(P, thinking) :- get(P, slot, stat_think, ST), send(ST, stop).
% internal message send by the timer
my_message(P) :->
% gtrace,
get(P, slot, status, Status),
next_status(P, Status).
% philosopher eating ==> thinking next_status(P, eating) :- get(P, slot, waiter, Waiter), get(P, slot, stat_eat, SE), send(SE, stop), get(P, slot, stat_think, ST), send(ST, start), send(Waiter, give_back_forks, P), send(P, slot, status, thinking), send(P, display_status), get(P, plate, Plate), send(Plate, fill_pattern, colour(white)), I is random(20)+ 10, get(P, slot, mytimer, Timer), send(Timer, interval, I), send(Timer, start, once).
next_status(P, thinking) :- get(P, slot, waiter, Waiter), send(P, slot, status, waiting), send(P, display_status), get(P, slot, stat_think, ST), send(ST, stop), get(P, slot, stat_wait, SW), send(SW, start), send(Waiter, want_forks, P).
% send by the waiter % philosopher can eat ! receive_forks(P) :-> get(P, slot, stat_wait, SW), send(SW, stop), get(P, slot, stat_eat, SE), send(SE, start), send(P, slot, status, eating), send(P, display_status), get(P, plate, Plate), send(Plate, fill_pattern, colour(black)), I is random(20)+ 5, get(P, slot, mytimer, Timer), send(Timer, interval, I), send(Timer, start, once).
display_status(P) :-> get(P, old_text, OT), free(OT), get(P, name, Name), get(Name, value, V), get(P, status, Status), choose_color(Status, Colour), sformat(A, '~w ~w', [V, Status]), get(P, window, W), get(P, pos, point(X, Y)), new(Text, text(A)), send(Text, font, font(times, bold, 16)), send(Text, colour, Colour), get(Text, string, Str), get(font(times, bold, 16), width(Str), M), (get(P, side, right) -> X1 is X - M; X1 = X), send(W, display, Text, point(X1, Y)), send(P, old_text, Text).
start(P) :->
I is random(10)+ 2,
get(P, slot, stat_think, ST),
send(ST, start),
send(P, mytimer, new(_, timer(I,message(P, my_message)))),
send(P?mytimer, start, once).
choose_color(eating, colour(blue)).
choose_color(thinking, colour(green)).
choose_color(waiting, colour(red)).
- - pce_end_class.
- - pce_begin_class(disk, ellipse, "disk with color ").
initialise(P, C, R, Col) :->
send(P, send_super, initialise, R, R),
send(P, center, C), send(P, pen, 0), send(P, fill_pattern, Col).
change_color(P, Col) :-> send(P, fill_pattern, Col).
- - pce_end_class.
- - pce_begin_class(my_stat , object, "statistics").
variable(name, string, both). variable(nb, number, both). variable(duration, real, both). variable(start, real, both).
initialise(P, Name) :-> send(P, name, Name), send(P, nb, 0), send(P, duration, 0.0).
start(P) :-> get_time(T), send(P, slot, start, T).
stop(P) :-> get_time(Fin),
get(P, slot, nb, N), send(N, plus,1), send(P, slot, nb, N),
get(P, slot, duration, D), get(P, slot, start, Deb),
get(D, value, VD), get(Deb, value, VDeb), X is VD + Fin - VDeb,
send(P, slot, duration, X).
statistics(P, W, L) :-> get(P, nb, N), get(N, value, VN), get(P, duration, D), get(D, value, VD), get(P, name, Name), get(Name, value, V), sformat(A, '~w~tnb :~13| ~t~w~17| duration : ~t~1f~35|', [V, VN, VD]), new(Text, text(A)), send(Text, font, font(screen, roman, 14)), Y is L * 30, send(W, display, Text, point(40, Y)).
- -pce_end_class.
% forks changes of place
- - pce_begin_class(fork, line, "to help philosopphers to eat").
variable(value, number, both, "0 => 4"). variable(side, object, both), "left / right". variable(status, object, both, "free / used").
initialise(P, Val) :-> send_super(P, initialise), send(P, slot, value, Val), send(P, slot, status, free), compute(Val, free, _, PS, PE), send(P, start, PS), send(P, end, PE).
free(P) :-> send(P, status, free), send(P, position).
used(P, Side) :->
send(P, status, used),
send(P, side, Side),
send(P, position).
position(P) :-> get(P, value, V), get(V, value, N), get(P, status, St), get(P, side, Side), compute(N, St, Side, PS, PE), send(P, start, PS), send(P, end, PE).
compute(N, free, _Side, point(XS,YS), point(XE,YE)) :-
A is N * pi / 2.5 + pi / 5,
XS is 400 + 100 * cos(A),
YS is 400 + 100 * sin(A),
XE is 400 + 180 * cos(A),
YE is 400 + 180 * sin(A).
compute(N, used, left, point(XS,YS), point(XE,YE)) :-
A is N * pi / 2.5 + pi / 5 - 2 * pi / 15,
XS is 400 + 100 * cos(A),
YS is 400 + 100 * sin(A),
XE is 400 + 180 * cos(A),
YE is 400 + 180 * sin(A).
compute(N, used, right, point(XS,YS), point(XE,YE)) :- A is N * pi / 2.5 + pi / 5 + 2 * pi / 15, XS is 400 + 100 * cos(A), YS is 400 + 100 * sin(A), XE is 400 + 180 * cos(A), YE is 400 + 180 * sin(A).
- - pce_end_class.
</lang>
PureBasic
My Philosophers are very polite, if one can not get both forks they then put down the first and waits a few breaths, then boldly tries in the opposite order.
<lang PureBasic>Macro Tell(Mutex, Message) ; Make a macro to easy send info back to main thread
LockMutex(Mutex) LastElement(Queue()) AddElement(Queue()) Queue() = Message SignalSemaphore(Semaphore) UnlockMutex(Mutex)
EndMacro
- Set up a data structure to pass needed info into the threads
Structure Thread_Parameters
Name.s fork1.i fork2.i
EndStructure
- Declare function to be used
Declare.i TryFork(n) Declare PutDownFork(n) Declare Invite(Namn.s, Fork1, Fork2) Declare _philosophers(*arg.Thread_Parameters)
Global Semaphore = CreateSemaphore() Global Mutex1 = CreateMutex() ; Eg. fork 1 Global Mutex2 = CreateMutex() ; Eg. fork 2 Global Mutex3 = CreateMutex() ; Eg. fork 3 Global Mutex4 = CreateMutex() ; Eg. fork 4 Global Mutex5 = CreateMutex() ; Eg. fork 5 Global Mutex_main = CreateMutex() ; locking communication with the main thread which do all output. Global NewList Queue.s()
If OpenConsole()
Invite("Aristotle",1,2) ; Get all Philosophers activated
Invite("Kant", 2,3)
Invite("Spinoza", 3,4)
Invite("Marx", 4,5)
Invite("Russell", 5,1)
CompilerIf #PB_Compiler_OS=#PB_OS_Windows
SetConsoleTitle_("Dining philosophers, by Jofur") ; Using a Windows-API here, so checking before
CompilerEndIf
; Wait and see if any Philosophers want to tell me anything
Repeat
WaitSemaphore(Semaphore)
LockMutex(Mutex_main)
ForEach Queue()
PrintN( Queue() ) ; Print what the Philosopher(s) told me
i-1
Next Queue()
ClearList(Queue())
UnlockMutex(Mutex_main)
ForEver
EndIf
Procedure TryFork(n) ; Se is fork #n is free and if so pick it up
Select n Case 1: ProcedureReturn TryLockMutex(Mutex1) Case 2: ProcedureReturn TryLockMutex(Mutex2) Case 3: ProcedureReturn TryLockMutex(Mutex3) Case 4: ProcedureReturn TryLockMutex(Mutex4) Default:ProcedureReturn TryLockMutex(Mutex5) EndSelect
EndProcedure
Procedure PutDownFork(n) ; put down fork #n and free it to be used by neighbors.
Select n Case 1: UnlockMutex(Mutex1) Case 2: UnlockMutex(Mutex2) Case 3: UnlockMutex(Mutex3) Case 4: UnlockMutex(Mutex4) Default:UnlockMutex(Mutex5) EndSelect
EndProcedure
Procedure Invite(Namn.s, Fork1, Fork2)
Protected *arg.Thread_Parameters ;create the structure containing the parameters Protected Thread *arg = AllocateMemory(SizeOf(Thread_Parameters)) *arg\Name = Namn *arg\fork1 = Fork1 *arg\fork2 = Fork2 Thread=CreateThread(@_philosophers(), *arg) ;send the thread a pointer to our structure ProcedureReturn Thread
EndProcedure
Procedure _philosophers(*arg.Thread_Parameters)
Protected Iam.s=*arg\Name, j=*arg\fork1, k=*arg\fork2
Protected f1, f2
ClearStructure(*arg, Thread_Parameters)
FreeMemory(*arg)
;
Repeat
Tell(Mutex_main,Iam+": Going to the table")
Repeat ;Trying to get my two forks
f1=TryFork(j)
If f1
f2=TryFork(k)
If Not f2 ; I got only one fork
PutDownFork(j)
f1=0
EndIf
EndIf
If Not f2
Delay(Random(100)) ; Take a short breath, then try the forks in the other order
Swap j,k
EndIf
Until f1 And f2
Tell(Mutex_main,Iam+": I have fork #"+Str(j)+" & #"+Str(k)+" and I'm eating now")
Delay(Random(1500)+15)
Tell(Mutex_main,Iam+": release fork #"+Str(j)+" & #"+Str(k)+"")
Delay(Random(45)+15)
PutDownFork(j)
PutDownFork(k)
f1=0:f2=0
Tell(Mutex_main,Iam+": Thinking about the nature of the universe...")
Delay(Random(2500)+25)
ForEver
EndProcedure</lang>
Python
This solution avoids deadlock by never waiting for a fork while having one in hand. If a philosopher acquires one fork but can't acquire the second, he releases the first fork before waiting to acquire the other (which then becomes the first fork acquired). <lang python>import threading import random import time
- Dining philosophers, 5 Phillies with 5 forks. Must have two forks to eat.
- Deadlock is avoided by never waiting for a fork while holding a fork (locked)
- Procedure is to do block while waiting to get first fork, and a nonblocking
- acquire of second fork. If failed to get second fork, release first fork,
- swap which fork is first and which is second and retry until getting both.
- See discussion page note about 'live lock'.
class Philosopher(threading.Thread):
running = True
def __init__(self, xname, forkOnLeft, forkOnRight):
threading.Thread.__init__(self)
self.name = xname
self.forkOnLeft = forkOnLeft
self.forkOnRight = forkOnRight
def run(self):
while(self.running):
# Philosopher is thinking (but really is sleeping).
time.sleep( random.uniform(3,13))
print '%s is hungry.' % self.name
self.dine()
def dine(self):
fork1, fork2 = self.forkOnLeft, self.forkOnRight
while self.running:
fork1.acquire(True)
locked = fork2.acquire(False)
if locked: break
fork1.release()
print '%s swaps forks' % self.name
fork1, fork2 = fork2, fork1
else:
return
self.dining()
fork2.release()
fork1.release()
def dining(self):
print '%s starts eating '% self.name
time.sleep(random.uniform(1,10))
print '%s finishes eating and leaves to think.' % self.name
def DiningPhilosophers():
forks = [threading.Lock() for n in range(5)]
philosopherNames = ('Aristotle','Kant','Buddha','Marx', 'Russel')
philosophers= [Philosopher(philosopherNames[i], forks[i%5], forks[(i+1)%5]) \
for i in range(5)]
random.seed(507129)
Philosopher.running = True
for p in philosophers: p.start()
time.sleep(100)
Philosopher.running = False
print ("Now we're finishing.")
DiningPhilosophers()</lang>
Racket
<lang racket>
- lang racket
- Racket has traditional semaphores in addition to several higher level
- synchronization tools. (Note that these semaphores are used for Racket's
- green-threads, there are also "future semaphores" which are used for OS
- threads, with a similar interface.)
- ----------------------------------------------------------------------------
- First, a bunch of code to run the experiments below
- Only two philosophers to make it deadlock very fast
(define philosophers '(Aristotle Kant #|Spinoza Marx Russell|#))
(define (run-philosopher name fork1 fork2)
(define (show what) (displayln (~a name " " what))) (define (loop) (show "thinks") (sleep (* 2 (random))) (show "is hungry") (grab-forks fork1 fork2 (λ() (show "eats") (sleep (random)))) (loop)) (thread loop))
(define (run:simple)
(define forks (for/list ([i philosophers]) (make-semaphore 1))) (for ([i philosophers] [fork1 forks] [fork2 (cons (last forks) forks)]) (run-philosopher i fork1 fork2)) (sleep (* 60 60 24 365)))
- ----------------------------------------------------------------------------
- This is the naive implementation, which can be used to try getting a
- deadlock.
(define (grab:naive fork1 fork2 eat!)
(semaphore-wait fork1) (sleep (random)) ; to make deadlocks probable (semaphore-wait fork2) (eat!) (semaphore-post fork1) (semaphore-post fork2))
- ----------------------------------------------------------------------------
- One way to solve it is to release the first fork if the second is busy and
- wait for a while.
(define (grab:release+wait fork1 fork2 eat!)
(semaphore-wait fork1)
(if (not (semaphore-try-wait? fork2))
;; couldn't grab the second fork, so release the first and wait
(begin (semaphore-post fork1)
(sleep (random))
(grab-forks fork1 fork2)) ; can swap them to improve chances
;; we have both forks
(begin (eat!)
(semaphore-post fork1)
(semaphore-post fork2))))
- ----------------------------------------------------------------------------
- Another solution is to label the forks and lock the lowest-id one first,
- which makes the naive solution work.
(define (run:labeled-forks)
(define forks (for/list ([i philosophers]) (make-semaphore 1)))
;; the simple run used forks as (1 2 3 4) (4 1 2 3) -- so to implement this,
;; we can swap the two first ones: (4 2 3 4) (1 1 2 3)
(for ([i philosophers]
[fork1 (cons (last forks) (cdr forks))]
[fork2 (cons (first forks) forks)])
(run-philosopher i fork1 fork2))
(sleep (* 60 60 24 365)))
- ----------------------------------------------------------------------------
- Homework
- implement the centralized waiter solution
- ...
- ----------------------------------------------------------------------------
- Uncomment one of the following pairs to try it
- (define grab-forks grab
- naive)
- (define run run
- simple)
- (define grab-forks grab
- release+wait)
- (define run run
- simple)
- (define grab-forks grab
- naive)
- (define run run
- labeled-forks)
(run) </lang>
Raku
(formerly Perl 6)
We use locking mutexes for the forks, and a lollipop to keep the last person who finished eating from getting hungry until the next person finishes eating, which prevents a cycle of dependency from forming. The algorithm should scale to many philosophers, and no philosopher need be singled out to be left-handed, so it's fair in that sense. <lang perl6>class Fork {
has $!lock = Lock.new;
method grab($who, $which) {
say "$who grabbing $which fork"; $!lock.lock;
}
method drop($who, $which) {
say "$who dropping $which fork"; $!lock.unlock;
}
}
class Lollipop {
has $!channel = Channel.new;
method mine($who) { $!channel.send($who) }
method yours { $!channel.receive }
}
sub dally($sec) { sleep 0.01 + rand * $sec }
sub MAIN(*@names) {
@names ||= <Aristotle Kant Spinoza Marx Russell>;
my @lfork = Fork.new xx @names;
my @rfork = @lfork.rotate;
my $lollipop = Lollipop.new;
start { $lollipop.yours; }
my @philosophers = do for flat @names Z @lfork Z @rfork -> $n, $l, $r {
start { sleep 1 + rand*4; loop { $l.grab($n,'left'); dally 1; # give opportunity for deadlock $r.grab($n,'right'); say "$n eating"; dally 10; $l.drop($n,'left'); $r.drop($n,'right');
$lollipop.mine($n); sleep 1; # lick at least once say "$n lost lollipop to $lollipop.yours(), now digesting";
dally 20; } }
} sink await @philosophers;
}</lang>
- Output:
Aristotle grabbing left fork Aristotle grabbing right fork Aristotle eating Marx grabbing left fork Marx grabbing right fork Marx eating Spinoza grabbing left fork Aristotle dropping left fork Aristotle dropping right fork Russell grabbing left fork Kant grabbing left fork Kant grabbing right fork Spinoza grabbing right fork Marx dropping left fork Marx dropping right fork Aristotle lost lollipop to Marx, now digesting Spinoza eating Spinoza dropping left fork Spinoza dropping right fork Kant eating Russell grabbing right fork Russell eating Marx lost lollipop to Spinoza, now digesting Kant dropping left fork Kant dropping right fork Spinoza lost lollipop to Kant, now digesting Russell dropping left fork Russell dropping right fork Kant lost lollipop to Russell, now digesting Spinoza grabbing left fork Spinoza grabbing right fork Spinoza eating Aristotle grabbing left fork Aristotle grabbing right fork Aristotle eating Aristotle dropping left fork Aristotle dropping right fork Russell lost lollipop to Aristotle, now digesting Spinoza dropping left fork Spinoza dropping right fork Aristotle lost lollipop to Spinoza, now digesting Russell grabbing left fork Marx grabbing left fork Russell grabbing right fork Russell eating Marx grabbing right fork Kant grabbing left fork Kant grabbing right fork Kant eating Russell dropping left fork Russell dropping right fork Spinoza lost lollipop to Russell, now digesting Marx eating Spinoza grabbing left fork Kant dropping left fork Kant dropping right fork Russell lost lollipop to Kant, now digesting Spinoza grabbing right fork ^C
REXX
Programming notes: This REXX version allows a specification of the names and numbers of dining philosophers (but no check was made for the number of philosophers less than two). The philosopher's names may have imbedded blanks in them, blanks are signified by an underscore (_). A random number seed can be specified to allow for repeatability. The duration of any of the activities the philosophers partake in are herein designated in minutes, but any consistent timer unit may be used. Intermediate steps (such as putting the forks down after finishing eating, leaving the dining room to contemplating the nature of the universe, then getting hungry and entering the dining room) are not shown. If the seed (first argument) has a leading plus sign (+), then no status trace is shown. A random selection of diners (for determining who gets to grab for the forks first) could've been implemented to make a more realistic simulation.
Deadlocks are eliminated by the method of acquiring the resources (forks): both forks are (attempted to be) grabbed at the same time (by both hands). <lang rexx>/*REXX program demonstrates a solution in solving the dining philosophers problem. */ signal on halt /*branches to HALT: (on Ctrl─break).*/ parse arg seed diners /*obtain optional arguments from the CL*/ if datatype(seed, 'W') then call random ,, seed /*this allows for random repeatability.*/ if diners= then diners = 'Aristotle, Kant, Spinoza, Marx, Russell'
tell=(left(seed, 1) \== '+') /*Leading + in SEED? Then no statistics*/
diners=space( translate(diners, , ',') ) /*change to an uncommatized diners list*/
#=words(diners); @.= 0 /*#: the number of dining philosophers.*/ eatL=15; eatH= 60 /*minimum & maximum minutes for eating.*/
thinkL=30; thinkH=180 /* " " " " " thinking*/ forks.=1 /*indicate that all forks are on table.*/
do tic=1 /*'til halted.*/ /*use "minutes" for time advancement.*/
call grabForks /*determine if anybody can grab 2 forks*/
call passTime /*handle philosophers eating|thinking. */
end /*tic*/ /* ··· and time marches on ··· */
/* [↓] this REXX program was halted,*/
halt: say ' ··· REXX program halted!' /*probably by Ctrl─Break or equivalent.*/ exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ fork: parse arg x 1 ox; x=abs(x); L=x - 1; if L==0 then L=# /*use "round Robin".*/
if ox<0 then do; forks.L=1; forks.x=1; return; end /*drop the forks. */
got2= forks.L & forks.x /*get 2 forks or not*/
if got2 then do; forks.L=0; forks.x=0; end /*obtained 2 forks. */
return got2 /*return with success ··· or failure. */
/*──────────────────────────────────────────────────────────────────────────────────────*/ grabForks: do person=1 for # /*see if any person can grab two forks.*/
if @.person.state\==0 then iterate /*this diner ain't in a waiting state. */
if \fork(person) then iterate /*this diner didn't grab two forks. */
@.person.state= 'eating' /*this diner is slurping spaghetti. */
@.person.dur=random(eatL, eatH) /*how long will this diner eat pasta ? */
end /*person*/ /* [↑] process the dining philosophers*/
return /*all the diners have been examined. */
/*──────────────────────────────────────────────────────────────────────────────────────*/ passTime: if tell then say /*display a handy blank line separator.*/
do p=1 for # /*handle each of the diner's activity. */
if tell then say right(tic, 9, .) right( word( diners, p), 20),
right(word(@.p.state 'waiting',1+(@.p.state==0)),9) right(@.p.dur,5)
if @.p.dur==0 then iterate /*this diner is waiting for two forks. */
@.p.dur= @.p.dur - 1 /*indicate single time unit has passed.*/
if @.p.dur\==0 then iterate /*Activity done? No, then keep it up.*/
if @.p.state=='eating' then do /*now, leave the table.*/
call fork -p /*drop the darn forks. */
@.p.state= 'thinking' /*status.*/
@.p.dur=random(thinkL, thinkH) /*length.*/
end /* [↓] a diner goes ──► the table. */
else if @.p.state=='thinking' then @.p.state=0
end /*p*/ /*[↑] P (person)≡ dining philosophers.*/
return</lang>
{{out|output|text= (some middle and end portions have been elided):
........1 Aristotle eating 42
........1 Kant waiting 0
........1 Spinoza eating 26
........1 Marx waiting 0
........1 Russell waiting 0
........2 Aristotle eating 41
........2 Kant waiting 0
........2 Spinoza eating 25
........2 Marx waiting 0
........2 Russell waiting 0
~~~elided~~~
.....2074 Aristotle thinking 20
.....2074 Kant thinking 56
.....2074 Spinoza thinking 157
.....2074 Marx eating 29
.....2074 Russell thinking 85
~~~elided~~~
Ruby
<lang ruby>require 'mutex_m'
class Philosopher
def initialize(name, left_fork, right_fork) @name = name @left_fork = left_fork @right_fork = right_fork @meals = 0 end
def go
while @meals < 5
think
dine
end
puts "philosopher #@name is full!"
end
def think puts "philosopher #@name is thinking..." sleep(rand()) puts "philosopher #@name is hungry..." end
def dine
fork1, fork2 = @left_fork, @right_fork
while true
pickup(fork1, :wait => true)
puts "philosopher #@name has fork #{fork1.fork_id}..."
if pickup(fork2, :wait => false)
break
end
puts "philosopher #@name cannot pickup second fork #{fork2.fork_id}..."
release(fork1)
fork1, fork2 = fork2, fork1
end
puts "philosopher #@name has the second fork #{fork2.fork_id}..."
puts "philosopher #@name eats..." sleep(rand()) puts "philosopher #@name belches" @meals += 1
release(@left_fork) release(@right_fork) end
def pickup(fork, opt)
puts "philosopher #@name attempts to pickup fork #{fork.fork_id}..."
opt[:wait] ? fork.mutex.mu_lock : fork.mutex.mu_try_lock
end
def release(fork)
puts "philosopher #@name releases fork #{fork.fork_id}..."
fork.mutex.unlock
end
end
n = 5
Fork = Struct.new(:fork_id, :mutex) forks = Array.new(n) {|i| Fork.new(i, Object.new.extend(Mutex_m))}
philosophers = Array.new(n) do |i|
Thread.new(i, forks[i], forks[(i+1)%n]) do |id, f1, f2|
ph = Philosopher.new(id, f1, f2).go
end
end
philosophers.each {|thread| thread.join}</lang>
Rust
A Rust implementation of a solution for the Dining Philosophers Problem. We prevent a deadlock by using Dijkstra's solution of making a single diner "left-handed." That is, all diners except one pick up the chopstick "to their left" and then the chopstick "to their right." The remaining diner performs this in reverse.
<lang rust>use std::thread; use std::sync::{Mutex, Arc};
struct Philosopher {
name: String, left: usize, right: usize,
}
impl Philosopher {
fn new(name: &str, left: usize, right: usize) -> Philosopher {
Philosopher {
name: name.to_string(),
left: left,
right: right,
}
}
fn eat(&self, table: &Table) {
let _left = table.forks[self.left].lock().unwrap();
let _right = table.forks[self.right].lock().unwrap();
println!("{} is eating.", self.name);
thread::sleep_ms(1000);
println!("{} is done eating.", self.name);
}
}
struct Table {
forks: Vec<Mutex<()>>,
}
fn main() {
let table = Arc::new(Table { forks: vec![
Mutex::new(()),
Mutex::new(()),
Mutex::new(()),
Mutex::new(()),
Mutex::new(()),
]});
let philosophers = vec![
Philosopher::new("Baruch Spinoza", 0, 1),
Philosopher::new("Gilles Deleuze", 1, 2),
Philosopher::new("Karl Marx", 2, 3),
Philosopher::new("Friedrich Nietzsche", 3, 4),
Philosopher::new("Michel Foucault", 0, 4),
];
let handles: Vec<_> = philosophers.into_iter().map(|p| {
let table = table.clone();
thread::spawn(move || {
p.eat(&table);
})
}).collect();
for h in handles {
h.join().unwrap();
}
}</lang>
Simula
<lang simula>COMMENT ! DEADLOCK IS PREVENTED BY REVERSING THE ORDER OF TAKING THE CHOPSTICKS FOR THE LAST PHILOSOPHER. ! THAT MEANS ALL PHILOSOPHERS FIRST TAKE THE LEFT CHOPSTICK, THEN THE RIGHT CHOPSTICK. ! BUT THE LAST PHILOSOPHER FIRST TAKES THE RIGHT CHOPSTICK, THEN THE LEFT. ! ! THE DETACH STATEMENT IN CLASS PHILOSOPHER GIVES CONTROL BACK TO THE MAIN BLOCK. ! THE MAIN BLOCK CALLS/RESUMES ALL THE PHILOSOPHERS USING THE RESUME(PHILOSOPHER) STATEMENT. ! THIS CONTINUES THE CODE IN THE PHILOSOPHER CLASS AFTER THE LAST DETACH STATEMENT. ! (ANOTHER NAME FOR THIS FEATURE IS THE CONCEPT OF A COROUTINE)
BEGIN
INTEGER N; INTEGER PNR, CNR; INTEGER SEED; SEED := ININT; N := 5; BEGIN
CLASS CHOPSTICK;
BEGIN
REF(PHILOSOPHER) OWNER;
INTEGER ID;
ID := CNR := CNR + 1;
END CHOPSTICK;
CLASS PHILOSOPHER(L,R);
REF(CHOPSTICK) L,R;
BEGIN
INTEGER ID;
ID := PNR := PNR + 1;
WHILE TRUE DO
BEGIN
DETACH;
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") THINKING...");
OUTIMAGE;
DETACH;
WHILE RANDINT(0,1,SEED) = 0 DO BEGIN
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") THINKING DEEPER...");
OUTIMAGE;
DETACH;
END;
WHILE L.OWNER =/= NONE DO BEGIN
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") WAITING FOR LEFT CHOPSTICK(");
OUTINT(L.ID, 0);
OUTTEXT(") ...");
OUTIMAGE;
DETACH;
END;
L.OWNER :- THIS PHILOSOPHER;
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") GRABBED LEFT CHOPSTICK(");
OUTINT(L.ID, 0);
OUTTEXT(")");
OUTIMAGE;
WHILE R.OWNER =/= NONE DO BEGIN
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") WAITING FOR RIGHT CHOPSTICK(");
OUTINT(R.ID, 0);
OUTTEXT(") ...");
OUTIMAGE;
DETACH;
END;
R.OWNER :- THIS PHILOSOPHER;
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") GRABBED RIGHT CHOPSTICK(");
OUTINT(R.ID, 0);
OUTTEXT(")");
OUTIMAGE;
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") EATING...");
OUTIMAGE;
WHILE RANDINT(0,1,SEED) = 0 DO BEGIN
DETACH;
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") STILL EATING...");
OUTIMAGE;
END;
L.OWNER :- NONE;
R.OWNER :- NONE;
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") RELEASED LEFT CHOPSTICK(");
OUTINT(L.ID, 0);
OUTTEXT(")");
OUTIMAGE;
OUTTEXT("PHILOSOPHER(");
OUTINT(ID, 0);
OUTTEXT(") RELEASED RIGHT CHOPSTICK(");
OUTINT(R.ID, 0);
OUTTEXT(")");
OUTIMAGE;
END;
END PHILOSOPHER;
!---------------------------------------|
! |
! |
! (3) |
! P2 P3 |
! |
! (2) (4) |
! |
! |
! P1 P4 |
! |
! |
! (1) (5) |
! |
! P5 | only P5 takes Right first (5), then Left (1)
! |
!---------------------------------------|
!;
REF(PHILOSOPHER) ARRAY PHILS (1:N);
REF(CHOPSTICK) L, R;
INTEGER I, LOOPS;
R :- NEW CHOPSTICK;
FOR I := 1 STEP 1 UNTIL N-1 DO
BEGIN
L :- NEW CHOPSTICK;
PHILS(I) :- NEW PHILOSOPHER(L,R);
R :- L;
END;
! REVERSED ORDER FOR THE LAST PHILOSOPHER ;
PHILS(N) :- NEW PHILOSOPHER(R,PHILS(1).R);
FOR I := 1 STEP 1 UNTIL N DO BEGIN
OUTTEXT("PHILOSOPHER(ID=");
OUTINT(PHILS(I).ID, 0);
OUTTEXT(", L=");
OUTINT(PHILS(I).L.ID, 0);
OUTTEXT(", R=");
OUTINT(PHILS(I).R.ID, 0);
OUTTEXT(")");
OUTIMAGE;
END;
FOR LOOPS := 1 STEP 1 UNTIL 10 DO BEGIN
FOR I := 1 STEP 1 UNTIL N DO BEGIN
RESUME(PHILS(I));
END;
END;
END;
END.</lang>
- Input:
12121
- Output:
PHILOSOPHER(ID=1, L=2, R=1) PHILOSOPHER(ID=2, L=3, R=2) PHILOSOPHER(ID=3, L=4, R=3) PHILOSOPHER(ID=4, L=5, R=4) PHILOSOPHER(ID=5, L=5, R=1) PHILOSOPHER(1) THINKING... PHILOSOPHER(2) THINKING... PHILOSOPHER(3) THINKING... PHILOSOPHER(4) THINKING... PHILOSOPHER(5) THINKING... PHILOSOPHER(1) THINKING DEEPER... PHILOSOPHER(2) GRABBED LEFT CHOPSTICK(3) PHILOSOPHER(2) GRABBED RIGHT CHOPSTICK(2) PHILOSOPHER(2) EATING... PHILOSOPHER(3) THINKING DEEPER... PHILOSOPHER(4) GRABBED LEFT CHOPSTICK(5) PHILOSOPHER(4) GRABBED RIGHT CHOPSTICK(4) PHILOSOPHER(4) EATING... PHILOSOPHER(5) WAITING FOR LEFT CHOPSTICK(5) ... PHILOSOPHER(1) WAITING FOR LEFT CHOPSTICK(2) ... PHILOSOPHER(2) STILL EATING... PHILOSOPHER(3) WAITING FOR LEFT CHOPSTICK(4) ... PHILOSOPHER(4) STILL EATING... PHILOSOPHER(5) WAITING FOR LEFT CHOPSTICK(5) ... PHILOSOPHER(1) WAITING FOR LEFT CHOPSTICK(2) ... PHILOSOPHER(2) STILL EATING... PHILOSOPHER(2) RELEASED LEFT CHOPSTICK(3) PHILOSOPHER(2) RELEASED RIGHT CHOPSTICK(2) PHILOSOPHER(3) WAITING FOR LEFT CHOPSTICK(4) ... PHILOSOPHER(4) STILL EATING... PHILOSOPHER(5) WAITING FOR LEFT CHOPSTICK(5) ... PHILOSOPHER(1) GRABBED LEFT CHOPSTICK(2) PHILOSOPHER(1) GRABBED RIGHT CHOPSTICK(1) PHILOSOPHER(1) EATING... PHILOSOPHER(1) RELEASED LEFT CHOPSTICK(2) PHILOSOPHER(1) RELEASED RIGHT CHOPSTICK(1) PHILOSOPHER(2) THINKING... PHILOSOPHER(3) WAITING FOR LEFT CHOPSTICK(4) ... PHILOSOPHER(4) STILL EATING... PHILOSOPHER(5) WAITING FOR LEFT CHOPSTICK(5) ... PHILOSOPHER(1) THINKING... PHILOSOPHER(2) GRABBED LEFT CHOPSTICK(3) PHILOSOPHER(2) GRABBED RIGHT CHOPSTICK(2) PHILOSOPHER(2) EATING... PHILOSOPHER(2) RELEASED LEFT CHOPSTICK(3) PHILOSOPHER(2) RELEASED RIGHT CHOPSTICK(2) PHILOSOPHER(3) WAITING FOR LEFT CHOPSTICK(4) ... PHILOSOPHER(4) STILL EATING... PHILOSOPHER(4) RELEASED LEFT CHOPSTICK(5) PHILOSOPHER(4) RELEASED RIGHT CHOPSTICK(4) PHILOSOPHER(5) GRABBED LEFT CHOPSTICK(5) PHILOSOPHER(5) GRABBED RIGHT CHOPSTICK(1) PHILOSOPHER(5) EATING... PHILOSOPHER(5) RELEASED LEFT CHOPSTICK(5) PHILOSOPHER(5) RELEASED RIGHT CHOPSTICK(1) PHILOSOPHER(1) GRABBED LEFT CHOPSTICK(2) PHILOSOPHER(1) GRABBED RIGHT CHOPSTICK(1) PHILOSOPHER(1) EATING... PHILOSOPHER(2) THINKING... PHILOSOPHER(3) GRABBED LEFT CHOPSTICK(4) PHILOSOPHER(3) GRABBED RIGHT CHOPSTICK(3) PHILOSOPHER(3) EATING... PHILOSOPHER(4) THINKING... PHILOSOPHER(5) THINKING... PHILOSOPHER(1) STILL EATING... PHILOSOPHER(1) RELEASED LEFT CHOPSTICK(2) PHILOSOPHER(1) RELEASED RIGHT CHOPSTICK(1) PHILOSOPHER(2) THINKING DEEPER... PHILOSOPHER(3) STILL EATING... PHILOSOPHER(4) THINKING DEEPER... PHILOSOPHER(5) THINKING DEEPER... PHILOSOPHER(1) THINKING... PHILOSOPHER(2) WAITING FOR LEFT CHOPSTICK(3) ... PHILOSOPHER(3) STILL EATING... PHILOSOPHER(3) RELEASED LEFT CHOPSTICK(4) PHILOSOPHER(3) RELEASED RIGHT CHOPSTICK(3) PHILOSOPHER(4) GRABBED LEFT CHOPSTICK(5) PHILOSOPHER(4) GRABBED RIGHT CHOPSTICK(4) PHILOSOPHER(4) EATING... PHILOSOPHER(5) THINKING DEEPER... PHILOSOPHER(1) THINKING DEEPER... PHILOSOPHER(2) GRABBED LEFT CHOPSTICK(3) PHILOSOPHER(2) GRABBED RIGHT CHOPSTICK(2) PHILOSOPHER(2) EATING... PHILOSOPHER(2) RELEASED LEFT CHOPSTICK(3) PHILOSOPHER(2) RELEASED RIGHT CHOPSTICK(2) PHILOSOPHER(3) THINKING... PHILOSOPHER(4) STILL EATING... PHILOSOPHER(4) RELEASED LEFT CHOPSTICK(5) PHILOSOPHER(4) RELEASED RIGHT CHOPSTICK(4) PHILOSOPHER(5) THINKING DEEPER...
Smalltalk
This solution is similar to the Python solution, except this uses Semaphore instances for forks and an OrderedCollection to hold sequence of forks and philosophers around the table. The method #pickUpForks releases and retries after a small delay till it has both. Special note: Smalltalk concurrent processes are created by sending the message #fork to a block. Do not confuse this method name with the forks of the problem domain.
<lang Smalltalk>'From Squeak3.7 of 4 September 2004 [latest update: #5989] on 13 October 2011 at 2:44:42 pm'! Object subclass: #Philosopher instanceVariableNames: 'table random name seat forks running' classVariableNames: poolDictionaries: category: 'rosettacode'!
!Philosopher methodsFor: 'private'! createfork ^ Semaphore forMutualExclusion! !
!Philosopher methodsFor: 'private'! displayState: aStateName Transcript show: name , ' is ' , aStateName; cr! !
!Philosopher methodsFor: 'private'! pickUpForkAt: relativePosition | fork pos | pos := self tableIndex: seat + relativePosition. (fork := table at: pos) ifNotNil: [fork critical: [(table at: pos) notNil ifTrue: [table at: pos put: nil] ifFalse: [fork := nil]]]. ^ (forks at: relativePosition put: fork) notNil! !
!Philosopher methodsFor: 'private'! putBackForkAt: aRelativePosition | fork | fork := forks at: aRelativePosition. fork ifNotNil: [table at: (self tableIndex: seat + aRelativePosition) put: fork. forks at: aRelativePosition put: nil]! !
!Philosopher methodsFor: 'private'! tableIndex: aNum ^ aNum - 1 \\ table size + 1! !
!Philosopher methodsFor: 'private'! waitRandomTime (Delay forMilliseconds: (random next * 4000) rounded) wait! !
!Philosopher methodsFor: 'dining'!
eat
self displayState: 'eating';
waitRandomTime;
putBackForkAt: -1;
putBackForkAt: 1! !
!Philosopher methodsFor: 'dining'! pickUpForks self displayState: 'trying to pick up forks'. [(self pickUpForkAt: -1) ifTrue: [(self pickUpForkAt: 1) ifFalse: [self putBackForkAt: -1]]. (forks at: 1) notNil] whileFalse: [(Delay forMilliseconds: 10) wait]! !
!Philosopher methodsFor: 'dining'! think self displayState: 'thinking'; waitRandomTime! !
!Philosopher methodsFor: 'initialize-release'!
beginDining: aName at: aTable
name := aName.
table := aTable.
forks := Dictionary new at: -1 put: nil;
at: 1 put: nil;
yourself.
random := Random new seed: name hash.
seat := table size + 1.
table add: self;
add: self createfork.
running := true.
[(Delay forSeconds: 20) wait.
running := false] fork.
[[running]
whileTrue: [self think; pickUpForks; eat].
nil] fork! !
"-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- "!
Philosopher class instanceVariableNames: !
!Philosopher class methodsFor: 'examples'! diningPhilosophersTest | diningTable | diningTable := OrderedCollection new. #('Aristotle' 'Kant' 'Buddha' 'Marx' 'Russel' ) do: [:aName | Philosopher new beginDining: aName at: diningTable]! ! </lang>
Tcl
<lang tcl>package require Thread
foreach name {Aristotle Kant Spinoza Marx Russel} {
lappend forks [thread::mutex create]
lappend tasks [set t [thread::create -preserved {
# Implement each task as a coroutine internally for simplicity of presentation
# This is because we want to remain able to receive messages so we can shut
# down neatly at the end of the program.
interp alias {} doTask {} coroutine t philosopher proc delay {expression} { yield [after [expr $expression] [info coroutine]] }
# Forks are mutexes...
proc pickUpFork fork {
thread::mutex lock $fork
}
proc putDownFork fork {
thread::mutex unlock $fork
}
# The actual implementation of the task
proc philosopher {f1 f2} { global name # Always acquire forks in order; prevents deadlock
# Uses the "natural" order of the lexicographical order of the fork names
if {$f1 > $f2} {
lassign [list $f1 $f2] f2 f1
}
# The classic "philosophers" loop
while {true} { puts "$name is thinking" delay {int(200*rand())}
puts "$name is hungry, getting fork in left hand" pickUpFork $f1 delay {int(2000*rand())} ;# Make deadlock likely if it is possible!
puts "$name is hungry, getting fork in right hand" pickUpFork $f2
puts "$name is eating" delay {int(2000*rand())}
puts "$name has finished eating; putting down forks" putDownFork $f2 putDownFork $f1
delay 100
} } thread::wait
}]] thread::send $t [list set name $name]
}
- Set the tasks going
foreach t $tasks {f1 f2} {0 1 1 2 2 3 3 4 4 0} {
thread::send -async $t [list \
doTask [lindex $forks $f1] [lindex $forks $f2]]
}
- Kill everything off after 30 seconds; that's enough for demonstration!
after 30000 puts "Completing..." foreach t $tasks {
thread::send -async $t thread::exit
}</lang>
VBA
Unfortunately, VBA is single threaded. The task is implemented by introducing a separate program counter for each individual philosopher. While the routine dine loops through all philosophers the routine philosopher execute the actions for each philosopher separately, as if in separate threads. Program terminates after max count is reached. Statistics show how many program ticks are spent at each step at the dining table. <lang vb>'The combination of holding to the second fork '(HOLDON=True) and all philosophers start 'with same hand (DIJKSTRASOLUTION=False) leads 'to a deadlock. To prevent deadlock 'set HOLDON=False, and DIJKSTRASOLUTION=True. Public Const HOLDON = False Public Const DIJKSTRASOLUTION = True Public Const X = 10 'chance to continue eating/thinking Public Const GETS = 0 Public Const PUTS = 1 Public Const EATS = 2 Public Const THKS = 5 Public Const FRSTFORK = 0 Public Const SCNDFORK = 1 Public Const SPAGHETI = 0 Public Const UNIVERSE = 1 Public Const MAXCOUNT = 100000 Public Const PHILOSOPHERS = 5 Public semaphore(PHILOSOPHERS - 1) As Integer Public positi0n(1, PHILOSOPHERS - 1) As Integer Public programcounter(PHILOSOPHERS - 1) As Long Public statistics(PHILOSOPHERS - 1, 5, 1) As Long Public names As Variant Private Sub init()
names = [{"Aquinas","Babbage","Carroll","Derrida","Erasmus"}]
For j = 0 To PHILOSOPHERS - 2
positi0n(0, j) = j + 1 'first fork in right hand
positi0n(1, j) = j 'second fork in left hand
Next j
If DIJKSTRASOLUTION Then
positi0n(0, PHILOSOPHERS - 1) = j ' first fork in left hand
positi0n(1, PHILOSOPHERS - 1) = 0 'second fork in right hand
Else
positi0n(0, PHILOSOPHERS - 1) = 0 'first fork in right hand
positi0n(1, PHILOSOPHERS - 1) = j 'second fork in left hand
End If
End Sub Private Sub philosopher(subject As Integer, verb As Integer, objekt As Integer)
statistics(subject, verb, objekt) = statistics(subject, verb, objekt) + 1
If verb < 2 Then
If semaphore(positi0n(objekt, subject)) <> verb Then
If Not HOLDON Then
'can't get a fork, release first fork if subject has it, and
'this won't toggle the semaphore if subject hasn't firt fork
semaphore(positi0n(FRSTFORK, subject)) = 1 - objekt
'next round back to try to get first fork
programcounter(subject) = 0
End If
Else
'just toggle semaphore and move on
semaphore(positi0n(objekt, subject)) = 1 - verb
programcounter(subject) = (programcounter(subject) + 1) Mod 6
End If
Else
'when eating or thinking, (100*(X-1)/X)% continue eating or thinking
'(100/X)% advance program counter
programcounter(subject) = IIf(X * Rnd > 1, verb, verb + 1) Mod 6
End If
End Sub Private Sub dine()
Dim ph As Integer
Do While TC < MAXCOUNT
For ph = 0 To PHILOSOPHERS - 1
Select Case programcounter(ph)
Case 0: philosopher ph, GETS, FRSTFORK
Case 1: philosopher ph, GETS, SCNDFORK
Case 2: philosopher ph, EATS, SPAGHETI
Case 3: philosopher ph, PUTS, FRSTFORK
Case 4: philosopher ph, PUTS, SCNDFORK
Case 5: philosopher ph, THKS, UNIVERSE
End Select
TC = TC + 1
Next ph
Loop
End Sub Private Sub show()
Debug.Print "Stats", "Gets", "Gets", "Eats", "Puts", "Puts", "Thinks"
Debug.Print "", "First", "Second", "Spag-", "First", "Second", "About"
Debug.Print "", "Fork", "Fork", "hetti", "Fork", "Fork", "Universe"
For subject = 0 To PHILOSOPHERS - 1
Debug.Print names(subject + 1),
For objekt = 0 To 1
Debug.Print statistics(subject, GETS, objekt),
Next objekt
Debug.Print statistics(subject, EATS, SPAGHETI),
For objekt = 0 To 1
Debug.Print statistics(subject, PUTS, objekt),
Next objekt
Debug.Print statistics(subject, THKS, UNIVERSE)
Next subject
End Sub Public Sub main()
init dine show
End Sub</lang>
- Output:
Stats Gets Gets Eats Puts Puts Thinks
First Second Spag- First Second About
Fork Fork hetti Fork Fork Universe
Aquinas 5595 1902 5843 550 550 5560
Babbage 5811 2360 5585 529 529 5186
Carroll 6445 2359 4929 523 523 5221
Derrida 6341 1828 5479 545 545 5262
Erasmus 5998 1556 5891 550 550 5455
Visual Basic .NET
This has three modes.
- In the first mode a dead lock will occur if each philosopher picks up their left fork before any pick up their right.
- In the second mode each philosopher will put down their left fork if they cannot pick up their right. This is susceptible to live lock, where each philosopher keeps retrying but never makes any progress.
- In the third mode, each fork is numbered. The philosopher will always pick up a lower number fork before a higher number fork, thus preventing a dead-lock while guaranteeing forward progress.
<lang vbnet>Imports System.Threading Module Module1
Public rnd As New Random
Sub Main()
'Aristotle, Kant, Spinoza, Marx, and Russel
Dim f1 As New Fork(1)
Dim f2 As New Fork(2)
Dim f3 As New Fork(3)
Dim f4 As New Fork(4)
Dim f5 As New Fork(5)
Console.WriteLine("1: Deadlock")
Console.WriteLine("2: Live lock")
Console.WriteLine("3: Working")
Select Console.ReadLine
Case "1"
Using _
Aristotle As New SelfishPhilosopher("Aristotle", f1, f2), _
Kant As New SelfishPhilosopher("Kant", f2, f3), _
Spinoza As New SelfishPhilosopher("Spinoza", f3, f4), _
Marx As New SelfishPhilosopher("Marx", f4, f5), _
Russel As New SelfishPhilosopher("Russel", f5, f1)
Console.ReadLine()
End Using
Case "2"
Using _
Aristotle As New SelflessPhilosopher("Aristotle", f1, f2), _
Kant As New SelflessPhilosopher("Kant", f2, f3), _
Spinoza As New SelflessPhilosopher("Spinoza", f3, f4), _
Marx As New SelflessPhilosopher("Marx", f4, f5), _
Russel As New SelflessPhilosopher("Russel", f5, f1)
Console.ReadLine()
End Using
Case "3"
Using _
Aristotle As New WisePhilosopher("Aristotle", f1, f2), _
Kant As New WisePhilosopher("Kant", f2, f3), _
Spinoza As New WisePhilosopher("Spinoza", f3, f4), _
Marx As New WisePhilosopher("Marx", f4, f5), _
Russel As New WisePhilosopher("Russel", f5, f1)
Console.ReadLine()
End Using
End Select
End Sub
End Module</lang>
<lang vbnet>Class Fork
Private ReadOnly m_Number As Integer
Public Sub New(ByVal number As Integer)
m_Number = number
End Sub
Public ReadOnly Property Number() As Integer
Get
Return m_Number
End Get
End Property
End Class</lang>
<lang vbnet>MustInherit Class PhilosopherBase
Implements IDisposable
Protected m_Disposed As Boolean
Protected ReadOnly m_Left As Fork
Protected ReadOnly m_Right As Fork
Protected ReadOnly m_Name As String
Public Sub New(ByVal name As String, ByVal right As Fork, ByVal left As Fork)
m_Name = name
m_Right = right
m_Left = left
Dim t As New Thread(AddressOf MainLoop)
t.IsBackground = True
t.Start()
End Sub
Protected Overridable Sub Dispose(ByVal disposing As Boolean)
m_Disposed = True
End Sub
Public Sub Dispose() Implements IDisposable.Dispose
Dispose(True)
GC.SuppressFinalize(Me)
End Sub
Public ReadOnly Property Name() As String
Get
Return m_Name
End Get
End Property
Public MustOverride Sub MainLoop()
End Class</lang>
Deadlock
<lang vbnet>Class SelfishPhilosopher
Inherits PhilosopherBase
Public Sub New(ByVal name As String, ByVal right As Fork, ByVal left As Fork)
MyBase.New(name, right, left)
End Sub
Public Overrides Sub MainLoop()
Do
Console.WriteLine(Name & " sat down")
SyncLock m_Left
Console.WriteLine(Name & " picked up fork " & m_Left.Number)
SyncLock m_Right
Console.WriteLine(Name & " picked up fork " & m_Right.Number)
Console.WriteLine(Name & " ate!!!!")
Console.WriteLine(Name & " put down fork " & m_Right.Number)
End SyncLock
Console.WriteLine(Name & " put down fork " & m_Left.Number)
End SyncLock
Console.WriteLine(Name & " stood up")
Thread.Sleep(rnd.Next(0, 10000))
Loop Until m_Disposed
End Sub
End Class</lang>
Live Lock
<lang vbnet>Class SelflessPhilosopher
Inherits PhilosopherBase
Public Sub New(ByVal name As String, ByVal right As Fork, ByVal left As Fork)
MyBase.New(name, right, left)
End Sub
Public Overrides Sub MainLoop()
Do
Console.WriteLine(Name & " sat down")
Dim needDelay = False
TryAgain:
If needDelay Then Thread.Sleep(rnd.Next(0, 10000))
Try
Monitor.Enter(m_Left)
Console.WriteLine(Name & " picked up fork " & m_Left.Number)
If Monitor.TryEnter(m_Right) Then
Console.WriteLine(Name & " picked up fork " & m_Right.Number)
Console.WriteLine(Name & " ate!!!!!!")
Console.WriteLine(Name & " put down fork " & m_Right.Number)
Monitor.Exit(m_Right)
Else
Console.WriteLine(Name & " is going to wait")
needDelay = True
GoTo TryAgain
End If
Finally
Console.WriteLine(Name & " put down fork " & m_Left.Number)
End Try
Console.WriteLine(Name & " stood up")
Thread.Sleep(rnd.Next(0, 10000))
Loop Until m_Disposed End Sub
End Class</lang>
Working
<lang vbnet>Class WisePhilosopher
Inherits PhilosopherBase
Public Sub New(ByVal name As String, ByVal right As Fork, ByVal left As Fork)
MyBase.New(name, right, left)
End Sub
Public Overrides Sub MainLoop()
Do
Console.WriteLine(Name & " sat down")
Dim first As Fork, second As Fork
If m_Left.Number > m_Right.Number Then
first = m_Left
second = m_Right
Else
first = m_Right
second = m_Left
End If
SyncLock first
Console.WriteLine(Name & " picked up fork " & m_Left.Number)
SyncLock second
Console.WriteLine(Name & " picked up fork " & m_Right.Number)
Console.WriteLine(Name & " ate!!!!")
Console.WriteLine(Name & " put down fork " & m_Right.Number)
End SyncLock
Console.WriteLine(Name & " put down fork " & m_Left.Number)
End SyncLock
Console.WriteLine(Name & " stood up")
Thread.Sleep(rnd.Next(0, 10000))
Loop Until m_Disposed
End Sub
End Class</lang>
zkl
The model used here is five seats, each with two forks. Sn:(Fn,Fn+1). Seat(n+1) shares a fork with seat n and so on. Fork are represented by an atomic bool, a bool that changes state atomically. Each philosopher is a thread. Each philosopher is ambidextrous, leading with a random hand. If the trailing hand can not get a fork, the already in hand fork is put down. <lang zkl>var [const] forks=(5).pump(List,Atomic.Bool.fp(False)), // True==fork in use
seats=(5).pump(List,'wrap(n){ List(forks[n],forks[(n+1)%5]) });
fcn sitAndEat(name,n){ // assigned seating
while(1){
fa,fb:=seats[n].shuffle(); // ambidextrous
if(fa.setIf(True,False)){ // got the first fork
if(fb.setIf(True,False)){ // got the other fork, nom nom time name.println(" is eating"); Atomic.sleep((1).random(5)); fa.set(False); fb.set(False); // put forks down return(); // leave the table } else{ fa.set(False); // put fork down, try again in a bit name.println(": Could not get two forks"); }
} else name.println(": Could not get first fork");
Atomic.sleep((1).random(2)); // sits for a bit
}
} fcn philo([(seat,name)]){ // a thread
while(1){ // eat and think forever
name.println(" is thinking."); Atomic.sleep((1).random(5));
sitAndEat(name,seat);
}
}</lang> The setIf method atomically sets the bool to the first value if it is currently the second value. <lang zkl>T("Aristotle", "Kant", "Spinoza", "Marx", "Russell").enumerate() .apply(philo.launch);
Atomic.sleep(100000); // hang out in the REPL, aka thread keep alive</lang>
- Output:
... Aristotle: Could not get two forks Russell: Could not get two forks Aristotle: Could not get two forks Russell: Could not get first fork Spinoza: Could not get first fork Kant is thinking. Aristotle is eating ...
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