Synchronous concurrency

From Rosetta Code
Task
Synchronous concurrency
You are encouraged to solve this task according to the task description, using any language you may know.

The goal of this task is to create two concurrent activities ("Threads" or "Tasks", not processes.) that share data synchronously. Your language may provide syntax or libraries to perform concurrency. Different languages provide different implementations of concurrency, often with different names. Some languages use the term threads, others use the term tasks, while others use co-processes. This task should not be implemented using fork, spawn, or the Linux/UNIX/Win32 pipe command, as communication should be between threads, not processes.

One of the concurrent units will read from a file named "input.txt" and send the contents of that file, one line at a time, to the other concurrent unit, which will print the line it receives to standard output. The printing unit must count the number of lines it prints. After the concurrent unit reading the file sends its last line to the printing unit, the reading unit will request the number of lines printed by the printing unit. The reading unit will then print the number of lines printed by the printing unit.

This task requires two-way communication between the concurrent units. All concurrent units must cleanly terminate at the end of the program.

Ada[edit]

This Ada example starts by creating a package defining a single instance of a printing task. Ada requires packages to be separated into two parts. The package specification defines the interface to all public members of the package.

package Synchronous_Concurrent is
task Printer is
entry Put(Item : in String);
entry Get_Count(Count : out Natural);
end Printer;
end Synchronous_Concurrent;

The package body contains the implementation of all the subprograms and tasks defined in the specification.

with Ada.Text_Io; use Ada.Text_Io;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
 
package body Synchronous_Concurrent is
 
task body Printer is
Num_Iter : Natural := 0;
Line  : Unbounded_String;
begin
loop
select
accept Put(Item : in String) do
Line := To_Unbounded_String(Item);
end Put;
Put_Line(To_String(Line));
Num_Iter := Num_Iter + 1;
or
accept Get_Count(Count : out Natural) do
Count := Num_Iter;
end Get_Count;
or terminate;
end select;
end loop;
end Printer;
 
end Synchronous_Concurrent;

Note that the task body contains an accept block for each entry defined in the task specification. When some other task calls an entry in the Printer task the communication between the tasks is synchronized.

This example uses an infinite loop in the printer task. There is no way to know ahead of time how many lines the printer task will need to print. Each iteration through the loop causes the task to execute a selective accept. That means that it can either accept a call on the Put entry, or it can accept a call on the Get_Count entry. The terminate option is execute only when the program contains no more tasks that can call the entries in the Printer task. If no task has called either entry the Printer task will wait for a task to call one of the entries, or for the terminate option to apply.

The next file contains the main procedure for this program. The main or entry-point procedure for a program always runs in the environment task. For this program, the environment task is takes on the role of the file reading concurrent unit while the Printer task takes on the role of the printing concurrent unit.

with Synchronous_Concurrent; use Synchronous_Concurrent;
with Ada.Text_Io; use Ada.Text_Io;
 
procedure Synchronous_Concurrent_Main is
Num_Strings : Natural;
The_File : File_Type;
Line : String(1..255);
Length : Natural;
begin
Open(File => The_File, Mode => In_File, Name => "input.txt");
while not End_Of_File(The_File) loop
Get_Line(File => The_File, Item => Line, Last => Length);
Printer.Put(Line(1..Length));
end loop;
Close(The_File);
Printer.Get_Count(Num_Strings);
New_Line;
Put_Line("The task wrote" & Natural'Image(Num_Strings) & " strings.");
end Synchronous_Concurrent_Main;

In this example only the environment task can call the entries on the Printer task. When the environment task completes the terminate option of the Printer task applies, terminating the Printer task. The environment task completes right after printing the number of lines sent through the Printer task. Because of the terminate option, the Printer task terminates just after the environment task prints the count.

Aikido[edit]

Aikido supports threads and monitors natively (built in to the language). There is also support for closures, but this example will use threads:

 
monitor Queue {
var items = []
public function put (item) {
items.append (item)
notify()
}
 
public function get() {
while (items.size() == 0) {
wait()
}
var item = items[0]
items <<= 1
return item
}
 
public function close {
items.append (none)
}
}
 
thread reader (queue) {
var numlines = 0
for (;;) {
var line = queue.get()
if (typeof(line) == "none") {
break
}
print (line)
numlines++
}
println ("Number of lines: " + numlines)
}
 
thread writer (queue, lines) {
foreach line lines {
queue.put (line)
}
queue.close()
}
 
var queue = new Queue()
var lines = readfile ("input.txt")
var r = reader(queue)
var w = writer(queue, lines)
 
join (r)
join (w)

ALGOL 68[edit]

(
STRING line;
INT count := 0, errno;
BOOL input complete := FALSE;
SEMA output throttle = LEVEL 0, input throttle = LEVEL 1;
 
FILE input txt;
errno := open(input txt, "input.txt", stand in channel);
 
PROC call back done = (REF FILE f) BOOL: ( input complete := TRUE );
on logical file end(input txt, call back done);
 
PAR (
WHILE
DOWN input throttle;
get(input txt,(line, new line));
UP output throttle;
NOT input complete
DO
count+:=1
OD
,
WHILE
DOWN output throttle;
NOT input complete
DO
print((line, new line));
UP input throttle
OD
);
print((count))
)

BCPL[edit]

// This is a BCPL implementation of the Rosettacode synchronous
// concurrency test using BCPL coroutines and a coroutine implementation
// of a Occum-style channels.
// BCPL is freely available from www.cl.cam.ac.uk/users/mr10
 
SECTION "coinout"
 
GET "libhdr.h"
 
GLOBAL {
tracing: ug
}
 
LET start() = VALOF
{ LET argv = VEC 50
LET in_co, out_co = 0, 0
LET channel = 0
LET filename = "input.txt"
 
UNLESS rdargs("-f,-t/S", argv, 50) DO
{ writef("Bad arguments for coinout*n")
RESULTIS 0
}
 
IF argv!0 DO filename := argv!0 // -f the source file
tracing := argv!1 // -t/S tracing option
 
in_co  := initco(infn, 500, @channel)
out_co := initco(outfn, 500, @channel)
 
UNLESS in_co & out_co DO
{ writef("Trouble creating the coroutines*n")
GOTO fin
}
 
IF tracing DO writef("*nBoth in and out coroutines created*n*n")
 
callco(in_co, filename)
 
fin:
IF in_co DO deleteco(in_co)
IF out_co DO deleteco(out_co)
 
IF tracing DO writef("Both in and out coroutines deleted*n*n")
 
RESULTIS 0
}
 
AND readline(line) = VALOF
{ LET ch, i = 0, 0
line%0 := 0
 
{ ch := rdch()
IF ch=endstreamch RESULTIS FALSE
i := i+1
line%0, line%i := i, ch
IF ch='*n' | i=255 BREAK
} REPEAT
 
RESULTIS TRUE
}
 
AND infn(args) BE
{ LET channelptr = args!0
LET name = cowait() // Get the file name
LET instream = findinput(name)
LET line = VEC 256/bytesperword
 
UNLESS instream DO
{ writef("*nTrouble with file: %s*n", name)
RETURN
}
 
selectinput(instream)
 
{ LET ok = readline(line)
UNLESS ok BREAK
IF tracing DO
writef("inco: Sending a line to outco*n")
cowrite(channelptr, line)
} REPEAT
 
IF tracing DO
writef("inco: Sending zero to outco*n")
 
writef("*nNumber of lines written was %n*n", cowrite(channelptr, 0))
 
endstream(instream)
}
 
AND outfn(args) BE
{ LET channelptr = args!0
LET linecount = 0
 
{ LET line = coread(channelptr)
UNLESS line BREAK
IF tracing DO writef("outfn: Received a line*n")
writes(line)
linecount := linecount + 1
} REPEAT
 
IF tracing DO
writef("outfn: Received zero, so sent count=%n back to inco*n",
linecount)
 
cowait(linecount)
}
 
// The following functions are a implementation of Occum-style channels
// using coroutines.
 
// The first coroutine to request a transfer through a channel becomes
// suspended and the second causes the data to be transfers and then allows
// both coroutines to resume (in some order). The channel word is either
// zero or points to a suspended (read or write) cocoutine.
 
// The use of resumeco in coread is somewhat subtle!
 
AND coread(ptr) = VALOF
{ LET cptr = !ptr
TEST cptr
THEN { !ptr := 0 // Clear the channel word
RESULTIS resumeco(cptr, currco)
}
ELSE { !ptr := currco // Set channel word to this coroutine
RESULTIS cowait() // Wait for value from cowrite
}
}
 
AND cowrite(ptr, val) BE
{ LET cptr = !ptr
TEST cptr
THEN { !ptr := 0
callco(cptr, val) // Send val to coread
}
ELSE { !ptr := currco
callco(cowait(), val)
}
}
 

C[edit]

Library: libco
#include <stdlib.h>	/* malloc(), realloc(), free() */
#include <stdio.h> /* fopen(), fgetc(), fwrite(), printf() */
 
#include <libco.h> /* co_create(), co_switch() */
 
void
fail(const char *message) {
perror(message);
exit(1);
}
 
/*
* These are global variables of this process. All cothreads of this
* process will share these global variables.
*/

cothread_t reader;
cothread_t printer;
struct {
char *buf; /* Not a C string. No terminating '\0'. */
size_t len; /* Length of line in buffer. */
size_t cap; /* Maximum capacity of buffer. */
} line;
size_t count; /* Number of lines printed. */
 
/*
* The reader cothread reads every line of an input file, passes each
* line to the printer cothread, and reports the number of lines.
*/

void
reader_entry(void)
{
FILE *input;
size_t newcap;
int c, eof, eol;
char *newbuf;
 
input = fopen("input.txt", "r");
if (input == NULL)
fail("fopen");
 
line.buf = malloc(line.cap = 4096); /* New buffer. */
if (line.buf == NULL)
fail("malloc");
line.len = 0; /* Start with an empty line. */
 
do {
c = fgetc(input); /* Read next character. */
if (ferror(input))
fail("fgetc");
 
eof = (c == EOF);
if (eof) {
/*
* End of file is also end of line,
` * unless the line would be empty.
*/

eol = (line.len > 0);
} else {
/* Append c to the buffer. */
if (line.len == line.cap) {
/* Need a bigger buffer! */
newcap = line.cap * 2;
newbuf = realloc(line.buf, newcap);
if (newbuf == NULL)
fail("realloc");
line.buf = newbuf;
line.cap = newcap;
}
line.buf[line.len++] = c;
 
/* '\n' is end of line. */
eol = (c == '\n');
}
 
if (eol) {
/* Pass our complete line to the printer. */
co_switch(printer);
line.len = 0; /* Destroy our line. */
}
} while (!eof);
 
free(line.buf);
line.buf = NULL; /* Stops a loop in the printer. */
 
printf("Printed %zu lines.\n", count);
co_switch(printer);
}
 
/*
* The printer cothread starts the reader cothread, prints every line
* line from the reader cothread, and counts the number of lines.
*/

int
main()
{
reader = co_create(4096, reader_entry);
printer = co_active();
count = 0;
 
for (;;) {
co_switch(reader);
if (line.buf == NULL)
break;
 
/* Print this line. Count it. */
fwrite(line.buf, 1, line.len, stdout);
count++;
}
 
co_delete(reader);
return 0;
}

C++[edit]

Works with: C++11
#include <future>
#include <iostream>
#include <fstream>
#include <mutex>
#include <queue>
#include <string>
#include <thread>
 
struct lock_queue
{
std::queue<std::string> q;
std::mutex mutex;
};
 
void reader(std::string filename, std::future<size_t> lines, lock_queue& out)
{
std::string line;
std::ifstream in(filename);
while(std::getline(in, line)) {
line += '\n';
std::lock_guard<std::mutex> lock(out.mutex);
out.q.push(line);
} {
std::lock_guard<std::mutex> lock(out.mutex);
out.q.push("");
}
lines.wait();
std::cout << "\nPrinted " << lines.get() << " lines\n";
}
 
void printer(std::promise<size_t> lines, lock_queue& in)
{
std::string s;
size_t line_n = 0;
bool print = false;
while(true) {
{
std::lock_guard<std::mutex> lock(in.mutex);
if(( print = not in.q.empty() )) { //Assignment intended
s = in.q.front();
in.q.pop();
}
}
if(print) {
if(s == "") break;
std::cout << s;
++line_n;
print = false;
}
}
lines.set_value(line_n);
}
 
int main()
{
lock_queue queue;
std::promise<size_t> promise;
std::thread t1(reader, "input.txt", promise.get_future(), std::ref(queue));
std::thread t2(printer, std::move(promise), std::ref(queue));
t1.join(); t2.join();
}
Output:
Lorem ipsum dolor sit amet, consectetur adipisicing elit, 
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 
Ut enim ad minim veniam, 
quis nostrud exercitation ullamco laboris 
nisi ut aliquip ex ea commodo consequat. 
Duis aute irure dolor in reprehenderit in voluptate velit 
esse cillum dolore eu fugiat nulla pariatur. 
Excepteur sint occaecat cupidatat non proident, 
sunt in culpa qui officia deserunt mollit anim id est laborum.

Printed 9 lines

C#[edit]

using System;
using System.Threading.Tasks;
using System.Collections.Concurrent;
using System.IO;
 
namespace SynchronousConcurrency
{
class Program
{
static void Main(string[] args)
{
BlockingCollection<string> toWriterTask = new BlockingCollection<string>();
BlockingCollection<int> fromWriterTask = new BlockingCollection<int>();
Task writer = Task.Factory.StartNew(() => ConsoleWriter(toWriterTask, fromWriterTask));
Task reader = Task.Factory.StartNew(() => FileReader(fromWriterTask, toWriterTask));
Task.WaitAll(writer, reader);
}
static void ConsoleWriter(BlockingCollection<string> input, BlockingCollection<int> output)
{
int nLines = 0;
string line;
while ((line = input.Take()) != null)
{
Console.WriteLine(line);
++nLines;
}
output.Add(nLines);
}
static void FileReader(BlockingCollection<int> input, BlockingCollection<string> output)
{
StreamReader file = new StreamReader("input.txt"); // TODO: check exceptions
string line;
while ((line = file.ReadLine()) != null)
{
output.Add(line);
 
}
output.Add(null); // EOF
Console.WriteLine("line count: " + input.Take());
}
}
}
Output:
foo
bar
baz
xenu 666
line count: 4

Clojure[edit]

The writer executes as an agent on a thread from a thread pool. The state of the agent is the count of written lines, initialized to 0. The reader will send the writer calls to the write-line function, which the agent will execute asynchronously. The state argument is the agent's state at the start of the call, and the last expression becomes the agent's new state.

(use '[clojure.java.io :as io])
 
(def writer (agent 0))
 
(defn write-line [state line]
(println line)
(inc state))

The reader executes on the main thread. It passes each line to the writer -- the send call returns immediately, waits until the writer has finished writing all the lines, gets the line count & prints it, and terminates the writer.

(with-open [r (io/reader "input.txt")]
(doseq [line (line-seq r)]
(send writer write-line line)))
(await writer)
(println "lines written:" @writer)
(shutdown-agents)

That's it!

Common Lisp[edit]

First, implement message-passing:

(defvar *self*)
 
(defclass queue ()
((condition :initform (make-condition-variable)
:reader condition-of)
(mailbox :initform '()
:accessor mailbox-of)
(lock :initform (make-lock)
:reader lock-of)))
 
(defun message (recipient name &rest message)
(with-lock-held ((lock-of recipient))
;; it would have been better to implement tail-consing or a LIFO
(setf (mailbox-of recipient)
(nconc (mailbox-of recipient)
(list (list* name message))))
(condition-notify (condition-of recipient)))
message)
 
(defun mklist (x)
(if (listp x)
x
(list x)))
 
(defun slurp-message ()
(with-lock-held ((lock-of *self*))
(if (not (endp (mailbox-of *self*)))
(pop (mailbox-of *self*))
(progn (condition-wait (condition-of *self*)
(lock-of *self*))
(assert (not (endp (mailbox-of *self*))))
(pop (mailbox-of *self*))))))
 
(defmacro receive-message (&body cases)
(let ((msg-name (gensym "MESSAGE"))
(block-name (gensym "BLOCK")))
`(let ((,msg-name (slurp-message)))
(block ,block-name
,@(loop for i in cases
for ((name . case) . body) = (cons (mklist (car i))
(cdr i))
when (typep i '(or (cons (eql quote)
t)
(cons (cons (eql quote) t)
t)))
do (warn "~S is a quoted form" i)
collect `(when ,(if (null name)
't
`(eql ',name (car ,msg-name)))
(destructuring-bind ,case
(cdr ,msg-name)
(return-from ,block-name
(progn ,@body)))))
(error "Unknown message: ~S" ,msg-name)))))
 
(defmacro receive-one-message (message &body body)
`(receive-message (,message . ,body)))
 
(defun queue () (make-instance 'queue))

Should be easy from now on:

(defun reader (pathname writer)
(with-open-file (stream pathname)
(loop for line = (read-line stream nil)
while line
do (message writer '|here's a line for you| line)
finally
(message writer '|how many lines?|)
(receive-one-message (|line count| count)
(format t "line count: ~D~%" count))
(message writer '|looks like i've got no more lines|))))
 
(defun writer (stream reader)
;; that would work better with ITERATE
(loop with line-count = 0 do
(receive-message
((|here's a line for you| line)
(write-line line stream)
(incf line-count))
(|looks like i've got no more lines|
(return))
(|how many lines?|
(message reader '|line count| line-count)))))
 
(defmacro thread (queue &body body)
`(make-thread (lambda (&aux (*self* ,queue))
,@body)))
 
(defun synchronous-concurrency (&key (pathname "input.txt"))
(let ((reader (queue))
(writer (queue)))
(thread reader (reader pathname writer))
(thread writer (writer *standard-output* reader)))
(values))

And now an example:

CL-USER> (synchronous-concurrency :pathname "/tmp/input.txt")
foo
bar
baz
xenu 666
line count: 4
; No value

Note that to run the example from the SLIME REPL you need to put:

 (setq swank:*globally-redirect-io* t)

in your ~/.swank.lisp

D[edit]

import std.algorithm, std.concurrency, std.stdio;
 
void main() {
auto printer = spawn(&printTask, thisTid);
auto f = File("input.txt","r");
foreach (string line; lines(f))
send(printer, line);
send(printer, true); //EOF
auto n = receiveOnly!(int)();
stdout.writefln("\n%d lines printed.", n);
}
 
void printTask(Tid reader) {
int n = 0;
for (bool eof = false; !eof;)
receive(
(string line) {stdout.write(line); n++;},
(bool) {send(reader, n); eof = true;}
);
}
 

Delphi[edit]

 
program Project2;
 
{$APPTYPE CONSOLE}
 
uses
SysUtils, Classes, Windows;
 
type
EThreadStackFinalized = class(Exception);
 
PLine = ^TLine;
TLine = record
Text: string;
end;
 
TThreadQueue = class
private
FFinalized: Boolean;
FQueue: THandle;
public
constructor Create;
destructor Destroy; override;
procedure Finalize;
procedure Push(Data: Pointer);
function Pop(var Data: Pointer): Boolean;
property Finalized: Boolean read FFinalized;
end;
 
TPrintThread = class(TThread)
private
FCount: Integer;
FTreminateEvent: THandle;
FDoneEvent: THandle;
FQueue: TThreadQueue;
public
constructor Create(aTreminateEvent, aDoneEvent: THandle; aQueue: TThreadQueue);
procedure Execute; override;
 
property Count: Integer read FCount;
end;
 
{ TThreadQueue }
 
constructor TThreadQueue.Create;
begin
FQueue := CreateIOCompletionPort(INVALID_HANDLE_VALUE, 0, 0, 0);
FFinalized := False;
end;
 
destructor TThreadQueue.Destroy;
begin
if FQueue <> 0 then
CloseHandle(FQueue);
inherited;
end;
 
procedure TThreadQueue.Finalize;
begin
PostQueuedCompletionStatus(FQueue, 0, 0, Pointer($FFFFFFFF));
FFinalized := True;
end;
 
function TThreadQueue.Pop(var Data: Pointer): Boolean;
var
A: Cardinal;
OL: POverLapped;
begin
Result := True;
if not FFinalized then
GetQueuedCompletionStatus(FQueue, A, Cardinal(Data), OL, INFINITE);
 
if FFinalized or (OL = Pointer($FFFFFFFF)) then begin
Data := nil;
Result := False;
Finalize;
end;
end;
 
procedure TThreadQueue.Push(Data: Pointer);
begin
if FFinalized then
raise EThreadStackFinalized.Create('Stack is finalized');
 
PostQueuedCompletionStatus(FQueue, 0, Cardinal(Data), nil);
end;
 
{ TPrintThread }
 
constructor TPrintThread.Create(aTreminateEvent, aDoneEvent: THandle; aQueue: TThreadQueue);
begin
inherited Create(True);
FCount := 0;
FreeOnTerminate := True;
FTreminateEvent := aTreminateEvent;
FDoneEvent := aDoneEvent;
FQueue := aQueue;
end;
 
procedure TPrintThread.Execute;
var
data: Pointer;
line: PLine;
begin
repeat
if FQueue.Pop(data) then begin
line := data;
try
Writeln(line^.Text);
if line^.Text = #0 then
SetEvent(FDoneEvent);
Inc(FCount);
finally
Dispose(line);
end;
end;
 
until False;
WaitForSingleObject(FTreminateEvent, INFINITE);
end;
 
var
PrintThread: TPrintThread;
Queue: TThreadQueue;
lines: TStrings;
i: Integer;
line: PLine;
TreminateEvent, DoneEvent: THandle;
begin
Queue := TThreadQueue.Create;
try
TreminateEvent := CreateEvent(nil, False, False, 'TERMINATE_EVENT');
DoneEvent := CreateEvent(nil, False, False, 'DONE_EVENT');
try
PrintThread := TPrintThread.Create(TreminateEvent, DoneEvent, Queue);
PrintThread.Start;
lines := TStringList.Create;
try
lines.LoadFromFile('input.txt');
for i := 0 to lines.Count - 1 do begin
New(line);
line^.Text := lines[i];
Queue.Push(line);
end;
 
New(line);
line^.Text := #0;
Queue.Push(line);
 
WaitForSingleObject(DoneEvent, INFINITE);
 
New(line);
line^.Text := IntToStr(PrintThread.Count);
Queue.Push(line);
 
SetEvent(TreminateEvent);
finally
lines.Free;
end;
finally
CloseHandle(TreminateEvent);
CloseHandle(DoneEvent)
end;
 
Readln;
finally
Queue.Free;
end;
end.
 

E[edit]

def printer := {
var count := 0
def printer {
to run(item) {
count += 1
println(item)
}
to getCount() {
return count
}
}
}
 
def sender(lines) {
switch (lines) {
match [] {
when (def count := printer <- getCount()) -> {
println(`$count lines were printed.`)
}
}
match [line] + rest {
when (printer <- run(line)) -> {
sender(rest)
}
}
}
}
 
# Stream IO in E is not finished yet, so this example just uses a list.
sender(<file:input.txt>.getText().split("\n"))

EchoLisp[edit]

This task uses two processes, reader/writer, synchronized by a semaphore S and its queue of messages. The reader sends write or reader-count-please messages. The writer responds with ack or count messages.

 
(require 'sequences)
(require 'tasks)
 
;; inter-tasks message : (op-code . data)
(define (is-message? op message)
(and message (equal? op (first message))))
 
;; reader task
(define (reader infile )
(wait S)
(define message (semaphore-pop S))
(when (is-message? 'count message ) (writeln 'reader-> message) (task-stop-all))
 
(if (first infile) ;; not EOF
(set! message (cons 'write (next infile)))
(set! message (list 'reader-count-please)))
 
(semaphore-push S message)
(signal S)
infile)
 
(define (writer count)
(wait S)
(define message (semaphore-pop S))
(when (is-message? 'write message )
(writeln (rest message))
(set! count (1+ count))
(set! message (cons 'ack count)))
 
(when (is-message? 'reader-count-please message )
(set! message (cons 'count count)))
(semaphore-push S message)
(signal S)
count)
 
 
Output:
;; input sequence
(define infile null) ;; input file handle
(define (set-infile name text) (set! infile (procrastinator (string-split text "\n"))))
(file->string set-infile "input.txt")

(define S (make-semaphore 1)) ;; semaphore to synchronize
(task-run (make-task reader infile))
(task-run (make-task writer 0))

    #task:id:165:running
    #task:id:166:running
    Well, it's one for the money    
    Two for the show    
    Three to get ready    
    Now go, cat, go    
    But don't you    
    Step on my blue suede shoes    
    You can do anything    
    But stay off of my blue suede shoes    
    Well, you can knock me down    
 [lines skipped ...]
    Blue, suede shoes, baby    
    Blue, blue    
    Blue suede shoes    
    Well, you can do anything    
    But stay off of my blue suede shoes!    
        
    reader->     (count . 53)    

Elixir[edit]

Translation of: Erlang
defmodule RC do
def start do
my_pid = self
pid = spawn( fn -> reader(my_pid, 0) end )
File.open( "input.txt", [:read], fn io ->
process( IO.gets(io, ""), io, pid )
end )
end
 
defp process( :eof, _io, pid ) do
send( pid, :count )
receive do
i -> IO.puts "Count:#{i}"
end
end
defp process( any, io, pid ) do
send( pid, any )
process( IO.gets(io, ""), io, pid )
end
 
defp reader( pid, c ) do
receive do
 :count -> send( pid, c )
any ->
IO.write any
reader( pid, c+1 )
end
end
end
 
RC.start

Erlang[edit]

-module(cc).
 
-export([start/0]).
 
start() ->
My_pid = erlang:self(),
Pid = erlang:spawn( fun() -> reader(My_pid, 0) end ),
{ok, IO } = file:open( "input.txt", [read] ),
process( io:get_line(IO, ""), IO, Pid ),
file:close( IO ).
 
process( eof, _IO, Pid ) ->
Pid ! count,
receive
I -> io:fwrite("Count:~p~n", [I])
end;
process( Any, IO, Pid ) ->
Pid ! Any,
process( io:get_line(IO, ""), IO, Pid ).
 
reader(Pid, C) ->
receive
count -> Pid ! C;
Any ->
io:fwrite("~s", [Any]),
reader(Pid, C+1)
end.

Euphoria[edit]

sequence lines
sequence count
lines = {}
count = {}
 
procedure read(integer fn)
object line
while 1 do
line = gets(fn)
if atom(line) then
exit
else
lines = append(lines, line)
task_yield()
end if
end while
 
lines = append(lines,0)
while length(count) = 0 do
task_yield()
end while
 
printf(1,"Count: %d\n",count[1])
end procedure
 
procedure write(integer fn)
integer n
object line
n = 0
while 1 do
while length(lines) = 0 do
task_yield()
end while
 
line = lines[1]
lines = lines[2..$]
if atom(line) then
exit
else
puts(fn,line)
n += 1
end if
end while
count = append(count,n)
end procedure
 
integer fn
atom reader, writer
constant stdout = 1
fn = open("input.txt","r")
reader = task_create(routine_id("read"),{fn})
writer = task_create(routine_id("write"),{stdout})
task_schedule(writer,1)
task_schedule(reader,1)
 
while length(task_list()) > 1 do
task_yield()
end while

F#[edit]

This code will read lines from the file on one thread, and print them to the console on one or more other threads from the ThreadPool, using a MailboxProcessor for lock-free communication between threads and tracking the line count without the use of mutable state.

 
open System.IO
 
type Msg =
| PrintLine of string
| GetCount of AsyncReplyChannel<int>
 
let printer =
MailboxProcessor.Start(fun inbox ->
let rec loop count =
async {
let! msg = inbox.Receive()
match msg with
| PrintLine(s) ->
printfn "%s" s
return! loop (count + 1)
| GetCount(reply) ->
reply.Reply(count)
return! loop count
}
loop 0
)
 
let reader (printAgent:MailboxProcessor<Msg>) file =
File.ReadLines(file)
|> Seq.iter (fun line -> PrintLine line |> printAgent.Post)
printAgent.PostAndReply(fun reply -> GetCount(reply))
|> printfn "Lines written: %i"
 
reader printer @"c:\temp\input.txt"
 

Go[edit]

package main
 
import (
"bufio"
"fmt"
"log"
"os"
)
 
func main() {
lines := make(chan string)
count := make(chan int)
go func() {
c := 0
for l := range lines {
fmt.Println(l)
c++
}
count <- c
}()
 
f, err := os.Open("input.txt")
if err != nil {
log.Fatal(err)
}
for s := bufio.NewScanner(f); s.Scan(); {
lines <- s.Text()
}
f.Close()
close(lines)
fmt.Println("Number of lines:", <-count)
}

Haskell[edit]

The following Haskell code uses simple MVars for thread communication. While the GHC libraries for concurrency give quite a wide design space for thread communication, I felt that the following was fairly reasonable.

For those who are unaware of MVars, they are essentially mutable cells which may be empty or hold a single value, and which have the following important properties:

  • takeMVar will get the contents of an MVar when it is full, emptying it.
  • takeMVar will block if the MVar is empty, until it has been filled by another thread.
  • putMVar will fill an empty MVar with a given value.
  • putMVar will block until the MVar is empty if it is full.

So MVars are essentially bounded channels which hold a maximum of one element at a time.

The code below defines various signals in terms of takeMVar and putMVar and then passes those to the parts of the code which should be permitted to use them. Note that this way, it is impossible for the reader process to take the current line, for example.

import Control.Concurrent
import Control.Concurrent.MVar
 
main =
do lineVar <- newEmptyMVar
countVar <- newEmptyMVar
 
let takeLine = takeMVar lineVar
putLine = putMVar lineVar . Just
putEOF = putMVar lineVar Nothing
takeCount = takeMVar countVar
putCount = putMVar countVar
 
forkIO $ writer takeLine putCount
reader putLine putEOF takeCount

The reader simply reads the file lazily, applying putLine to each of the lines in turn, which blocks until the writer has taken the line. It then signals that it is finished with putEOF, and then takes the count and prints it.

reader putLine putEOF takeCount =
do ls <- fmap lines (readFile "input.txt")
mapM putLine ls
putEOF
n <- takeCount
print n

The writer gets the lines in a loop with takeLine until it receives Nothing, at which point it uses putCount to tell the reader how many lines there were.

writer takeLine putCount = loop 0
where loop n = do l <- takeLine
case l of
Just x -> do putStrLn x
loop (n+1)
Nothing -> putCount n

Icon and Unicon[edit]

Icon does not support concurrent programming. Co-expressions can be used in Icon to mimic some of the behavior, but they are not truly concurrently executed.

Unicon supports concurrent programming. A Unicon solution is:

procedure main(A)
fName := A[1]|"index.txt"
p := thread produce(fName)
c := thread consume(p)
every wait(p | c)
end
 
procedure produce(fName)
every !open(fName)@>> # drop messages in p's outbox (blocking whenever box is full)
@>> # Signal consumer that p is done
write("count is ",<<@) # block until message in p's inbox
end
 
procedure consume(p)
i := 0
while write(\<<@p) do (i+:=1) # loop until empty message in p's outbox
# (blocking until each message arrives)
i@>>p # put row count into p's inbox
end

Sample output:

->pc
procedure main()
   n := 5134
   x := *n
   write(x)
end
count is 5
->

J[edit]

Implementation:

input=: 1 :0
nlines=: 0
u;._2@fread 'input.txt'
smoutput nlines
)
 
output=: 3 :0
nlines=: nlines+1
smoutput y
)

Usage:

   output input

Note that we are not using OS threads here, but instead - as specified by this task - are using two concurrent activities which share data synchronously.

Java[edit]

import java.io.BufferedReader;
import java.io.FileReader;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
 
class SynchronousConcurrency
{
public static void main(String[] args) throws Exception
{
final AtomicLong lineCount = new AtomicLong(0);
final BlockingQueue<String> queue = new LinkedBlockingQueue<String>();
final String EOF = new String();
 
final Thread writerThread = new Thread(new Runnable() {
public void run()
{
long linesWrote = 0;
while (true)
{
try
{
String line = queue.take();
// Reference equality
if (line == EOF)
break;
System.out.println(line);
linesWrote++;
}
catch (InterruptedException ie)
{ }
}
lineCount.set(linesWrote);
}
}
);
writerThread.start();
 
// No need to start a third thread for the reader, just use this thread
BufferedReader br = new BufferedReader(new FileReader("input.txt"));
String line;
while ((line = br.readLine()) != null)
queue.put(line);
br.close();
queue.put(EOF);
writerThread.join();
// AtomicLong is not needed here due to memory barrier created by thread join, but still need a mutable long since lineCount must be final to access it from an anonymous class
System.out.println("Line count: " + lineCount.get());
return;
}
}
 

Logtalk[edit]

The following example can be found in the Logtalk distribution and is used here with permission. The "input.txt" file contains the terms "a(0)..a(9)", one per line. Works when using SWI-Prolog, XSB, or YAP as the backend compiler.

 
:- object(team).
 
:- threaded.
 
:- public(start/0).
start :-
threaded((
reader,
writer(0)
)).
 
reader :-
open('input.txt', read, Stream),
repeat,
read_term(Stream, Term, []),
threaded_notify(term(Term)),
Term == end_of_file,
!,
close(Stream),
threaded_wait(lines(Lines)),
write('Number of lines: '), write(Lines), nl.
 
writer(N0) :-
threaded_wait(term(Term)),
( Term == end_of_file ->
threaded_notify(lines(N0))
; N is N0 + 1,
write(Term), nl,
writer(N)
).
 
:- end_object.
 

Output:

 
| ?- ?- team::start.
a(0)
a(1)
a(2)
a(3)
a(4)
a(5)
a(6)
a(7)
a(8)
a(9)
Number of lines: 10
true.
 

Lua[edit]

function ReadFile()
local fp = io.open( "input.txt" )
assert( fp ~= nil )
 
for line in fp:lines() do
coroutine.yield( line )
end
 
fp:close()
end
 
co = coroutine.create( ReadFile )
 
while true do
local status, val = coroutine.resume( co )
if coroutine.status( co ) == "dead" then break end
print( val )
end
 

Mercury[edit]

:- module synchronous_concurrency.
:- interface.
:- import_module io.
 
:- pred main(io::di, io::uo) is cc_multi.
 
:- implementation.
:- import_module int, list, string, thread, thread.channel, thread.mvar.
 
:- type line_or_stop
---> line(string)
 ; stop.
 
main(!IO) :-
io.open_input("input.txt", Res, !IO),
(
Res = ok(Input),
channel.init(Channel, !IO),
mvar.init(MVar, !IO),
thread.spawn(writer(Channel, MVar, 0), !IO),
reader(Input, Channel, MVar, !IO)
 ;
Res = error(Err),
io.format("Error opening file: %s\n", [s(io.error_message(Err))], !IO)
).
 
:- pred reader(io.text_input_stream::in, channel(line_or_stop)::in, mvar(int)::in,
io::di, io::uo) is det.
 
reader(Input, Channel, MVar, !IO) :-
io.read_line_as_string(Input, Res, !IO),
(
Res = ok(Line),
channel.put(Channel, line(Line), !IO),
reader(Input, Channel, MVar, !IO)
 ;
Res = eof,
channel.put(Channel, stop, !IO),
mvar.take(MVar, Count, !IO),
io.format("%d lines printed.\n", [i(Count)], !IO)
 ;
Res = error(Err),
channel.put(Channel, stop, !IO),
io.format("Error reading file: %s\n", [s(io.error_message(Err))], !IO)
).
 
:- pred writer(channel(line_or_stop)::in, mvar(int)::in, int::in,
io::di, io::uo) is cc_multi.
 
writer(Channel, MVar, Count, !IO) :-
channel.take(Channel, LineOrStop, !IO),
(
LineOrStop = line(Line),
io.write_string(Line, !IO),
writer(Channel, MVar, Count + 1, !IO)
 ;
LineOrStop = stop,
mvar.put(MVar, Count, !IO)
).

Nim[edit]

Compile with nim --threads:on c sync.nim:

Translation of: Python

Two threads are spun up that communicate over a channel.

var msgs: Channel[string]
var count: Channel[int]
 
const FILE = "input.txt"
 
proc read() {.thread.} =
var file = open(FILE)
for line in file.lines:
msgs.send(line)
msgs.send(nil)
file.close()
echo count.recv()
count.close()
 
proc print() {.thread.} =
var n = 0
while true:
var msg = msgs.recv()
if msg == nil:
break
echo msg
n += 1
msgs.close()
count.send(n)
 
var reader_thread = Thread[void]()
var printer_thread = Thread[void]()
 
msgs.open()
count.open()
createThread(reader_thread, read)
createThread(printer_thread, print)
joinThreads(reader_thread, printer_thread)

OCaml[edit]

Using only the standard library[edit]

We use the built-in Event module to provide communication channels between threads.

open Event

The reader is a simple loop. It starts by opening a file, then reads lines from that file until there is nothing left to read. After each line, it sends Some v on channel lines_dest, where v is the contents of the line. Once there are no lines anymore, exception End_of_file is raised and we send None on that same channel. After that, it's just the matter of waiting for one message on count_source, closing the file and printing the result:

let reader count_source lines_dest =
let file = open_in "input.txt" in
let rec aux () =
let line = try Some (input_line file)
with End_of_file -> None in
sync (send lines_dest line);
match line with
| Some _ -> aux ()
| None -> let printed = sync (receive count_source) in
Printf.printf "The task wrote %i strings\n" printed;
close_in file
in aux ()

The printer is also a simple loop. It keeps receiving messages on lines_source. If a message has structure Some v, then v is a line, print it and increment the counter. Otherwise, the message has structure None, which means that we're done, just send the number of lines on count_dest:

let printer lines_source count_target =
let rec aux i =
match sync (receive lines_source) with
| Some line -> print_endline line; aux ( i + 1 )
| None -> sync (send count_target i)
in aux 0

Finally, our main program creates both communication channels and backgrounds treatment of printer:

let _ =
let count = new_channel ()
and lines = new_channel ()
in
let _ = Thread.create (printer lines) count
in reader count lines

Note that, had we decided to background treatment of reader instead, an additional synchronization would have been necessary to prevent the program from leaving once the main thread is over.

Oforth[edit]

Oforth implements concurrency with tasks and communication between tasks using channels. Here, we create 2 channels (one for print, one to retrieve the number of lines).

When the file has been read and sent to channel chPrint, this channel is closed. All tasks waiting for an object in this channel are released and receive null. Then printing task returns number of printed lines into chCount channel for the main task.

import: parallel
 
: printing(chPrint, chCount)
0 while( chPrint receive dup notNull ) [ println 1+ ] drop
chCount send drop ;
 
: concurrentPrint(aFileName)
| chPrint chCount line |
Channel new ->chPrint
Channel new ->chCount
 
#[ printing(chPrint, chCount) ] &
 
aFileName File new forEach: line [ chPrint send(line) drop ]
chPrint close
chCount receive "Number of lines printed : " print println ;

ooRexx[edit]

 
queue = .workqueue~new
input = .stream~new("jabberwocky.txt")
output = .output
 
reader = .filereader~new(input, queue)
writer = .filewriter~new(output, queue)
 
::class workQueue
::method init
expose queue stopped actionpending
queue = .queue~new
stopped = .false
actionPending = .false
 
-- add an item to the work queue. This is a
-- guarded method, which means this is a synchronized access
::method addItem guarded
expose queue actionPending
use arg item
-- add the item to the queue
queue~queue(item)
-- indicate there's something new. This is a condition variable
-- that any will wake up any thread that's waiting on access. They'll
-- be able to get access once we exit
actionPending = .true
 
-- another method for coordinating access with the other thread. This indicates
-- it is time to shut down
::method stop guarded
expose actionPending stopped
-- indicate this has been stopped and also flip the condition variable to
-- wake up any waiters
stopped = .true
actionPending = .true
 
-- read the next item off of the queue. .nil indicates we've reached
-- the last item on the queue. This is also a guarded method, but we'll use
-- the GUARD ON instruction to wait for work if the queue is currently empty
::method nextItem
expose queue stopped actionPending
-- we might need to loop a little to get an item
do forever
-- if there's something on the queue, pull the front item and return
if \queue~isEmpty then return queue~pull
-- if the other thread says it is done sending is stuff, time to shut down
if stopped then return .nil
-- nothing on the queue, not stopped yet, so release the guard and wait until
-- there's something pending to work on.
guard on when actionPending
end
 
-- one half of the synchronization effort. This will read lines and
-- add them to the work queue. The thread will terminate once we hit end-of-file
::class filereader
::method init
-- accept a generic stream...the data source need not be a file
use arg stream, queue
 
reply -- now multithreaded
 
signal on notready
loop forever
queue~addItem(stream~linein)
end
-- we come here on an EOF condition. Indicate we're done and terminate
-- the thread
notready:
queue~stop
 
-- the other end of this. This class will read lines from a work queue
-- and write it to a stream
::class filewriter
::method init
-- accept a generic stream...the data source need not be a file
use arg stream, queue
 
reply -- now multithreaded
 
loop forever
item = queue~nextItem
-- .nil means last item received
if item == .nil then return
-- write to the stream
stream~lineout(item)
end
 

Oz[edit]

declare
%% Helper function to read a file lazily.
%% Returns a lazy list of lines.
fun {ReadLines FN}
F = {New class $ from Open.file Open.text end init(name:FN)}
fun lazy {ReadNext}
case {F getS($)} of
false then nil
[] Line then
Line|{ReadNext}
end
end
in
%% close file when handle becomes unreachable
{Finalize.register F proc {$ F} {F close} end}
{ReadNext}
end
 
Count %% Will receive the number of lines
PrinterPort
in
%% Printer thread
thread
Stream
Counter = {NewCell 0} %% mutable variable
in
PrinterPort = {NewPort ?Stream}
for Line in Stream do
case Line of eof then
Count = @Counter
else
{System.showInfo Line}
Counter := @Counter + 1
end
end
end
 
%% Send all lines to printer thread; make sure that eof is sent.
try
for Line in {ReadLines "input.txt"} do
{Send PrinterPort Line}
end
finally
{Send PrinterPort eof}
end
 
%% Sync on Count and print its value.
{Wait Count}
{Show Count}

Perl[edit]

use threads;
use Thread::Queue qw();
 
my $q1 = Thread::Queue->new;
my $q2 = Thread::Queue->new;
 
my $reader = threads->create(sub {
my $q1 = shift;
my $q2 = shift;
 
open my $fh, '<', 'input.txt';
$q1->enqueue($_) while <$fh>;
close $fh;
$q1->enqueue(undef);
 
print $q2->dequeue;
}, $q1, $q2);
 
my $printer = threads->create(sub {
my $q1 = shift;
my $q2 = shift;
 
my $count;
while (my $line = $q1->dequeue) {
print $line;
$count++;
};
 
$q2->enqueue($count);
}, $q1, $q2);
 
$reader->join;
$printer->join;

Perl 6[edit]

Works with: rakudo version 2013-02-27
sub MAIN ($infile) {
$infile.IO.lines ==> printer() ==> my $count;
say "printed $count lines";
}
 
sub printer(*@lines) {
my $lines;
for @lines {
.say;
++$lines;
}
$lines;
}

Concurrent units are established by use of the feed operator, which works much like an in-process object pipe. Since the final feed goes to a variable declaration that belongs to the outer thread, it serves as a backchannel from the printer thread. In this case the outer thread signals the desire for a line count by terminating the pipe to the printing thread. (Note: soon these will be implemented with real threads in Perl 6, but this is currently emulated with coroutine semantics, aka lazy lists.)

Phix[edit]

Busy wait version (using threads). This program is included in the distribution as demo\rosetta\Synchronous_concurrency.exw, which also contains single step-lock and queue-less versions of this code.

atom frThread,  -- file reader thread
lcThread -- line counter thread
 
sequence queue = {}
integer qlock = init_cs()
 
integer linecount = 1
 
procedure readfile()
object line
integer fn = open("input.txt","r")
while 1 do
line = gets(fn)
enter_cs(qlock)
queue = append(queue,line)
line = atom(line) -- kill refcount!
leave_cs(qlock)
if line then exit end if
end while
close(fn)
wait_thread(lcThread)
printf(1,"Lines read: %d\n",linecount)
exit_thread(0)
end procedure
 
procedure countlines()
object line
linecount = 0
while 1 do
enter_cs(qlock)
if length(queue)=0 then
leave_cs(qlock)
-- sleep(0.1)
else
line = queue[1]
queue = queue[2..$]
leave_cs(qlock)
if atom(line) then exit end if
--  ?line
linecount += 1
end if
end while
exit_thread(0)
end procedure
 
frThread = create_thread(routine_id("readfile"),{})
lcThread = create_thread(routine_id("countlines"),{})
 
wait_thread(frThread)
puts(1,"done")
{} = wait_key()

PicoLisp[edit]

PicoLisp has no threads, but synchronous background tasks and asynchronous signal handlers, or coroutines.

Using background tasks and signals[edit]

The following two tasks communicate via UDP, so in fact they don't need to run within the same process and not even the same machine. "input.txt" would rather be a device (like a named pipe or socket) than a plain file.

# Reading task (synchronous)
(task (open "input.txt")
(let Fd @
(if (in Fd (line T)) # More lines?
(udp "localhost" 4444 @) # Yes: Send next line
(task (port T 4445) # Else install handler
(prinl (udp @) " lines") # to receive and print count
(task (close @)) )
(udp "localhost" 4444 T) # Send 'T' for "Done"
(task (close Fd)) ) ) ) # Stop the task
 
# Printing task (asynchronous)
(sigio (setq "Sock" (port T 4444))
(job '((Cnt . 0))
(let? X (udp "Sock")
(if (=T X) # Done?
(prog
(udp "localhost" 4445 Cnt) # Yes: Send count
(sigio (close "Sock")) ) # and stop the task
(println X) # Else print line to stdout
(inc 'Cnt) ) ) ) ) # and increment count

If the two cases of 'sigio' in the printing task are replaced with 'task', that task would also be synchronous. The resulting behavior is the same.

Using coroutines[edit]

Coroutines are available only in the 64-bit version.

(co 'unit1
(yield) # Allow 'unit2' to start
(in "input.txt" # Read the file
(while (line T) # Send each line
(yield @ 'unit2) ) ) # to 'unit2'
(prinl
(yield NIL 'unit2) # Send 'NIL' for "Done", receive count
" lines" ) )
 
(co 'unit2
(let Cnt 0 # Init counter
(while (yield NIL 'unit1) # Receive line
(println @) # Print it
(inc 'Cnt) ) # Increment count
(yield Cnt 'unit1) ) ) # Send count to 'unit1'

PureBasic[edit]

PureBasic uses Semaphores and Mutex's to coordinate threads.

Enumeration 
#Write
#Done
EndEnumeration
 
Structure commblock
txtline.s
Order.i
EndStructure
 
Global MessageSent=CreateSemaphore()
Global LineWritten=CreateSemaphore()
Global LinesWritten, com.commblock
 
Procedure Writer(arg)
Repeat
WaitSemaphore(MessageSent)
If com\Order=#Write
PrintN(com\txtline)
LinesWritten+1
EndIf
SignalSemaphore(LineWritten)
Until com\Order=#Done
EndProcedure
 
Procedure Reader(arg)
Protected File=ReadFile(#PB_Any,OpenFileRequester("","input.txt","",0))
While file And Not Eof(file)
com\txtline=ReadString(File)
com\Order=#Write
SignalSemaphore(MessageSent)
WaitSemaphore(LineWritten)
Wend
com\Order=#Done
SignalSemaphore(MessageSent)
WaitSemaphore(LineWritten)
PrintN(Str(LinesWritten)+" lines written.")
EndProcedure
 
If OpenConsole()
Define Thread1=CreateThread(@Reader(),0)
Define Thread2=CreateThread(@Writer(),0)
WaitThread(Thread1) And WaitThread(Thread2)
Print("Press Enter to exit"):Input()
EndIf

Python[edit]

Notes: instead of hardcoding the input and output files in the units, each unit is created with a file and read or write the given file.

import sys
from Queue import Queue
from threading import Thread
 
lines = Queue(1)
count = Queue(1)
 
def read(file):
try:
for line in file:
lines.put(line)
finally:
lines.put(None)
print count.get()
 
def write(file):
n = 0
while 1:
line = lines.get()
if line is None:
break
file.write(line)
n += 1
count.put(n)
 
reader = Thread(target=read, args=(open('input.txt'),))
writer = Thread(target=write, args=(sys.stdout,))
reader.start()
writer.start()
reader.join()
writer.join()

Using generators[edit]

Translation of: Ruby
count = 0
 
def reader():
for line in open('input.txt'):
yield line.rstrip()
print('Printed %d lines.' % count)
 
r = reader()
# printer
for line in r:
print(line)
count += 1

Note that the above accesses a variable from both paths of execution. To be more in the spirit of the task, we can actually communicate the count from the printer to the reader:

Works with: Python version 2.5+
def reader():
for line in open('input.txt'):
yield line.rstrip()
count = yield None
print('Printed %d lines.' % count)
 
r = reader()
 
# printer
for count, line in enumerate(r):
if line is None:
break
print(line)
try:
r.send(count)
except StopIteration:
pass

Racket[edit]

 
(define reader->printer-channel (make-channel))
(define printer->reader-channel (make-channel))
 
(define (sync-line-counter filename)
(define (reader)
(define file-port (open-input-file filename))
(let loop ([line (read-line file-port)])
(when (not (eof-object? line))
(begin
(channel-put reader->printer-channel line)
(loop (read-line file-port)))))
(channel-put reader->printer-channel eof)
(let ([num-lines (channel-get printer->reader-channel)])
(printf "Number of lines printed = ~a~%" num-lines)))
 
(define (printer)
(define count 0)
(let loop ([line (channel-get reader->printer-channel)])
(when (not (eof-object? line))
(begin
(printf "~a~%" line)
(set! count (add1 count))
(loop (channel-get reader->printer-channel)))))
(channel-put printer->reader-channel count))
 
(thread reader)
(thread printer))
 
(sync-line-counter "input.txt")
 

Raven[edit]

'input.txt' as src_file
 
class Queue
 
new list as items
condition as ready
 
define item_put
items push ready notify
 
define item_get
items empty if ready wait
items shift
 
Queue as lines
Queue as count
 
thread reader
"file://r:%(src_file)s" open each lines.item_put
NULL lines.item_put count.item_get "reader: %d\n" print
 
thread writer
0 repeat lines.item_get dup while
"writer: %s" print 1+
drop count.item_put
 
reader as r
writer as w

Ruby[edit]

The task is to refactor this program to use two concurrent units.

count = 0
IO.foreach("input.txt") { |line| print line; count += 1 }
puts "Printed #{count} lines."

The above program has no concurrency, because the printer is a block { |line| print line; count += 1 } that can print only one line. (The reader calls block more than one time.) After the refactoring, the printer will print all lines, and the program will jump between the reader and the printer. For the refactoring, Fibers might be best, Continuations work if our Ruby is too old for Fibers, and Threads are another alternative.



Fibers. Ruby 1.9 gives Fiber to us. Fibers provide concurrency. Each fiber has its own stack (to hold nested function calls). A pair of fibers has a boss-vassal relationship: the boss fiber calls vassal.resume to jump to the vassal, and the vassal fiber calls Fiber.yield to jump to the boss.

Works with: Ruby version 1.9
count = 0
reader = Fiber.new do
IO.foreach("input.txt") { |line| Fiber.yield line }
puts "Printed #{count} lines."
nil
end
 
# printer
while line = reader.resume
print line
count += 1
end
  • We create a fiber as the reader, and we use the current fiber as the printer.
  • Our reader is a generator, because reader.resume generates a line, and Fiber.yield passes the generated value.
  • This generator allows Fiber.yield inside nested function calls (because a fiber has its own stack). We nested three calls: we have Fiber.yield inside our line block, inside IO.foreach, inside our fiber block.
  • Because the reader and printer are in the same process, they can share the count variable. An alternate way is to use reader.resume(count) to pass the count, so Fiber.yield would return the count.
  • If IO.foreach raises an IO error, then the reader dies, and reader.resume raises the same error in the printer. This is what we want. If we run this program with no input.txt file, then we see the error.



Continuations. Ruby 1.8 gives Continuation to us. (Ruby 1.9 still gives Continuation if we require 'continuation' from the standard library.) The trick is that you can continue a function call after you leave it. Continuations can provide concurrency. The problem is that continuations make spaghetti code with very confusing control flow.

Library: continuation
require 'continuation' unless defined? Continuation
 
count = 0
reader = proc do |cont|
IO.foreach("input.txt") { |line| cont = callcc { |cc| cont[cc, line] }}
puts "Printed #{count} lines."
cont[nil]
end
 
# printer
while array = callcc { |cc| reader[cc] }
reader, line = array
print line
count += 1
end
  • The above program uses continuations almost like fibers. The reader continues the printer, and the printer continues the reader.
  • The first call to reader[c] uses Proc#[] to start the reader; but later calls to reader[c] use Continuation#[] to continue the reader.
  • The reader and printer share the same stack. The control flow when IO.foreach raises an IO error is very strange. The reader dies, and the original call to Proc#[] raises the same error in the printer.



Threads. Both Ruby 1.8 and Ruby 1.9 give Thread to us. Threads provide preemptive concurrency. The scheduler preempts threads and switches between threads, seemingly at random. Threads seem worse than continuations, because threads have unpredictable control flow, but we can use a Queue to restore some order. We use Thread with Queue.

Library: thread
require 'thread'
 
counts = Queue.new
lines = Queue.new
reader = Thread.new do
begin
File.foreach("input.txt") { |line| lines << line }
lines << :EOF
puts "Printed #{counts.pop} lines."
ensure
lines << nil
end
end
 
# writer
count = 0
while line = lines.pop
case line
when String
print line
count += 1
when :EOF
counts << count
end
end
reader.join
  • We create a thread as the reader, and use the current thread as the writer.
  • If a thread tries to pop an empty queue, then the thread waits until some other thread queues something.
  • The queue of lines can become long; the worst case allows the reader to read the entire file before the printer pops the first line! If you wanted to prevent a long queue, a SizedQueue.new(5) would hold only 5 elements.
  • If IO.reader raises an IO error, then the reader dies. The writer would deadlock on an empty queue after the reader dies. To prevent this deadlock, the reader ensures to queue a final nil before it dies. The writer uses this nil to break its loop and call reader.join. If the reader dies with an IO error, then reader.join raises the same error.

Rust[edit]

Works with: rustc 1.4.0-nightly version f84d53ca0 2015-09-06
use std::fs::File;
use std::io::BufReader;
use std::io::BufRead;
 
use std::thread::spawn;
use std::sync::mpsc::{SyncSender, Receiver, sync_channel};
 
fn main() {
let (tx, rx): (SyncSender<String>, Receiver<String>) = sync_channel::<String>(0);
 
// Reader thread.
spawn(move || {
let file = File::open("input.txt").unwrap();
let reader = BufReader::new(file);
 
for line in reader.lines() {
match line {
Ok(msg) => tx.send(msg).unwrap(),
Err(e) => println!("{}", e)
}
}
 
drop(tx);
});
 
// Writer thread.
spawn(move || {
let mut loop_count: u16 = 0;
 
loop {
let recvd = rx.recv();
 
match recvd {
Ok(msg) => {
println!("{}", msg);
loop_count += 1;
},
Err(_) => break // rx.recv() will only err when tx is closed.
}
}
 
println!("Line count: {}", loop_count);
}).join().unwrap();
}

Scala[edit]

A possible implementation using Actors

case class HowMany(asker: Actor)
 
val printer = actor {
var count = 0
while (true) {
receive {
case line: String =>
print(line); count = count + 1
case HowMany(asker: Actor) => asker ! count; exit()
}
}
}
 
def reader(printer: Actor) {
scala.io.Source.fromFile("c:\\input.txt").getLines foreach { printer ! _ }
printer ! HowMany(
actor {
receive {
case count: Int => println("line count = " + count)
}
})
}
 
reader(printer)

Swift[edit]

Using Grand Central Dispatch and NSNotificationCenter

Reader.swift

//
// Reader.swift
//
 
import Foundation
 
class Reader: NSObject {
let inputPath = "~/Desktop/input.txt".stringByExpandingTildeInPath
var gotNumberOfLines = false
 
override init() {
super.init()
NSNotificationCenter.defaultCenter().addObserver(self, selector: "linesPrinted:",
name: "LinesPrinted", object: nil)
}
 
deinit {
NSNotificationCenter.defaultCenter().removeObserver(self)
}
 
// Selector for the number of lines printed
func linesPrinted(not:NSNotification) {
println(not.object!)
self.gotNumberOfLines = true
exit(0)
}
 
func readFile() {
var err:NSError?
let fileString = NSString(contentsOfFile: self.inputPath,
encoding: NSUTF8StringEncoding, error: &err)
 
if let lines = fileString?.componentsSeparatedByString("\n") {
for line in lines {
NSNotificationCenter.defaultCenter().postNotificationName("Line", object: line)
}
NSNotificationCenter.defaultCenter().postNotificationName("LineNumberRequest", object: nil)
 
while !self.gotNumberOfLines {
sleep(1 as UInt32)
}
}
}
}

Printer.swift

//
// Printer.swift
//
 
import Foundation
 
class Printer: NSObject {
var numberOfLines = 0
var gotRequestLineNumber = false
 
override init() {
super.init()
NSNotificationCenter.defaultCenter().addObserver(self, selector: "gotLine:",
name: "Line", object: nil)
NSNotificationCenter.defaultCenter().addObserver(self, selector: "lineNumberRequest:",
name: "LineNumberRequest", object: nil)
}
 
deinit {
NSNotificationCenter.defaultCenter().removeObserver(self)
}
 
func gotLine(not:NSNotification) {
println(not.object!)
self.numberOfLines++
}
 
func lineNumberRequest(not:NSNotification) {
self.gotRequestLineNumber = true
NSNotificationCenter.defaultCenter().postNotificationName("LinesPrinted", object: self.numberOfLines)
}
 
func waitForLines() {
while !self.gotRequestLineNumber {
sleep(1 as UInt32)
}
}
}

main.swift

//
// main.swift
//
 
import Foundation
 
dispatch_async(dispatch_get_global_queue(0, 0)) {
let printer = Printer()
printer.waitForLines()
}
 
dispatch_async(dispatch_get_global_queue(0, 0)) {
let reader = Reader()
reader.readFile()
}
 
CFRunLoopRun()
 

SystemVerilog[edit]

program main;
 
mailbox#(bit) p2c_cmd = new;
mailbox#(string) p2c_data = new;
mailbox#(int) c2p_data = new;
 
initial begin
int fh = $fopen("input.txt", "r");
string line;
int count;
while ($fgets(line, fh)) begin
p2c_cmd.put(0);
p2c_data.put(line);
end
p2c_cmd.put(1);
c2p_data.get(count);
$display( "COUNT: %0d", count );
end
 
initial begin
bit done;
int count;
while (!done) begin
p2c_cmd.get(done);
if (done) begin
c2p_data.put(count);
end
else begin
string line;
p2c_data.get(line);
$display( "LINE: %s", line);
count++;
end
end
end
 
endprogram

Tcl[edit]

Uses the Thread package.

package require Thread
 
# Define the input thread
set input [thread::create {
proc readFile {filename receiver} {
set f [open $filename]
while {[gets $f line] >= 0} {
thread::send $receiver [list line $line]
}
close $f
thread::send $receiver lineCount lines
puts "got $lines lines"
}
thread::wait
}]
# Define the output thread
set output [thread::create {
set lines 0
proc line {string} {
puts $string
incr ::lines
}
proc lineCount {} {return $::lines}
thread::wait
}]
 
# Connect everything together and start the processing
thread::send $input [list readFile "input.txt" $output]

TXR[edit]

Using delimited-continuation-based obtain and yield-from to simulate co-routines, wrapped in some OOP. A thread base class is derived into consumer and producer, both of which provide run methods. The consumer has a counter also, and producer holds a reference to a consumer.

When the objects are instantiated, their co-routines auto-start, thanks to the :postinit hook.

To get things going, we resume the producer via pro.(resume), because we started that in a suspended state. This is actually not necessary; if we remove the suspended t from the new expression which instantiates the producer, we can remove this line. However, this means that the body of the let doesn't receive control. The producer finishes producing and then the pro variable is bound, and the final (put-line ...) expression evaluates. Starting the producer suspended lets us insert some logic prior to dispatching the producer. We implicitly start the consumer, though, because it immediately suspends to wait for an item, saving its context in a continuation and relinquishing control.

(defstruct thread nil
suspended
cont
(:method resume (self)
[self.cont])
(:method give (self item)
[self.cont item])
(:method get (self)
(yield-from run nil))
(:method start (self)
(set self.cont (obtain self.(run)))
(unless self.suspended
self.(resume)))
(:postinit (self)
self.(start)))
 
(defstruct consumer thread
(count 0)
(:method run (self)
(whilet ((item self.(get)))
(prinl item)
(inc self.count))))
 
(defstruct producer thread
consumer
(:method run (self)
(whilet ((line (get-line)))
self.consumer.(give line))))
 
(let* ((con (new consumer))
(pro (new producer suspended t consumer con)))
pro.(resume)
(put-line `count = @{con.count}`))

UnixPipes[edit]

This task has been flagged for clarification. Code on this page in its current state may be flagged incorrect once this task has been clarified. See this page's Talk page for discussion.

the main process is the one started by cat file. The subshell created by >(...) is the secondary process the main pipeline waits for the secondary process to finish, collects and prints the count. This falls a-foul of the requirement that fork should not be used as every thing in pipes is done using forks.

rm -f node ; mkfifo node
cat file | tee >(wc -l > node ) | cat - node

Visual Basic .NET[edit]

This can be improved by adding a blocking Dequeue instead of spinning on TryDequeue.

Imports System.Threading
 
Module Module1
 
Sub Main()
Dim t1 As New Thread(AddressOf Reader)
Dim t2 As New Thread(AddressOf Writer)
t1.Start()
t2.Start()
t1.Join()
t2.Join()
End Sub
 
Sub Reader()
For Each line In IO.File.ReadAllLines("input.txt")
m_WriterQueue.Enqueue(line)
Next
m_WriterQueue.Enqueue(Nothing)
 
Dim result As Integer
Do Until m_ReaderQueue.TryDequeue(result)
Thread.Sleep(10)
Loop
 
Console.WriteLine(result)
 
End Sub
 
Sub Writer()
Dim count = 0
Dim line As String = Nothing
Do
Do Until m_WriterQueue.TryDequeue(line)
Thread.Sleep(10)
Loop
If line IsNot Nothing Then
Console.WriteLine(line)
count += 1
Else
m_ReaderQueue.Enqueue(count)
Exit Do
End If
Loop
End Sub
 
Private m_WriterQueue As New SafeQueue(Of String)
Private m_ReaderQueue As New SafeQueue(Of Integer)
 
End Module
 
Class SafeQueue(Of T)
Private m_list As New Queue(Of T)
Public Function TryDequeue(ByRef result As T) As Boolean
SyncLock m_list
If m_list.Count = 0 Then Return False
result = m_list.Dequeue
Return True
End SyncLock
End Function
Public Sub Enqueue(ByVal value As T)
SyncLock m_list
m_list.Enqueue(value)
End SyncLock
End Sub
End Class

zkl[edit]

fcn reader(fileName,out){
n:=0; foreach line in (File(fileName)) { out.write(line); n+=1; }
out.close(); // signal done
Atomic.waitFor(out.Property("isOpen")); // wait for other thread to reopen Pipe
out.write(n);
}
fcn writer(in){
Utils.zipWith(fcn(n,line){ "%3d: %s".fmt(n,line).print() },[1..],in);
in.open(); // signal other thread to send num lines read
println("Other thread read ",in.read()," lines");
}
 
p:=Thread.Pipe(); // NOT Unix pipes, thread safe channel between threads
reader.launch("input.txt",p);
writer.future(p).noop(); // noop forces eval, ie sleep until writer finished

Light on error handling, easily fixed.

Output:

input.txt is just a code file

  1: to := Thread.Pipe(); from := Thread.Pipe();
  2: a:=fcn(to,from){
 ...
 15: }.launch(to,from);
 16: 
 17: a.noop();
Other thread read 17 lines