Fork

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

In this task, the goal is to spawn a new process which can run simultaneously with, and independently of, the original parent process.

ALGOL 68

Translation of: C

Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9 - "fork" is not part of the standard's prelude.

<lang algol68>main: (

 INT pid;
 IF (pid:=fork)=0 THEN
   print("This is new process")
 ELIF pid>0 THEN
   print("This is the original process")
 ELSE
   print("ERROR: Something went wrong")
 FI

)</lang> Output:

This is new process
This is the original process

AutoHotkey

This example is untested. Please check that it's correct, debug it as necessary, and remove this message.

<lang AutoHotkey>instancenum = %1%+1 MsgBox, 4, Fork Process, %instancenum% number: run another? IfMsgBox, Yes

  Run, %A_ScriptFullName% %instancenum%

ExitApp</lang>

C

Library: POSIX

<lang c>#include <stdio.h>

1. include <unistd.h>

int main() {

 pid_t pid;

 if( ( pid = fork() ) == 0) {
   printf("This is new process\n");
 } else if (pid > 0) {
   printf("This is the original process\n");
 } else {
   printf("ERROR: Something went wrong\n");
 }

 return 0;

}</lang>

C++

Translation of: C

Library: POSIX

<lang cpp>#include<iostream>

1. include<unistd.h>

int main() {

 pid_t pid = fork();

 if (pid == 0)
 {
   std::cout << "This is the new process\n";
 }
 else if (pid > 0)
 {
   std::cout << "This is the original process\n";
 }
 else
 {
   std::cerr << "ERROR: Something went wrong\n";
 }

 return 0;

}</lang>

Clojure

This example is incorrect. It does not accomplish the given task. Please fix the code and remove this message.

In Clojure, a computation can be done asynchronously, with Clojure managing the thread pool, by creating an agent with an initial step. Computational requests are sent using 'send', and the agent can be dereferenced to obtain its current state. To make sure the computation has finished, await can be used, or await-for for a bounded wait.

<lang lisp>user=> (def child (agent (iterate inc 1)))

1. 'user/child

user=> (send child #(reduce + (take 1000 %)))

1. <Agent@215f7107: 500500>

user=> (await child) nil user=> @child 500500</lang>

Common Lisp

There's not a standard way to fork, but some implementations have built-in bindings for POSIX fork.

Translation of: C

Works with: SBCL

<lang lisp>(let ((pid (sb-posix:fork)))

 (cond
  ((zerop pid) (write-line "This is the new process."))
  ((plusp pid) (write-line "This is the original process."))
  (t           (error "Something went wrong while forking."))))</lang>

Erlang

<lang erlang>-module(fork). -export([start/0]).

start() ->

   spawn(fork,child,[]),
   io:format("This is the original process~n").

child() ->

   io:format("This is the new process~n").</lang>

Then you can compile your code and execute it:

<lang erlang>c(fork). fork:start().</lang>

Haskell

This example is incorrect. It does not accomplish the given task. Please fix the code and remove this message.

<lang haskell>import Control.Concurrent

main = do

 forkIO (putStrLn "This is the new process")
 putStrLn "This is the original process"</lang>

OCaml

<lang ocaml>#load "unix.cma";; let pid = Unix.fork ();; if pid > 0 then

 print_endline "This is the original process"

else

 print_endline "This is the new process";;</lang>

Perl

Works with: Perl version 5.x

In the child code, you may have to re-open database handles and such.

<lang perl>FORK: if ($pid = fork()) {

   # parent code

} elsif (defined($pid)) {

   setsid; # tells apache to let go of this process and let it run solo
   # disconnect ourselves from input, output, and errors
   close(STDOUT);
   close(STDIN);
   close(STDERR);    
   # re-open to /dev/null to prevent irrelevant warn messages.
   open(STDOUT, '>/dev/null');
   open(STDIN, '>/dev/null');
   open(STDERR, '>>/home/virtual/logs/err.log');
   
   # child code
   
   exit; # important to exit

} elsif($! =~ /emporar/){

   warn '[' . localtime() . "] Failed to Fork - Will try again in 10 seconds.\n";
   sleep(10);
   goto FORK;

} else {

   warn '[' . localtime() . "] Unable to fork - $!";
   exit(0);

}</lang>

Obviously you could do a Fork in a lot less lines, but this code covers all the bases

PHP

Translation of: C

<lang php><?php $pid = pcntl_fork(); if ($pid == 0)

 echo "This is the new process\n";

else if ($pid > 0)

 echo "This is the original process\n";

else

 echo "ERROR: Something went wrong\n";

?></lang>

Pop11

<lang pop11>lvars ress; if sys_fork(false) ->> ress then

  ;;; parent
  printf(ress, 'Child pid = %p\n');

else

  printf('In child\n');

endif;</lang>

Python

Works with: Python version 2.5

<lang python>import os

pid = os.fork() if pid > 0:

# parent code

else:

# child code</lang>

Ruby

<lang ruby>pid = fork if pid

# parent code

else

# child code

end</lang> or <lang ruby>fork do

 # child code

end

1. parent code</lang>

Slate

The following built-in method uses the cloneSystem primitive (which calls fork()) to fork code. The parent and the child both get a socket from a socketpair which they can use to communicate. The cloneSystem is currently unimplemented on windows (since there isn't a fork() system call).

<lang slate>p@(Process traits) forkAndDo: b [ | ret |

 ret: (lobby cloneSystem).
 ret first ifTrue: [p pipes addLast: ret second. ret second]
           ifFalse: [[p pipes clear. p pipes addLast: ret second. b applyWith: ret second] ensure: [lobby quit]]

].</lang>

Smalltalk

<lang smalltalk>'Here I am' displayNl. |a| a := [

 (Delay forSeconds: 2) wait . 
 1 to: 100 do: [ :i | i displayNl ]

] fork. 'Child will start after 2 seconds' displayNl. "wait to avoid terminating first the parent;

a better way should use semaphores"

(Delay forSeconds: 10) wait.</lang>

Standard ML

<lang sml>case Posix.Process.fork () of

  SOME pid => print "This is the original process\n"
| NONE     => print "This is the new process\n";</lang>

Tcl

(from the Tcl Wiki)

Fork is one of the primitives used for process creation in Unixy systems. It creates a copy of the process that calls it, and the only difference in internal state between the original and the copy is in the return value from the fork call (0 in the copy, but the pid of the copy in the parent).

The

Library: Expect

package includes a fork. So does the

Library: TclX

package.

Example:

<lang tcl>package require Expect

1. or

package require Tclx

for {set i 0} {$i < 100} {incr i} {

   set pid [fork]
   switch $pid {
       -1 {
           puts "Fork attempt #$i failed."
       }
       0 {
           puts "I am child process #$i."
           exit
       }
       default {
           puts "The parent just spawned child process #$i."
       }
   }

}</lang>

In most cases though, one is not interested in spawning a copy of the process one already has, but rather wants a different process. When using POSIX APIs, this has to be done by first forking and then having the child use the exec system call to replace itself with a different program. The Tcl exec command does this fork&exec combination — in part because non-Unix OSs typicallly don't have "make a copy of parent process" as an intermediate step when spawning new processes.

Note that fork is only supported at all on unthreaded builds of Tcl. This is because the POSIX threads library does not sit well with the fork() system call.

Toka

<lang toka>needs shell getpid is-data PID [ fork getpid PID = [ ." Child PID: " . cr ] [ ." In child\n" ] ifTrueFalse ] invoke</lang>

UnixPipes

Demonstrating a subshell getting forked, and running concurrently with the original process

<lang bash>(echo "Process 1" >&2 ;sleep 5; echo "1 done" ) | (echo "Process 2";cat;echo "2 done")</lang>