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Last updated: Thu, 21 Aug 2003

LXXXIII. Process Control Functions


Process Control support in PHP implements the Unix style of process creation, program execution, signal handling and process termination. Process Control should not be enabled within a webserver environment and unexpected results may happen if any Process Control functions are used within a webserver environment.

This documentation is intended to explain the general usage of each of the Process Control functions. For detailed information about Unix process control you are encouraged to consult your systems documentation including fork(2), waitpid(2) and signal(2) or a comprehensive reference such as Advanced Programming in the UNIX Environment by W. Richard Stevens (Addison-Wesley).

PCNTL now uses ticks as the signal handle callback mechanism, which is much faster than the previous mechanism. This change follows the same semantics as using "user ticks". You use the declare() statement to specify the locations in your program where callbacks are allowed to occur. This allows you to minimize the overhead of handling asynchronous events. In the past, compiling PHP with pcntl enabled would always incur this overhead, whether or not your script actually used pcntl.

There is one adjustment that all pcntl scripts prior to PHP 4.3.0 must make for them to work which is to either to use declare() on a section where you wish to allow callbacks or to just enable it across the entire script using the new global syntax of declare().

Note: This extension is not available on Windows platforms.


No external libraries are needed to build this extension.


Process Control support in PHP is not enabled by default. You have to compile the CGI or CLI version of PHP with --enable-pcntl configuration option when compiling PHP to enable Process Control support.

Note: Currently, this module will not function on non-Unix platforms (Windows).

Runtime Configuration

This extension has no configuration directives defined in php.ini.

Resource Types

This extension has no resource types defined.

Predefined Constants

The following list of signals are supported by the Process Control functions. Please see your systems signal(7) man page for details of the default behavior of these signals.

WNOHANG (integer)

WUNTRACED (integer)

SIG_IGN (integer)

SIG_DFL (integer)

SIG_ERR (integer)

SIGHUP (integer)

SIGINT (integer)

SIGQUIT (integer)

SIGILL (integer)

SIGTRAP (integer)

SIGABRT (integer)

SIGIOT (integer)

SIGBUS (integer)

SIGFPE (integer)

SIGKILL (integer)

SIGUSR1 (integer)

SIGSEGV (integer)

SIGUSR2 (integer)

SIGPIPE (integer)

SIGALRM (integer)

SIGTERM (integer)

SIGSTKFLT (integer)

SIGCLD (integer)

SIGCHLD (integer)

SIGCONT (integer)

SIGSTOP (integer)

SIGTSTP (integer)

SIGTTIN (integer)

SIGTTOU (integer)

SIGURG (integer)

SIGXCPU (integer)

SIGXFSZ (integer)

SIGVTALRM (integer)

SIGPROF (integer)

SIGWINCH (integer)

SIGPOLL (integer)

SIGIO (integer)

SIGPWR (integer)

SIGSYS (integer)

SIGBABY (integer)


This example forks off a daemon process with a signal handler.

Example 1. Process Control Example


$pid = pcntl_fork();
if ($pid == -1) {
     die("could not fork"); 
} else if ($pid) {
     exit(); // we are the parent 
} else {
     // we are the child

// detatch from the controlling terminal
if (!posix_setsid()) {
    die("could not detach from terminal");

// setup signal handlers
pcntl_signal(SIGTERM, "sig_handler");
pcntl_signal(SIGHUP, "sig_handler");

// loop forever performing tasks
while(1) {

    // do something interesting here


function sig_handler($signo) {

     switch($signo) {
         case SIGTERM:
             // handle shutdown tasks
         case SIGHUP:
             // handle restart tasks
             // handle all other signals



See Also

A look at the section about POSIX functions may be useful.

Table of Contents
pcntl_exec --  Executes specified program in current process space
pcntl_fork -- Forks the currently running process
pcntl_signal -- Installs a signal handler
pcntl_waitpid -- Waits on or returns the status of a forked child
pcntl_wexitstatus --  Returns the return code of a terminated child
pcntl_wifexited --  Returns TRUE if status code represents a successful exit
pcntl_wifsignaled --  Returns TRUE if status code represents a termination due to a signal
pcntl_wifstopped --  Returns TRUE if child process is currently stopped
pcntl_wstopsig --  Returns the signal which caused the child to stop
pcntl_wtermsig --  Returns the signal which caused the child to terminate

add a note add a note User Contributed Notes
Process Control Functions
jeremy at nirvani dot net
16-Oct-2003 10:54
#!/usr/local/bin/php -q

# Jeremy Brand <>

# ./configure --enable-pcntl --enable-sigchild
# make
# make install

# This code example shows how to use a script to do multiprocessing.  Each time
# this script is ran, the result is 5 (in this example) processes running to
# accomplish a specified task.

# Examples could be a messaging queue.  You could get the number of messages in
# a queue and handle any or all of them asynchronously.

# Get the number of children you want to be born by running this
# script once.
$children 5# likely a function call here.

for ($i=1$i<=$children$i++)

$pid pcntl_fork();
  if (
$pid == -1)
"could not fork\n");
  else if (
# If we are the parent, we did our job of giving birth,
    # now lets finish our job and die!
# Since we are the child, fork so that init will become our parent.  Init
    # is good at wait()ing on children - ie: reaping.
$cpid pcntl_fork();
    if (
$cpid == -1)
"could not fork in child process\n");
    if (!
# We have now forked off from our parent and also are not waiting on any
      # other children to process, however other children are running
      # simutaniously to us.  Make sure the code you write here is safe to run
      # in this environment of multiprocessing - ie: proper locking, etc.
      # Write the custom code here that you want to multiprocess.

print "we are child number $i\n";

# Don't forget to exit after the child processing is done.  Certainly
      # change this exit code if you need to however.

David Koopman
15-Sep-2003 03:32
I had a hard time finding a complete example of using PHP as a multi-process (or multi-threaded - I don't understand the difference in these two terms) daemon using connection pooling.  I put pieces of the puzzle together and came up with the program below.  I hope it helps someone.  Notes about making this work:

1) I rebuilt PHP on my machine with these config options:
./configure --enable-sockets --enable-pcntl --enable-sigchild
make install

2) I have problems when tried to handle SIGTERM and SIGHUP myself, so I removed these from my code, don't use them unless you have a special need for this:
pcntl_signal(SIGTERM, "sig_handler");
pcntl_signal(SIGHUP, "sig_handler");

What I do is:
1. start the program, then fork to detach from the terminal (kill the parent and make the child the session leader).

2. bind to address and port and start listening.

3. fork $poolNum times, creating $poolNum children (this is the pool of daemons running.  The children handle the incoming connections).

4. keep the parent process running in a loop, constantly checking to see if it should create a new child.  It will always keep $poolNum spare children ready (as long as the total pooled connections doesn't exceed $maxDaemon).  As connections come in, more children are spawned.

5. When a new connection comes in, it is handed off to the first child.  This child then sends a SIGUSR1 signal back to the parent.  The parent has a signal handler for SIGUSR1, which will increment the $numActive variable by one.  The loop that is running (see 4 above) will note the increment in $numActive and automatically create a new child process to keep the process pool going.

I have to post the code in the next note, the note engine on this site won't allow such a long note to be posted, but I think this code example is well worth a comment on this...
schst at php dot net
01-Sep-2003 09:39
To get rid of the zombies when child processes terminate you do not have to write a lot of code that uses complex stuff like message queues.
Instead you only set a signal handler:

pcntl_signal(SIGCHLD, SIG_IGN);

Patrice Levesque
15-Jun-2003 03:11
So, you want to create multiple child processes and don't want any zombies, don't you?

You can use IPC to achieve just that.  Every child that is spawned has to tell its parent that time has come for him to be terminated.  So, yes, zombies will be created, but once in a while the parent will 'clean up' after his kids.  Code:

declare(ticks 1);
// create a IPC message queue
$msgqueue msg_get_queue(ftok("/tmp/php_msgqueue.stat"'R'),0666 IPC_CREAT);
// loop for 1000 children
for ($c 0$c 1000$c++) {
// fork
$pcid pcntl_fork();
    if (
$pcid == -1) {
"Could not fork!");
    elseif (
$pcid) { // we are the parent, look for zombie kids and terminate 'em
      // look in the IPC message queue if there are any entries
$currentqueue msg_stat_queue($msgqueue);
$n $currentqueue['msg_qnum']; // number of messages (number of kids to terminate)
if ($n 0) {
"There are $n kids to terminate.\n";
        for (
$i 0$i $n$i++) {
// pop the kid's PID from the IPC message queue
if (!msg_receive ($msgqueue1$msg_type16384$msgtrue0$msg_error)) {
"MSG_RECV ERROR: $errmsg \n"// something has gone wrong
          else {
pcntl_waitpid($msg$tmpstat0); // terminate kid for real.
    else { 
// we are the child!
if (!posix_setsid()) { die ("Could not detach"); }; // detach
echo "I am child number $c\n";
sleep(5); // do something useful
      // tell dad I'm finished via IPC: send him my PID
if (!msg_send($msgqueue1posix_getpid(), truetrue$errmsg)) {
"MSG_SEND ERROR: $errmsg \n";
// become a zombie until dad kills me                           
11-Oct-2002 09:53
Forking your PHP daemon will cause it to zombie on exit.

...or so I've seen on:
FreeBSD (PHP4.2.x)
Debian (PHP4.3.0-dev)
Darwin (PHP4.3.0-dev)

This was tested with the example code above and other scripts created for evaluation.

Seems adding --enable-sigchild to your configure will get rid of the problem.

Hope that saves some hair tearing :]
30-May-2002 12:10
You have to use socket_select before socket_accept, so your code will wait for connection with select. socket_select can be interrupted by signals easily. Below is an example from my library (methods of class TNetSocket):
  //-- select
  function select($aread=NULL,$awrite=NULL,$aexcept=NULL,$timeout=NULL)
      $res=socket_select($aread, $awrite, $aexcept, $timeout);

      // if errno===0 it means what select was interrrupted by SysV signal
      if($res===false && socket_last_error($this->socket())!==0)
      { // error occured, interrupted not by a signal

  //-- accept, wait for incomming connection
  function accept()

    if ($this->select($a=&$aread)===false)

    if($child_socket <= 0)
    { // error occured



    if($res <= 0)
    { // error occured

    TLogManager::phpserv("Connection accepted. ADDRESS $this->address, PORT $this->port","net_socket",__FILE__,__LINE__);

    return(true); // return new object of TNetSocket type
christophe dot dirac at swissphone dot ch
27-May-2002 05:42
The signal handler pcntl_signal() does not
work if you are waiting for socket connections
with socket_accept(). The signal handler is
installed, but does not do anything if a signal
arrives. Anyone got a solution for that?
28-Feb-2002 07:48
This piece of code helped me to find out what signals are being sent to my process:

function sig_identify($signo) {
  switch($signo) {
    case SIGFPE:    return 'SIGFPE';
    case SIGSTOP:   return 'SIGSTOP';
    case SIGHUP:    return 'SIGHUP';
    case SIGINT:    return 'SIGINT';
    case SIGQUIT:   return 'SIGQUIT';
    case SIGILL:    return 'SIGILL';
    case SIGTRAP:   return 'SIGTRAP';
    case SIGABRT:   return 'SIGABRT';
    case SIGIOT:    return 'SIGIOT';
    case SIGBUS:    return 'SIGBUS';
    case SIGPOLL:   return 'SIGPOLL';
    case SIGSYS:    return 'SIGSYS';
    case SIGCONT:   return 'SIGCONT';
    case SIGUSR1:   return 'SIGUSR1';
    case SIGUSR2:   return 'SIGUSR2';
    case SIGSEGV:   return 'SIGSEGV';
    case SIGPIPE:   return 'SIGPIPE';
    case SIGALRM:   return 'SIGALRM';
    case SIGTERM:   return 'SIGTERM';
    case SIGSTKFLT: return 'SIGSTKFLT';
    case SIGCHLD:   return 'SIGCHLD';
    case SIGCLD:    return 'SIGCLD';
    case SIGIO:     return 'SIGIO';
    case SIGKILL:   return 'SIGKILL';
    case SIGTSTP:   return 'SIGTSTP';
    case SIGTTIN:   return 'SIGTTIN';
    case SIGTTOU:   return 'SIGTTOU';
    case SIGURG:    return 'SIGURG';
    case SIGXCPU:   return 'SIGXCPU';
    case SIGXFSZ:   return 'SIGXFSZ';
    case SIGVTALRM: return 'SIGVTALRM';
    case SIGPROF:   return 'SIGPROF';
    case SIGWINCH:  return 'SIGWINCH';
    case SIGPWR:    return 'SIGPWR';

function sig_handler($signo) {
  echo "Caught " . sig_identify($signo) . " (" . $signo  . ") on " . posix_getpid() . "\n";

pcntl_signal(SIGFPE, "sig_handler");
pcntl_signal(SIGHUP, "sig_handler");
// pcntl_signal(SIGINT, "sig_handler");
pcntl_signal(SIGQUIT, "sig_handler");
pcntl_signal(SIGILL, "sig_handler");
pcntl_signal(SIGTRAP, "sig_handler");
pcntl_signal(SIGABRT, "sig_handler");
pcntl_signal(SIGIOT, "sig_handler");
pcntl_signal(SIGBUS, "sig_handler");
pcntl_signal(SIGPOLL, "sig_handler");
pcntl_signal(SIGSYS, "sig_handler");
pcntl_signal(SIGCONT, "sig_handler");
pcntl_signal(SIGUSR1, "sig_handler");
pcntl_signal(SIGUSR2, "sig_handler");
pcntl_signal(SIGSEGV, "sig_handler");
pcntl_signal(SIGPIPE, "sig_handler");
pcntl_signal(SIGALRM, "sig_handler");
pcntl_signal(SIGTERM, "sig_handler");
pcntl_signal(SIGSTKFLT, "sig_handler");
pcntl_signal(SIGCHLD, "sig_handler");
pcntl_signal(SIGCLD, "sig_handler");
pcntl_signal(SIGIO, "sig_handler");
pcntl_signal(SIGTSTP, "sig_handler");
pcntl_signal(SIGTTIN, "sig_handler");
pcntl_signal(SIGTTOU, "sig_handler");
pcntl_signal(SIGURG, "sig_handler");
pcntl_signal(SIGXCPU, "sig_handler");
pcntl_signal(SIGXFSZ, "sig_handler");
pcntl_signal(SIGVTALRM, "sig_handler");
pcntl_signal(SIGPROF, "sig_handler");
pcntl_signal(SIGWINCH, "sig_handler");
pcntl_signal(SIGPWR, "sig_handler");

I commented out SIGNIT, as it is the signal which is sent to your process when you press CTRL-C. If you catch this signal, you must handle it properly:

function sig_handler($signo) {
  switch($signo) {
    case SIGINT:
      // customized cleanup code
      exit; // now exit

Otherwise the only possibility to stop your process is by sending a SIGKILL signal - you can do this on the shell by typing "kill -9 PID" (where -9 is the numerical value for SIGKILL).

Note: You cannot add a handler (i.e. ignore signals) for SIGSTOP and SIGKILL - for obvious reasons.
rfrew at mxm dot com
23-Feb-2002 07:59
If you use Zend Optimizer 1.20, it is not compatible with PCNTL.  You'll get an error message stating: '[Zend Optimizer] Zend Optimizer 1.2.0 is incompatible with pcntl 1.0 in Unknown on line 0'.

You get this error message when you compile using the --enable-pcntl option.

 Last updated: Thu, 21 Aug 2003
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