A problem with pcntl_fork

[Thomas]

I'm having some issues with forking and defunct children. This is my code:

#!/usr/bin/php
<?php
$socket = stream_socket_server("tcp://0.0.0.0:8000", $errno, $errstr);

/*
* LISTEN FOR CONNECTIONS
*/
while(TRUE){
while($conn = stream_socket_accept($socket,-1)){
$pid = pcntl_fork();
if($pid==-1){
fwrite(STDOUT,"Fork failed. Exiting!\n");
exit;
}elseif($pid){
/*
* We are parent.
* Close connection
*/
fclose($conn);
}else{
fwrite($conn, "Hello!\n");
fwrite($conn, "I am PID: ".posix_getpid()."\n");
sleep(2);
fclose($conn);
exit;
}
}
}

function sig_handler($signo){
switch ($signo) {
case SIGTERM:
exit;
break;
case SIGCHLD:
exit;
break;
default:
// handle all other signals
}
}

// setup signal handlers
pcntl_signal(SIGTERM, "sig_handler");
pcntl_signal(SIGCHLD, "sig_handler");
?>

When the child exits (after the 2 seconds sleep), it leaves a defunct
zombie running ([scriptname] <defunct>), and I can't get rid of it
unless I kill the parent script.

What I'm hoping to accomplish is removing the child completely on exit.
How can I do this?

I've read the manual on pcntl_fork on php.net (and all the comments),
but nothing seems to work. I know it's me doing something completely
wrong, I just can't figure out what.

I hope someone can point me in the right direction. :o)

Regards
Thomas

[petersprc]

Hi,

The OS keeps the zombie around in case the parent process eventually
decides to retrieve the child's exit status information. You can tell
the OS you're not interested and to not keep zombies around using this:

pcntl_signal(SIGCHLD, SIG_IGN);

You could also trap SIGCHLD and check the child's exit status:

function sigchld($signo) {
while (($pid = pcntl_waitpid(-1, $status, WNOHANG)) 0) {
echo "Child with PID $pid returned " .
pcntl_wexitstatus($status) . ".\n";
}
}

pcntl_signal(SIGCHLD, 'sigchld');

Thomas wrote:

I'm having some issues with forking and defunct children. This is my code:

#!/usr/bin/php
<?php
$socket = stream_socket_server("tcp://0.0.0.0:8000", $errno, $errstr);

/*
* LISTEN FOR CONNECTIONS
*/
while(TRUE){
while($conn = stream_socket_accept($socket,-1)){
$pid = pcntl_fork();
if($pid==-1){
fwrite(STDOUT,"Fork failed. Exiting!\n");
exit;
}elseif($pid){
/*
* We are parent.
* Close connection
*/
fclose($conn);
}else{
fwrite($conn, "Hello!\n");
fwrite($conn, "I am PID: ".posix_getpid()."\n");
sleep(2);
fclose($conn);
exit;
}
}
}

function sig_handler($signo){
switch ($signo) {
case SIGTERM:
exit;
break;
case SIGCHLD:
exit;
break;
default:
// handle all other signals
}
}

// setup signal handlers
pcntl_signal(SIGTERM, "sig_handler");
pcntl_signal(SIGCHLD, "sig_handler");
?>

When the child exits (after the 2 seconds sleep), it leaves a defunct
zombie running ([scriptname] <defunct>), and I can't get rid of it
unless I kill the parent script.

What I'm hoping to accomplish is removing the child completely on exit.
How can I do this?

I've read the manual on pcntl_fork on php.net (and all the comments),
but nothing seems to work. I know it's me doing something completely
wrong, I just can't figure out what.

I hope someone can point me in the right direction. :o)

Regards
Thomas

[Thomas]

Hi Peter,

I ended up with this solution:

function sig_handler($signo) {
switch($signo){
case SIGCHLD:
$child = pcntl_wait($status,WNOHANG);
if($child 0){
// Do something
}
}
}

pcntl_signal(SIGCHLD, "sig_handler");

It seems to work like a charm, but it looks a bit different from your
solutions. You sound like you know A LOT more about this than I do, so
perhaps I could trouble you for a short explanation on the differences
between the three solutions? :o)

From my chair you second solution looks like the most "correct" way to
go. Am I right?

Sincerely,
Thomas
petersprc wrote:

Hi,

The OS keeps the zombie around in case the parent process eventually
decides to retrieve the child's exit status information. You can tell
the OS you're not interested and to not keep zombies around using this:

pcntl_signal(SIGCHLD, SIG_IGN);

You could also trap SIGCHLD and check the child's exit status:

function sigchld($signo) {
while (($pid = pcntl_waitpid(-1, $status, WNOHANG)) 0) {
echo "Child with PID $pid returned " .
pcntl_wexitstatus($status) . ".\n";
}
}

pcntl_signal(SIGCHLD, 'sigchld');

[petersprc]

Hi,

pcntl_wait is actually equivalent to calling pcntl_waitpid with -1, so
that will work no problem.

Thomas wrote:

Hi Peter,

I ended up with this solution:

function sig_handler($signo) {
switch($signo){
case SIGCHLD:
$child = pcntl_wait($status,WNOHANG);
if($child 0){
// Do something
}
}
}

pcntl_signal(SIGCHLD, "sig_handler");

It seems to work like a charm, but it looks a bit different from your
solutions. You sound like you know A LOT more about this than I do, so
perhaps I could trouble you for a short explanation on the differences
between the three solutions? :o)

From my chair you second solution looks like the most "correct" way to
go. Am I right?

Sincerely,
Thomas
petersprc wrote:

Hi,

The OS keeps the zombie around in case the parent process eventually
decides to retrieve the child's exit status information. You can tell
the OS you're not interested and to not keep zombies around using this:

pcntl_signal(SIGCHLD, SIG_IGN);

You could also trap SIGCHLD and check the child's exit status:

function sigchld($signo) {
while (($pid = pcntl_waitpid(-1, $status, WNOHANG)) 0) {
echo "Child with PID $pid returned " .
pcntl_wexitstatus($status) . ".\n";
}
}

pcntl_signal(SIGCHLD, 'sigchld');

[Thomas]

Ok Peter.

Thanks a bunch for the help. IMHO this stuff is quite hard to understand
- it's nice when someone steps up and lends a hand. :o)

Regards,
Thomas
petersprc wrote:

Hi,

pcntl_wait is actually equivalent to calling pcntl_waitpid with -1, so
that will work no problem.