pcntl_alarm

(PHP 4 >= 4.3.0, PHP 5, PHP 7, PHP 8)

pcntl_alarm — Set an alarm clock for delivery of a signal

Description

pcntl_alarm(int $seconds): int

Creates a timer that will send a SIGALRM signal to the process after the given number of seconds. Any call to pcntl_alarm() will cancel any previously set alarm.

Parameters

seconds: The number of seconds to wait. If seconds is zero, no new alarm is created.

Return Values

Returns the time in seconds that any previously scheduled alarm had remaining before it was to be delivered, or 0 if there was no previously scheduled alarm.

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User Contributed Notes 4 notes

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kuba at valentine dot dev ¶

4 years ago

This is that universal timeout functionality you dreamed about and always wanted to have and guess what - it's as reliable as it gets, it's basically bulletproof. It can interrupt absolutely anything you throw at it and more, you name it - socket_connect(), socket_read(), fread(), infinite while() loops, sleep(), semaphores - seriously, any blocking operation. You can specify your own handler and just get over anything that normally would make your code unresponsive.



<?php

/**

 * Because we shouldn't handle asynchronous

 * events in synchronous manner.

 */

pcntl_async_signals(TRUE);



/**

 * Some flag we can change to know for sure

 * that our operation timed out.

 */

$timed_out = FALSE;



/**

 * Register SIGALRM signal handler to avoid

 * getting our process killed when signal arrives.

 */

pcntl_signal(SIGALRM, function($signal) use (&$timed_out) {

  $timed_out = TRUE;

});



/**

 * Now we set our timeout for 2 seconds, but it's not set in stone

 * we can call pcntl_alarm() anytime to extend or to turn if off.

 */

pcntl_alarm(2);



/**

 * Here we do something with unpredictable outcome that could

 * possibly block our program for a very long time.

 * I like sockets as an example, but it can be anything.

 */

$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);

$connection = socket_connect($socket, 'irc.ircnet.com', 6667);



/**

 * If our blocking operation didn't timed out then 

 * timer is still ticking, we should turn it off ASAP.

 */

$timed_out || pcntl_alarm(0);



/**

 * And now we do whatever we want to do.

 */

$status = $connection ? 'Connected.' : ($timed_out ? 'Timed out.' : socket_strerror(socket_last_error($socket)));

echo 'STATUS: '. $status . PHP_EOL;

down

kuba at valentine dot dev ¶

1 year ago

Keep in mind that you can easily and selectively prevent signal types of your choosing (including SIGALRM) from interrupting blocking operations you don't want interrupted. Use pcntl_sigprocmask() to shield any part of your code, whenever needed and without affecting expected signal behavior for the rest of it.

Here is simple example of shielding one blocking sleep operation from being interrupted while allowing it to happen to another - right next to each other.

<?php
$alarm = 2;
$sleep = 10;
$stamp = time();

pcntl_async_signals(TRUE);

/**
 * Alarm signal handler.
 */
pcntl_signal(SIGALRM, function(int $sig) use ($alarm, &$stamp) {
  $late = (($now = time()) - $stamp) - $alarm;
  $stamp = $now;
  echo '* ALARM SIGNAL HANDLER * Fired '. ($late ? ($late .' seconds late') : 'on schedule') .' *'. PHP_EOL;
  pcntl_alarm($alarm);
});

/**
 * Sleeping function.
 */
function get_some_sleep(int $duration, string $info) {
  $start = time();
  echo PHP_EOL . $duration .' seconds sleep - '. $info . PHP_EOL;
  sleep($duration);
  $early = $duration - (time() - $start);
  echo 'Sleep was '. ($early ? ('interrupted. Woke up '. $early .' seconds early.') : 'uninterrupted.') . PHP_EOL;
}

/**
 * Here we schedule first alarm.
 */
pcntl_alarm($alarm);

while (TRUE) {
  /**
   * This sleep can and will be interrupted by alarm signal.
   */
  get_some_sleep($sleep, 'without a shield');

  /**
   * Now we set our shield. From now on blocked signals will be ignored.
   * Each blocked signal type that fires while shield is up is put on hold.
   */
  pcntl_sigprocmask(SIG_BLOCK, [SIGALRM]);
  get_some_sleep($sleep, 'protected by sigprocmask()');

  /**
   * Now we take off our shield. If any blocked signal fired while shield
   * was up now it's handler will be executed immediately, single time.
   * This is very useful behaviour because we protect what needs protecting but also
   * don't miss out on anything - without being flooded thanks to "once per type" queue.
   */
  pcntl_sigprocmask(SIG_UNBLOCK, [SIGALRM]);
}

down

molsavsky1 at gmail dot com ¶

2 years ago

Beware that pcntl_signal will interrupt (u)sleep calls which will not be resumed once the handler is completed.

It's a documented behaviour (https://www.php.net/manual/en/function.sleep.php) although it may look like a bug when encountered for the first time.

From the docs:
"If the call was interrupted by a signal, sleep() returns a non-zero value. On Windows, this value will always be 192 (the value of the WAIT_IO_COMPLETION constant within the Windows API). On other platforms, the return value will be the number of seconds left to sleep."

```
<?php

$interval = 1;
pcntl_async_signals(true);

pcntl_signal(SIGALRM, function () use ($interval): void {
    echo 'SIGALRM called' . PHP_EOL;
    pcntl_alarm($interval);
});

pcntl_alarm($interval);

echo 'Sleep (will be interrupted) started' . PHP_EOL;

sleep(100000000000);

echo 'Sleep ended soon due to interrupt' . PHP_EOL;

$sleepTimeout = 10;
echo "Proper sleep for {$sleepTimeout} seconds" . PHP_EOL;

$startedAt = time();
while ($sleepTimeout > 0 && ($sleepTimeout = sleep($sleepTimeout)) !== true) {
    echo "Sleep interrupted, {$sleepTimeout} seconds remaining" . PHP_EOL;
}

$elapsed = time() - $startedAt;
echo "Sleep finished after {$elapsed} seconds" . PHP_EOL;
```

down

-4

Gao,Shengwei ¶

6 years ago

Use pcntl_signal_dispatch() to catch the signal, don't use declare(ticks=1) because it is ineffcient

<?php
pcntl_signal(SIGALRM, function () {
    echo 'Received an alarm signal !' . PHP_EOL;
}, false);

pcntl_alarm(5);

while (true) {
    pcntl_signal_dispatch();
    sleep(1);
}

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