PHP: Can pcntl_alarm() and socket_select() peacefully exist in the same thread?

Question

I have a PHP CLI script mostly written that functions as a chat server for chat clients to connect to (don't ask me why I'm doing it in PHP, thats another story haha).

My script utilizes the socket_select() function to hang execution until something happens on a socket, at which point it wakes up, processes the event, and waits until the next event. Now, there are some routine tasks that I need performed every 30 seconds or so (check if tempbanned users should be unbanned, save user databases, other assorted things).

From what I can tell, PHP doesn't have very great multi-threading support at all. My first thought was to compare a timestamp every time the socket generates an event and gets the program flowing again, but this is very inconsistent since the server could very well sit idle for hours and not have any of my cleanup routines executed.

I came across the PHP pcntl extensions, and it lets me use assign a time interval for SIGALRM to get sent and a function get executed every time it's sent. This seems like the ideal solution to my problem, however pcntl_alarm() and socket_select() clash with each other pretty bad. Every time SIGALRM is triggered, all sorts of crazy things happen to my socket control code.

My program is fairly lengthy so I can't post it all here, but it shouldn't matter since I don't believe I'm doing anything wrong code-wise. My question is: Is there any way for a SIGALRM to be handled in the same thread as a waiting socket_select()? If so, how? If not, what are my alternatives here?

Here's some output from my program. My alarm function simply outputs "Tick!" whenever it's called to make it easy to tell when stuff is happening. This is the output (including errors) after allowing it to tick 4 times (there were no actual attempts at connecting to the server despite what it says):

[05-28-10 @ 20:01:05] Chat server started on 192.168.1.28 port 4050

[05-28-10 @ 20:01:05] Loaded 2 users from file

PHP Notice: Undefined offset: 0 in /home/danny/projects/PHPChatServ/ChatServ.php on line 112

PHP Warning: socket_select(): unable to select [4]: Interrupted system call in /home/danny/projects/PHPChatServ/ChatServ.php on line 116

[05-28-10 @ 20:01:15] Tick!

PHP Warning: socket_accept(): unable to accept incoming connection [4]: Interrupted system call in /home/danny/projects/PHPChatServ/ChatServ.php on line 126

[05-28-10 @ 20:01:25] Tick! PHP Warning: socket_getpeername() expects parameter 1 to be resource, boolean given in /home/danny/projects/PHPChatServ/ChatServ.php on line 129

[05-28-10 @ 20:01:25] Accepting socket connection from PHP Notice: Undefined offset: 1 in /home/danny/projects/PHPChatServ/ChatServ.php on line 112

PHP Warning: socket_select(): unable to select [4]: Interrupted system call in /home/danny/projects/PHPChatServ/ChatServ.php on line 116

[05-28-10 @ 20:01:35] Tick!

PHP Warning: socket_accept(): unable to accept incoming connection [4]: Interrupted system call in /home/danny/projects/PHPChatServ/ChatServ.php on line 126

[05-28-10 @ 20:01:45] Tick!

PHP Warning: socket_getpeername() expects parameter 1 to be resource, boolean given in /home/danny/projects/PHPChatServ/ChatServ.php on line 129

[05-28-10 @ 20:01:45] Accepting socket connection from

PHP Notice: Undefined offset: 2 in /home/danny/projects/PHPChatServ/ChatServ.php on line 112

Answer 1

pcntl_alarm and socket_select can coexist, but you need to be aware of how to do it right.

In particular, if the alarm goes off while socket_select() is waiting, then after the alarm is handled the socket_select() will return immediately with an error indication. The error is "Interrupted System Call", which is what you're seeing in the output. You need to specifically check for that error, and retry the socket_select().

Alternatively, you can just forget about using the alarm and instead use the timeout facility of socket_select(). This is what the tv_sec parameter is for - it gives a timeout in seconds, after which the socket_select() will return even if no sockets are ready. You can then do your regular processing.