socket_select returns false immediately buth without error code

Question

I'm playing with socket_select, but on one hosting, this function does strange things:

• returns immediately, instead of waiting 5 seconds
• returns false, indicating some error
• but socket_last_error() returns 0 (success).

phpinfo() of this server: http://jsfiddle.net/Lmrfe/embedded/result/

$server = socket_create( AF_UNIX, SOCK_STREAM, 0 );
$r = socket_bind( $server, '/some/file/somewhere');
$r = socket_listen( $server );
// none of the above socket_* returns false

$t = microtime(true);
socket_clear_error();

$read = array( $server ); $write = null; $except = null;
$read = array( $server ); $write = array(); $except = array();
$read = array(); $write = array(); $except = array();
$read = null; $write = null; $except = null;

$changed = socket_select( $read, $write, $except, 5,0 );
$changed = socket_select( $read, $write, $except, null );
$changed = socket_select( $read, $write, $except, 5000000 );
$changed = socket_select( $read, $write, $except, 5000000, 0 );
$changed = socket_select( $read, $write, $except, 5000000, 5000000 );

/* Results:

   microtime(true) - $t  == almost zero

   $changed === false

   socket_last_error() === 0
   socket_strerror(socket_last_error()) === Success

   $read === array(1) {
     [0]=>
     resource(2) of type (Socket)
   }
   $write  === NULL
   $except === NULL
*/

$s = socket_read( $server, 1024, PHP_BINARY_READ );
// $s === false

What is happening here?

Updated test script: still runs in an instant:

header('Content-Type: text/plain; charset=utf-8');
error_reporting(-1);

for( $i = 0; $i < 4; $i++ ){
for( $j = 0; $j < 5; $j++ ){
  echo "\n\n\n\n\n[i,j]=[{$i},{$j}]\n";

  $socket = socket_create( AF_UNIX, SOCK_STREAM, 0 );
  var_dump('socket_create', $socket ); echo "\n"; if( $socket === false ) continue;

  $socket_file = dirname(__FILE__)."/test_socket_i{$i}_j{$j}";
  if( file_exists( $socket_file )) unlink( $socket_file );

  $r = socket_bind( $socket, $socket_file );
  var_dump('socket_bind', $r ); echo "\n"; if( $r === false ) continue;

  $r = socket_listen( $socket );
  var_dump('socket_listen', $r ); echo "\n"; if( $r === false ) continue;

  $t = microtime(true);
  socket_clear_error();

  if($i==0){ $read = null;             $write = null;    $except = null; }
  if($i==1){ $read = array();          $write = array(); $except = array(); }
  if($i==2){ $read = array( $socket ); $write = null;    $except = null; }
  if($i==3){ $read = array( $socket ); $write = array(); $except = array(); }

  if($j==0){ $changed = socket_select( $read, $write, $except, 5,0 ); } // 5 seconds
  if($j==1){ $changed = socket_select( $read, $write, $except, null ); } // forever
  if($j==2){ $changed = socket_select( $read, $write, $except, 5000000 ); }
  if($j==3){ $changed = socket_select( $read, $write, $except, 5000000, 0 ); }
  if($j==4){ $changed = socket_select( $read, $write, $except, 5000000, 5000000 ); }

  var_dump('•socket_select returned:', $changed );echo "\n";
  var_dump('•$read/$write/$except:',$read,$write,$except);echo "\n";
  var_dump('•error:', socket_last_error(), socket_strerror(socket_last_error()) );echo "\n";
  var_dump('time: ', microtime(true) - $t );echo "\n";               // almost zero
}}

Answer 1

If you run into this bug, you could use something similar to work it around:

$socket_select_timeout_seconds = 5;

while(true){
  $time_start = microtime(true);
  $c = socket_select($r,$w,$e, $socket_select_timeout_seconds );

  if( /* something happened */ ){
    // handle socket events
  }else if( microtime(true) - $time_start < 0.01){
    // nothing happened, but buggy socket_select didn't wait
    // wait manually
    usleep( $socket_select_timeout_seconds * 1000000 );
  }

}

Answer 2

You might want to use stream_* functions instead of socket.

Stream functions are more generic and are part of PHP core, whereas Socket support needs to be installed. Stream functions give you basically more control.

http://www.php.net/manual/en/intro.stream.php

Answer 3

check this http://php.net/manual/en/function.socket-select.php

Due a limitation in the current Zend Engine it is not possible to pass a constant modifier like NULL directly as a parameter to a function which expects this parameter to be passed by reference. Instead use a temporary variable or an expression with the leftmost member being a temporary variable:

so the problem is that you are initializing $write = null

Answer 4

As @Amez pointed out you should not pass null values to select. Instead, create an empty array for the unused socket_select() parameters:

$read    = array( $server );
$write   = array();
$except  = array();
$changed = socket_select( $read, $write, $except, 5,0 );

I just recently stumbled upon the same issue in python (I have no clue why PHP/Python won't simply replace null values wit empty arrays internally as for select this certainly seems to be expected behaviour).