How to check whether stream has any data?

Question

This what I'm trying to do:

$output = '';
$stream = popen("some-long-running-command 2>&1", 'r');
while (!feof($stream)) {
  $meta = stream_get_meta_data($stream);
  if ($meta['unread_bytes'] > 0) {
    $line = fgets($stream);
    $output .= $line;
  }
  echo ".";
}
$code = pclose($stream);

Looks like this code is not correct, since it gets stuck at the call to stream_get_meta_data(). What is the right way to check whether the stream has some data to read? The whole point here is to avoid locking at fgets().

Answer 1

The correct way to do this is with stream_select():

$stream = popen("some-long-running-command 2>&1", 'r');
while (!feof($stream)) {
  $r = array($stream);
  $w = $e = NULL;

  if (stream_select($r, $w, $e, 1)) {
    // there is data to be read
  }
}
$code = pclose($stream);

One thing to note though (I'm not sure about this) is that it may be the feof() check that is "blocking" - it may be that the loop never ends because the child process does not close its STDOUT descriptor.