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().

Fuddyduddy

fgets() locks because it waits for a "new-line" character. Use stream_get_contents() with the length argument instead: $line = stream_get_contents($stream, $meta['unread_bytes']); – Necrotomy

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.

Douville

It is important to note that, as @Douville has done, if you are going to use NULL you MUST assign it to a variable in order to avoid trouble with passing a non-variable by reference (which is how the first three parameters are declared). Read the notes at php.net/manual/en/function.stream-select.php – Eighteenmo

Answer 2

If you want to test whether or not your cli command has something in STDIN without reading it, you can also use stream_select.

$r = [STDIN];
$w = $e = NULL;
$hasStdin = stream_select($r, $w, $e, 0);
if ($hasStdin) {
  //do something
}

Steeplejack