curl_multi_select

Alex Palmer (2012-12-17 16:08:05)

On php 5.3.18+ be aware that curl_multi_select() may return -1 forever until you call curl_multi_exec().
See https://bugs.php.net/bug.php?id=63411 for more information.

vigo dot von dot harrach at gmx dot de (2012-06-06 10:53:29)

curl_multi_select($mh, $timeout) simply blocks for $timeout seconds while curl_multi_exec() returns CURLM_CALL_MULTI_PERFORM. Otherwise, it works as intended, and blocks until at least one connection has completed or $timeout seconds, whatever happens first.

For that reason, curl_multi_exec() should always be wrapped:

<?php
  function full_curl_multi_exec($mh, &$still_running) {
    do {
      $rv = curl_multi_exec($mh, $still_running);
    } while ($rv == CURLM_CALL_MULTI_PERFORM);
    return $rv;
  }
?>

With that, the core of "multi" processing becomes (ignoring error handling for brevity):

<?php
  full_curl_multi_exec($mh, $still_running); // start requests
  do { // "wait for completion"-loop
    curl_multi_select($mh); // non-busy (!) wait for state change
    full_curl_multi_exec($mh, $still_running); // get new state
    while ($info = curl_multi_info_read($mh)) {
      // process completed request (e.g. curl_multi_getcontent($info['handle']))
    }
  } while ($still_running);
?>

Note that after starting requests, retrieval is done in the background - one of the better shots at parallel processing in PHP.

Anonymous (2011-08-27 05:52:49)

if you are using a non zts php build, expect it to block on connect() calls when using curl multi, if you plan to use curl multi to realtime fetch multiple pages for a user, have a localhost squid setup to funnel them through so as connect() only blocks for a ms or 2

Anonymous (2010-07-24 08:33:03)

Since the docs are still lacking, here's an example of how to use the function. The following code will keep checking all active threads until one of them returns the HTTP 200 Ok status code, or simply end. On success, it will return the URL that worked.

<?php
$running=null;
do {
    curl_multi_exec($mh,$running);
    $ready=curl_multi_select($mh); // this will pause the loop
    if($ready>0){
        while($info=curl_multi_info_read($mh)){
            $status=curl_getinfo($info['handle'],CURLINFO_HTTP_CODE);
            if($status==200){
                $successUrl=curl_getinfo($info['handle'],CURLINFO_EFFECTIVE_URL);
                break 2;
            }
        }
    }
} while ($running>0 && $ready!=-1);
?>

The question for the $ready variable is, if it will return the value before or after the timeout has occurred. From my tests it appears that it will return the value immediately, and only then pause the execution. That's because it's always zero the first time through the loop, even if the time limit is as high as 10sec. I've expected it to wait and only then return the value so that came unexpected to me.

As stated by someone else, it also doesn't seem to return the overall count of threads in the handle, but only that of the currently active ones.

public at grik dot net (2008-08-10 06:38:10)

This function blocks the calling process until there is activity on any of the connections opened by the curl_multi interface, or until the timeout period has expired.
In other words, it waits for data to be received in the opened connections.
Internally it fetches socket pointers with "curl_multi_fdset()" and runs the "select()" C function.
It returns in 3 cases:
1. Activity is detected on any socket;
2. Timeout has ended (second parameter);
3. Process received any signal (#man kill).
The function returns an integer:
* In case of activity it returns a number, usually 1.
I suppose, it returns the number of connections with activity detected.
* If timeout expires it returns 0
* In case of error it returns -1
Thanks for attention, hope this helps.