curl_multi_select

    (PHP 5, PHP 7, PHP 8)

    curl_multi_select — Wait until reading or writing is possible for any cURL multi handle connection

    Description

    curl_multi_select(CurlMultiHandle $multi_handle, float $timeout = 1.0): int

    Blocks the execution of the script until a cURL handle attached to the cURL multi handle would be able to make progress on the next call to curl_multi_exec() or until the timeout strikes (whichever comes first).

    Parameters

    multi_handle

    A cURL multi handle returned by curl_multi_init().

    timeout

    Time, in seconds, to wait for a response from the active cURL multi handle connections.

    Return Values

    On success, returns the number of active descriptors contained in the descriptor sets. This may be 0 if there was no activity on any of the descriptors. On failure, this function will return -1 on a select failure (from the underlying select() system call).

    Errors/Exceptions

    Throws a ValueError if timeout is less than 0 or greater than PHP_INT_MAX.

    Changelog

    Version Description
    8.4.0 Now throws a ValueError if timeout is less than 0 or greater than PHP_INT_MAX.
    8.0.0 multi_handle expects a CurlMultiHandle instance now; previously, a resource was expected.

    See Also

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    User Contributed Notes 8 notes

    up
    41
    nevil85 at gmail dot com
    10 years ago
    Even after so many distro releases, in PHP v5.5.9 and libcurl v7.35.0, curl_multi_select still returns -1. The only work around here is to wait and proceed like nothing ever happened. You have to determine the "wait" required here.

    In my application a very small interval like usleep(1) worked. For example:

    <?php
    // While we're still active, execute curl
    $active = null;
    do {
    $mrc = curl_multi_exec($multi, $active);
    } while (    $mrc == CURLM_CALL_MULTI_PERFORM);

    while (    $active && $mrc == CURLM_OK) {
    // Wait for activity on any curl-connection
    if (curl_multi_select($multi) == -1) {
    usleep(1);
    }

    // Continue to exec until curl is ready to
    // give us more data
    do {
    $mrc = curl_multi_exec($multi, $active);
    } while (    $mrc == CURLM_CALL_MULTI_PERFORM);
    }
    ?>

    Internally php curl_multi_select uses libcurl curl_multi_fdset function and its libcurl documentation says :
    http://curl.haxx.se/libcurl/c/curl_multi_fdset.html

    "When libcurl returns -1 in max_fd, it is because libcurl currently does something that isn't possible for your application to monitor with a socket and unfortunately you can then not know exactly when the current action is completed using select(). When max_fd returns with -1, you need to wait a while and then proceed and call curl_multi_perform anyway. How long to wait? I would suggest 100 milliseconds at least, but you may want to test it out in your own particular conditions to find a suitable value.

    When doing select(), you should use curl_multi_timeout to figure out how long to wait for action."

    Untill PHP implements curl_multi_timeout() we have to play with our application and determine the "wait".
    up
    3
    bishop at php dot net
    5 years ago
    Before PHP 7.1.23 and 7.2.11, curl_multi_select() would return -1 if cURL had no open file descriptors. Since then, assuming libcurl 7.28 or higher, curl_multi_select() will return 0 if cURL has no open file descriptors and -1 on error.

    See https://bugs.php.net/bug.php?id=76480

    This is a BC break. See https://bugs.php.net/bug.php?id=77030
    up
    16
    vigo dot von dot harrach at gmx dot de
    12 years ago
    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.
    up
    10
    public at grik dot net
    16 years ago
    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.
    up
    4
    xxavalanchexx at gmail dot com
    10 years ago
    According to https://bugs.php.net/bug.php?id=61141:

    On Windows setups using libcurl version 7.24 or later (which seems to correspond to PHP 5.3.10 or later), you may find that this always returns -1. This is, apparently, not strictly a bug: according to the libcurl documentation, you should add your own sleep if curl_multi_select returns -1.

    For example:
    <?php
    /* setup $mh */

    $active = null;
    do {
    $mrc = curl_multi_exec($mh, $active);
    } while (    $mrc == CURLM_CALL_MULTI_PERFORM);

    while (    $active && $mrc == CURLM_OK) {
    if (    curl_multi_select($mh) == -1) {
    usleep(100);
    }
    do {
    $mrc = curl_multi_exec($mh, $active);
    } while (    $mrc == CURLM_CALL_MULTI_PERFORM);
    }

    /* close $mh */
    ?>
    up
    6
    Anonymous
    11 years ago
    In 5.3.9+, curl_multi_select always returns -1. If this is your case, just wait a bit and then proceed like nothing ever happened:
    <?php
    do {
    $mrc = curl_multi_exec($multi, $active);
    } while (    $mrc == CURLM_CALL_MULTI_PERFORM);

    while (    $active && $mrc == CURLM_OK) {
    //check for results and execute until everything is done

    if (curl_multi_select($multi) == -1) {
    //if it returns -1, wait a bit, but go forward anyways!
    usleep(100);
    }

    //do something with the return values
    while(($info = curl_multi_info_read($multi)) !== false){
    if (    $info["result"] == CURLE_OK){
    $content = curl_multi_getcontent($info["handle"]);
    do_something($content);
    }
    }
    do {
    $mrc = curl_multi_exec($multi, $active);
    } while (    $mrc == CURLM_CALL_MULTI_PERFORM);
    }
    ?>
    up
    1
    gib-o-master
    3 years ago
    NOTE: current implementation of `curl_multi_select` doesn't block and doesn't respect timeout parameter (maybe will be fixed later, in this case just remove the usleep call)

    here, i would like to post my working/tested example of proper usage of this function:

    ```
    $running = 1;
    while ($running)
    {
    # execute request
    if ($a = curl_multi_exec($this->murl, $running)) {
    throw BotError::text("curl_multi_exec[$a]: ".curl_multi_strerror($a));
    }
    # check finished
    if (!$running) {
    break;
    }
    # wait for activity
    while (!$a)
    {
    if (($a = curl_multi_select($this->murl, $timeout)) < 0)
    {
    throw BotError::text(
    ($a = curl_multi_errno($this->murl))
    ? "curl_multi_select[$a]: ".curl_multi_strerror($a)
    : 'system select failed'
    );
    }
    usleep($timeout * 1000000);# should be <=1
    }
    }
    ```
    up
    6
    Alex Palmer
    11 years ago
    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.
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