curl_multi_select
(PHP 5)
curl_multi_select — Возвращает сокеты, созданные модулем cURL
Описание
int curl_multi_select
( resource $mh
[, float $timeout
] )
Внимание
К настоящему времени эта функция еще не была документирована; для ознакомления доступен только список аргументов.
См. также описание функции curl_multi_init().
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Коментарии
This function blocks the calling process until there is activity on any of the connections opened by the curl_multi interface, or until the timeout period has expired.
In other words, it waits for data to be received in the opened connections.
Internally it fetches socket pointers with "curl_multi_fdset()" and runs the "select()" C function.
It returns in 3 cases:
1. Activity is detected on any socket;
2. Timeout has ended (second parameter);
3. Process received any signal (#man kill).
The function returns an integer:
* In case of activity it returns a number, usually 1.
I suppose, it returns the number of connections with activity detected.
* If timeout expires it returns 0
* In case of error it returns -1
Thanks for attention, hope this helps.
curl_multi_select($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.
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 */
?>
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".
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