#include <curl/curl.h>

CURLMcode curl_multi_socket_action(CURLM * multi_handle, 
                                   curl_socket_t sockfd, int ev_bitmask,
                                   int *running_handles);

CURLMcode curl_multi_socket(CURLM * multi_handle, curl_socket_t sockfd,
                            int *running_handles);

CURLMcode curl_multi_socket_all(CURLM *multi_handle,
                                int *running_handles);

At return, the int running_handles points to will contain the number of still running easy handles within the multi handle. When this number reaches zero, all transfers are complete/done. Note that when you call curl_multi_socket_action(3) on a specific socket and the counter decreases by one, it DOES NOT necessarily mean that this exact socket/transfer is the one that completed. Use curl_multi_info_read(3) to figure out which easy handle that completed.

The curl_multi_socket functions inform the application about updates in the socket (file descriptor) status by doing none, one or multiple calls to the socket callback function set with the CURLMOPT_SOCKETFUNCTION option to curl_multi_setopt(3). They update the status with changes since the previous time this function was called.

Force libcurl to (re-)check all its internal sockets and transfers instead of just a single one by calling curl_multi_socket_all(3). Note that there should rarely be reasons to use this function!

Get the timeout time - how long to wait for socket actions at most before doing the timeout action: call the curl_multi_socket(3) function with the sockfd argument set to CURL_SOCKET_TIMEOUT, by setting the CURLMOPT_TIMERFUNCTION option with curl_multi_setopt(3). You can also use the curl_multi_timeout(3) function to poll the value at any given time, but for an event-based system using the callback is far better than relying on polling the timeout value.

Usage of curl_multi_socket(3) is deprecated, whereas the function is equivalent to curl_multi_socket_action(3), when ev_bitmask is set to 0.

The socket callback function uses a prototype like this

  int curl_socket_callback(CURL *easy,      /* easy handle */
                           curl_socket_t s, /* socket */
                           int action,      /* see values below */
                           void *userp,    /* private callback pointer */
                           void *socketp); /* private socket pointer */

The easy argument is a pointer to the easy handle that deals with this particular socket. Note that a single handle may work with several sockets simultaneously.

The s argument is the actual socket value as you use it within your system.

The action argument to the callback has one of five values:

CURL_POLL_NONE (0)

register, not interested in readiness (yet)

CURL_POLL_IN (1)

register, interested in read readiness

CURL_POLL_OUT (2)

register, interested in write readiness

CURL_POLL_INOUT (3)

register, interested in both read and write readiness

CURL_POLL_REMOVE (4)

unregister

The socketp argument is a private pointer you have previously set with curl_multi_assign(3) to be associated with the s socket. If no pointer has been set, socketp will be NULL. This argument is of course a service to applications that want to keep certain data or structs that are strictly associated to the given socket.

The userp argument is a private pointer you have previously set with curl_multi_setopt(3) and the CURLMOPT_SOCKETDATA option.  

If you receive CURLM_CALL_MULTI_PERFORM, this basically means that you should call curl_multi_socket(3) again, before you wait for more actions on libcurl's sockets. You don't have to do it immediately, but the return code means that libcurl may have more data available to return or that there may be more data to send off before it is "satisfied".

NOTE that this only returns errors etc regarding the whole multi stack. There might still have occurred problems on individual transfers even when this function returns OK.  

2. Set the socket callback with CURLMOPT_SOCKETFUNCTION

3. Set the timeout callback with CURLMOPT_TIMERFUNCTION, to get to know what timeout value to use when waiting for socket activities.

4. Add easy handles with curl_multi_add_handle()

5. Provide some means to manage the sockets libcurl is using, so you can check them for activity. This can be done through your application code, or by way of an external library such as libevent or glib.

6. Wait for activity on any of libcurl's sockets, use the timeout value your callback has been told

7, When activity is detected, call curl_multi_socket_action() for the socket(s) that got action. If no activity is detected and the timeout expires, call curl_multi_socket_action(3) with CURL_SOCKET_TIMEOUT

8. Go back to step 6.  

curl_multi_socket(3) is deprecated, use curl_multi_socket_action(3) instead!  

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