NAME

       curl_multi_socket_action - reads/writes available data given an action

SYNOPSIS

       #include <curl/curl.h>

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

DESCRIPTION

       When  the  application  has detected action on a socket handled by libcurl, it should call
       curl_multi_socket_action(3) with the sockfd argument set to the socket  with  the  action.
       When  the events on a socket are known, they can be passed as an events bitmask ev_bitmask
       by first setting ev_bitmask to 0, and then adding using bitwise OR (|) any combination  of
       events  to  be chosen from CURL_CSELECT_IN, CURL_CSELECT_OUT or CURL_CSELECT_ERR. When the
       events on a socket are unknown, pass 0 instead,  and  libcurl  will  test  the  descriptor
       internally.  It is also permissible to pass CURL_SOCKET_TIMEOUT to the sockfd parameter in
       order to initiate the whole process or when a timeout occurs.

       At return, the integer running_handles points to will contain the number of  running  easy
       handles  within  the  multi  handle.  When  this  number  reaches  zero, all transfers are
       complete/done. 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_action(3)  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 the callback was called.

       Get   the   timeout   time   by   setting   the   CURLMOPT_TIMERFUNCTION    option    with
       curl_multi_setopt(3).  Your  application will then get called with information on how long
       to  wait  for  socket  actions  at  most  before  doing  the  timeout  action:  call   the
       curl_multi_socket_action(3)  function with the sockfd argument set to CURL_SOCKET_TIMEOUT.
       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.

CALLBACK DETAILS

       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,
                                                     NULL if not
                                                     previously assigned with
                                                     curl_multi_assign(3) */

       The callback MUST return 0.

       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.

RETURN VALUE

       CURLMcode type, general libcurl multi interface error code.

       Before  version 7.20.0: If you receive CURLM_CALL_MULTI_PERFORM, this basically means that
       you should call curl_multi_socket_action(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".

       The  return  code  from  this function is for the whole multi stack.  Problems still might
       have occurred on individual transfers even when one of these functions return OK.

TYPICAL USAGE

       1. Create a multi handle

       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.   Call   curl_multi_socket_action(...,   CURL_SOCKET_TIMEOUT,  0,  ...)   to  kickstart
       everything. To get one or more callbacks called.

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

       8,  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.

AVAILABILITY

       This function was added in libcurl 7.15.4, and is deemed stable since 7.16.0.

SEE ALSO

       curl_multi_cleanup(3),  curl_multi_init(3),  curl_multi_fdset(3), curl_multi_info_read(3),
       the hiperfifo.c example