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Updated: Thursday, June 13, 2019

curl_multi_socket_action (3)


curl_multi_socket_action - reads/writes available data given an action


#include <curl/curl.h>

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


libcurl Manual                                     curl_multi_socket_action(3)

       curl_multi_socket_action - reads/writes available data given an action

       #include <curl/curl.h>

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

       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 com-
       bination 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, running_handles points to 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  spe-
       cific  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 CURL-
       MOPT_SOCKETFUNCTION(3) 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(3) 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.

       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

       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)

       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 ser-
       vice 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(3) option.

       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.  Prob-
       lems still might have occurred on individual transfers even when one of
       these functions return OK.

       1. Create a multi handle

       2. Set the socket callback with CURLMOPT_SOCKETFUNCTION(3)

       3.  Set  the timeout callback with CURLMOPT_TIMERFUNCTION(3), 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.

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

       See attributes(7) for descriptions of the following attributes:

       |Availability   | web/curl         |
       |Stability      | Uncommitted      |
       curl_multi_cleanup(3),     curl_multi_init(3),     curl_multi_fdset(3),
       curl_multi_info_read(3), the hiperfifo.c example

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source   was    downloaded    from     https://github.com/curl/curl/ar-

       Further information about this software can be found on the open source
       community website at http://curl.haxx.se/.

libcurl 7.16.0                    9 Jul 2006