curl_multi_socket_action(3)
curl_multi_socket_action(3libcurl Manuacurl_multi_socket_action(3)
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 descrip-
tor internally. It is also permissible to pass
CURL_SOCKET_TIMEOUT to the sockfd parameter in order to ini-
tiate 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 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 appli-
cation 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(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) func-
tion 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.
libcurl 7.58.0 Last change: June 07, 2016 1
curl_multi_socket_action(3libcurl Manuacurl_multi_socket_action(3)
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 readi-
ness
CURL_POLL_REMOVE (4)
unregister
The socketp argument is a private pointer you have previ-
ously 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.
libcurl 7.58.0 Last change: June 07, 2016 2
curl_multi_socket_action(3libcurl Manuacurl_multi_socket_action(3)
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 call-
backs 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
libcurl 7.58.0 Last change: June 07, 2016 3
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