Manual Reference Pages - CURL_MULTI_SOCKET_ACTION (3)
curl_multi_socket_action - reads/writes available data given an action
CURLMcode curl_multi_socket_action(CURLM * multi_handle,
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.
The socket callback function uses a prototype like this
The callback MUST return 0.
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
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
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:
register, not interested in readiness (yet)
register, interested in read readiness
register, interested in write readiness
register, interested in both read and write readiness
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.
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 libcurls sockets. You dont 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
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.
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 libcurls 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 7.16.0.
the hiperfifo.c example
|libcurl 7.16.0 ||CURL_MULTI_SOCKET_ACTION (3) ||9 Jul 2006 |
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