curl_multi_socket_action(3) libcurl Manual curl_multi_socket_action(3)NAMEcurl_multi_socket_action - reads/writes available data given an action
CURLMcode curl_multi_socket_action(CURLM * multi_handle,
curl_socket_t sockfd, int ev_bitmask,
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.
At return, the integer running_handles points to will contain the num‐
ber 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_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 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 infor‐
mation 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 */
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
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 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 option.
CURLMcode type, general libcurl multi interface error code.
Legacy: 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 immedi‐
ately, but the return code means that libcurl may have more data avail‐
able to return or that there may be more data to send off before it is
In modern libcurls, CURLM_CALL_MULTI_PERFORM or CURLM_CALL_MULTI_SOKCET
should not be returned and no application needs to care about them.
NOTE that the return code 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. 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.
This function was added in libcurl 7.15.4, and is deemed stable since
SEE ALSOcurl_multi_cleanup(3), curl_multi_init(3), curl_multi_fdset(3),
curl_multi_info_read(3), the hiperfifo.c example
libcurl 7.16.0 9 Jul 2006 curl_multi_socket_action(3)