curl_multi_socket(3) libcurl Manual curl_multi_socket(3)NAMEcurl_multi_socket - reads/writes available data
CURLMcode curl_multi_socket_action(CURLM * multi_handle,
curl_socket_t sockfd, int ev_bitmask,
CURLMcode curl_multi_socket(CURLM * multi_handle, curl_socket_t sockfd,
CURLMcode curl_multi_socket_all(CURLM *multi_handle,
Alternative versions of curl_multi_perform(3) that allows the applica‐
tion to pass in the file descriptor/socket that has been detected to
have "action" on it and let libcurl perform. This allows libcurl to not
have to scan through all possible file descriptors to check for action.
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 int running_handles points to will contain the number 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 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_SOCKETFUNC‐
TION option to curl_multi_setopt(3). They update the status with
changes since the previous time this function was called.
Force libcurl to (re-)check all its internal sockets and transfers
instead of just a single one by calling curl_multi_socket_all(3). Note
that there should rarely be reasons to use this function!
Get the timeout time - how long to wait for socket actions at most
before doing the timeout action: call the curl_multi_socket(3) function
with the sockfd argument set to CURL_SOCKET_TIMEOUT, by setting the
CURLMOPT_TIMERFUNCTION option with curl_multi_setopt(3). 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.
Usage of curl_multi_socket(3) is deprecated, whereas the function is
equivalent to curl_multi_socket_action(3), when ev_bitmask is set to 0.
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.
If you receive CURLM_CALL_MULTI_PERFORM, this basically means that you
should call curl_multi_socket(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 "satis‐
NOTE that this only returns errors etc regarding the whole multi stack.
There might still have occurred problems on individual transfers even
when this function returns 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, although deemed stable since
curl_multi_socket(3) is deprecated, use curl_multi_socket_action(3)
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(3)