curl_multi_socket_action(3) libcurl Manual curl_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 ar-
gument 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 or-
der 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) function informs 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.
When this function returns error, the state of all transfers are uncer-
tain and they cannot be continued. curl_multi_socket_action(3) should
not be called again on the same multi handle after an error has been
returned, unless first removing all the handles and adding new ones.
TYPICAL USAGE
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.
EXAMPLE
/* the event-library gets told when there activity on the socket 'fd',
which we translate to a call to curl_multi_socket_action() */
int running;
rc = curl_multi_socket_action(multi_handle, fd, EVENT,
&running);
AVAILABILITY
This function was added in libcurl 7.15.4, and is deemed stable since
7.16.0.
RETURN VALUE
CURLMcode type, general libcurl multi interface error code. See
libcurl-errors(3)
SEE ALSO
curl_multi_cleanup(3), curl_multi_init(3), curl_multi_fdset(3),
curl_multi_info_read(3), the hiperfifo.c example
libcurl 7.81.0 December 09, 2021 curl_multi_socket_action(3)
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