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.TH curl_multi_socket_action 3 "June 30, 2018" "libcurl 7.61.1" "libcurl Manual"

.SH NAME
curl_multi_socket_action \- reads/writes available data given an action
.SH SYNOPSIS
.nf
#include <curl/curl.h>

CURLMcode curl_multi_socket_action(CURLM * multi_handle,
                                   curl_socket_t sockfd,
                                   int ev_bitmask,
                                   int *running_handles);
.fi
.SH DESCRIPTION
When the application has detected action on a socket handled by libcurl, it
should call \fIcurl_multi_socket_action(3)\fP with the \fBsockfd\fP argument
set to the socket with the action. When the events on a socket are known, they
can be passed as an events bitmask \fBev_bitmask\fP by first setting
\fBev_bitmask\fP 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 \fBsockfd\fP parameter in order to initiate the
whole process or when a timeout occurs.

At return, \fBrunning_handles\fP 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
\fIcurl_multi_socket_action(3)\fP 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 \fIcurl_multi_info_read(3)\fP to figure out
which easy handle that completed.

The \fIcurl_multi_socket_action(3)\fP 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
\fICURLMOPT_SOCKETFUNCTION(3)\fP option to \fIcurl_multi_setopt(3)\fP. They
update the status with changes since the previous time the callback was
called.

Get the timeout time by setting the \fICURLMOPT_TIMERFUNCTION(3)\fP option
with \fIcurl_multi_setopt(3)\fP. 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 \fIcurl_multi_socket_action(3)\fP function with the
\fBsockfd\fP argument set to CURL_SOCKET_TIMEOUT. You can also use the
\fIcurl_multi_timeout(3)\fP 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.
.SH "CALLBACK DETAILS"

The socket \fBcallback\fP function uses a prototype like this
.nf

  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,
                                              \fBNULL\fP if not
                                              previously assigned with
                                              \fIcurl_multi_assign(3)\fP */

.fi
The callback MUST return 0.

The \fIeasy\fP 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 \fIs\fP argument is the actual socket value as you use it within your
system.

The \fIaction\fP argument to the callback has one of five values:
.RS
.IP "CURL_POLL_NONE (0)"
register, not interested in readiness (yet)
.IP "CURL_POLL_IN (1)"
register, interested in read readiness
.IP "CURL_POLL_OUT (2)"
register, interested in write readiness
.IP "CURL_POLL_INOUT (3)"
register, interested in both read and write readiness
.IP "CURL_POLL_REMOVE (4)"
unregister
.RE

The \fIsocketp\fP argument is a private pointer you have previously set with
\fIcurl_multi_assign(3)\fP to be associated with the \fIs\fP 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 \fIuserp\fP argument is a private pointer you have previously set with
\fIcurl_multi_setopt(3)\fP and the \fICURLMOPT_SOCKETDATA(3)\fP option.
.SH "RETURN VALUE"
CURLMcode type, general libcurl multi interface error code.

Before version 7.20.0: If you receive \fICURLM_CALL_MULTI_PERFORM\fP, this
basically means that you should call \fIcurl_multi_socket_action(3)\fP 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.
.SH "TYPICAL USAGE"
1. Create a multi handle

2. Set the socket callback with \fICURLMOPT_SOCKETFUNCTION(3)\fP

3. Set the timeout callback with \fICURLMOPT_TIMERFUNCTION(3)\fP, 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 \fIcurl_multi_socket_action(3)\fP with \fICURL_SOCKET_TIMEOUT\fP.
.SH AVAILABILITY
This function was added in libcurl 7.15.4, and is deemed stable since 7.16.0.
.SH "SEE ALSO"
.BR curl_multi_cleanup "(3), " curl_multi_init "(3), "
.BR curl_multi_fdset "(3), " curl_multi_info_read "(3), "
.BR "the hiperfifo.c example"