## NAME

curl_multi_socket - reads/writes available data

## SYNOPSIS

#include <curl/curl.h>

CURLMcode curl_multi_socket(CURLM * multi_handle, curl_socket_t sockfd, int *running_handles);

CURLMcode curl_multi_socket_all(CURLM *multi_handle, int *running_handles);

## DESCRIPTION

Alternative versions of curl_multi_perform(3) that allows the application to pass in one of the file descriptors/sockets that have been detected to have "action" on them 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(3) with the sockfd argument set to the socket with the action.

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(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_SOCKETFUNCTION option to curl_multi_setopt(3). They update the status with changes since the previous time this function was called.

To force libcurl to (re-)check all its internal sockets and transfers instead of just a single one, you call curl_multi_socket_all(3). This is typically done as the first function call before the application has any knowledge about what sockets libcurl uses.

Applications should call curl_multi_timeout(3) to figure out 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.

## CALLBACK DETAILS

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 sockets
     simultaneously.
     The s argument is the actual socket value as you use it within your
     system.
     The action argument to the callback has one of five values:
     CURL_POLL_NONE (0) register, not interested in readiness (yet)
     CURL_POLL_IN (1) register, interested in read readiness
     CURL_POLL_OUT (2) register, interested in write readiness
     CURL_POLL_INOUT (3) register, interested in both read and write readiness
     CURL_POLL_REMOVE (4) deregister
     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.

## RETURN VALUE

CURLMcode type, general libcurl multi interface error code.

If you receive CURLM_CALL_MULTI_PERFORM, this basically means that you should call curl_multi_perform 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".

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.

## TYPICAL USAGE

1. Create a multi handle

2. Set the socket callback with CURLMOPT_SOCKETFUNCTION

3. Add easy handles

4. Call curl_multi_socket_all() first once

5. Setup a "collection" of sockets to supervise when your socket callback is called.

6. Use curl_multi_timeout() to figure out how long to wait for action

7. Wait for action on any of libcurl's sockets

8, When action happens, call curl_multi_socket() for the socket(s) that got action.

9. Go back to step 6.

## AVAILABILITY

This function was added in libcurl 7.15.4, although not deemed stable yet.

## SEE ALSO

curl_multi_cleanup (3) curl_multi_init (3) curl_multi_fdset (3) curl_multi_info_read (3)

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