This function creates an association in the multi handle between the given
socket and a private pointer of the application. This is designed for
When set, the sockptr pointer will be passed to all future socket
callbacks for the specific sockfd socket.
If the given sockfd isnt already in use by libcurl, this function will
return an error.
libcurl only keeps one single pointer associated with a socket, so calling
this function several times for the same socket will make the last set pointer
The idea here being that this association (socket to private pointer) is
something that just about every application that uses this API will need and
then libcurl can just as well do it since it already has an internal hash
table lookup for this.
In a typical application you allocate a struct or at least use some kind of
semi-dynamic data for each socket that we must wait for action on when using
the curl_multi_socket_action(3) approach.
When our socket-callback gets called by libcurl and we get to know about yet
another socket to wait for, we can use curl_multi_assign(3) to point out
the particular data so that when we get updates about this same socket again,
we dont have to find the struct associated with this socket by ourselves.