a problem about get auto-allocate port from IGD

hello,everybody!
I want to make a UPnP cp to UPnP IGD(I use linuxigd as my  UPnP IGD).In fact
,it has been done.

In this UPnP,AddPortMapping is fixed port. This means, in UPnP client,I
specify a port,
through invoke these functions following:
UpnpMakeAction(),UpnpAddToAction(),UpnpSendAction() (UPnP SDK proffer these)
I ask UPnP IGD my start port,if IGD agree and return UPNP_E_SUCCESS,I use
this port for my other Application.

but now I want to modify my UPnP CP code to auto get the port from the IGD
allocating .
I don't know how to ask IGD auto-allocate port for UPnP CP and how to get
this  port from IGD.
which functions (UPNPSDK proffer) will I invoke to complete this work?