How to handle C functions with variable number of args

I read this somewhere in a Pyrex document (maybe) but cannot find it now.

In my .pyx file I have this declaration:

cdef extern int xosd_display(xosd * osd, int line, xosd_command
command, ...) except -1

I want to have a method in this class:

cdef class XOSD:
    cdef xosd *xosdWin
...

like this (but this doesn't work):

    def displaytext(self, line, command,  *arg):
         #and then call the xosd_display function declared above

Where is handling C varargs in Pyrex documented, btw?
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