Receive a series of strings (until NULL) from multipart data.
Each string is allocated and filled with string data; if there
are not enough frames, unallocated strings are set to NULL.
Returns -1 if the message could not be read, else returns the
number of strings filled, zero or more. Free each returned string
using zstr_free(). If not enough strings are provided, remaining
multipart frames in the message are dropped.

def recvx(source, string_p, *args):
        """
        Receive a series of strings (until NULL) from multipart data.
Each string is allocated and filled with string data; if there
are not enough frames, unallocated strings are set to NULL.
Returns -1 if the message could not be read, else returns the
number of strings filled, zero or more. Free each returned string
using zstr_free(). If not enough strings are provided, remaining
multipart frames in the message are dropped.
        """
        return lib.zstr_recvx(source, byref(c_char_p.from_param(string_p)), *args)