int nng_pipe_id(nng_pipe p);
nng_pipe_id() function returns a positive identifier for the pipe p,
if it is valid.
Otherwise it returns
A pipe is considered valid if it was ever created by the socket.
Pipes that are allocated on the stack or statically should be
initialized with the macro
NNG_PIPE_INITIALIZER to ensure that
they cannot be confused with a valid pipe.
This function returns the positive value for the pipe identifier, or
-1 if the pipe is invalid.