Supplying a PIN for an HSM with PKCS 11

    I am trying to understand how to supply a PIN to generate a key pair in an HSM using the PKCS #11 interface in GnuTLS - and, so far, failing miserably. My function does (in essence) the following:

    1) Initialize the PKCS #11 interface with

       gnutls_pkcs11_init(GNUTLS_PKCS11_FLAG_MANUAL, NULL)

    2) Add the provider with 

        gnutls_pkcs11_add_provider("<lib-path>", NULL) ;

where <lib-path> is the pathname of the shared library for the client component of the HSM.

     After the above, I tried to generate a keypair as follows:

     gnutls_pkcs11_privkey_generate3(<hsm-url>,
                                                                     GNUTLS_PK_RSA,
                                                                     2048,
                                                                     "First Key",
                                                                     &cid,
                                                                     0,
                                                                     &pubKey,
                                                                     0,
                                                                     GNUTLS_PKCS11_OBJ_FLAG_MARK_SENSITIVE | GNUTLS_PKCS11_OBJ_FLAG_LOGIN) ;

where <hsm-url>  is the URL for the HSM that I am using.

     The above doesn't work. Both the init and provider calls do work, but generate3 returns error -300 - PKCS11 error. This is not surprising, for one needs to supply credentials in order to access the HSM.

     The GnuTLS documentation says that, when a PIN is required for PKCS 11 operations, one should install a callback by means of gnutls_pkcs11_set_pin_function(). I added such a thing, but the callback does not get invoked. Even if it were, I am confused as to where the arguments for the callback are coming from.  The callback has the prototype

typedef int (*gnutls_pin_callback_t) (void *userdata, int attempt,
                                                                  const char *token_url,
                                                                  const char *token_label,
                                                                  unsigned int flags,
                                                                  char *pin, size_t pin_max);

Who is supplying them? SInce they involve the PIN (among other things) I would have thought it is up to me. I don't know how. And, as I mentioned, the callback does not get invoked when I call the generate3 function. 

      Any help to clarify these issues would be much appreciated.