void gnutls_ia_set_client_avp_function(gnutls_ia_client_credentials_t cred, gnutls_ia_avp_func avp_func);
The AVP callback is called to process AVPs received from the server, and to get a new AVP to send to the server.
The callback's function form is: int (*avp_func) (gnutls_session_t session, void *ptr, const char *last, size_t lastlen, char **next, size_t *nextlen);
The session parameter is the gnutls_session_t structure corresponding to the current session. The ptr parameter is the application hook pointer, set through gnutls_ia_set_client_avp_ptr(). The AVP received from the server is present in last of lastlen size, which will be NULL on the first invocation. The newly allocated output AVP to send to the server should be placed in *next of *nextlen size.
The callback may invoke gnutls_ia_permute_inner_secret() to mix any generated session keys with the TLS/IA inner secret.
Return 0 (GNUTLS_IA_APPLICATION_PAYLOAD) on success, or a negative error code to abort the TLS/IA handshake.
Note that the callback must use allocate the next parameter using gnutls_malloc(), because it is released via gnutls_free() by the TLS/IA handshake function.
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