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There are several cases where GnuTLS may need some out of band input from your program. This is now implemented using some callback functions, which your program is expected to register.
An example of this type of functions are the push and pull callbacks which are used to specify the functions that will retrieve and send data to the transport layer.
Other callback functions such as the one set by gnutls_srp_set_server_credentials_function, may require more complicated input, including data to be allocated. These callbacks should allocate and free memory using the functions shown below.