Function signal_context_set_log_function 

pub unsafe extern "C" fn signal_context_set_log_function(
    context: *mut signal_context,
    log: Option<unsafe extern "C" fn(level: c_int, message: *const c_char, len: usize, user_data: *mut c_void)>,
) -> c_int

Set the log function to be used by the Signal Protocol library for logging.

@return 0 on success, negative on failure