Declare the client-side plugin descriptor with the mysql_declare_client_plugin() and mysql_end_client_plugin macros (see Section 4.4.2.3, “Client Plugin Descriptors”). For the auth_simple plugin, the descriptor looks like this:

mysql_declare_client_plugin(AUTHENTICATION)
  "auth_simple",                        /* plugin name */
  "Author Name",                        /* author */
  "Any-password authentication plugin", /* description */
  {1,0,0},                              /* version = 1.0.0 */
  "GPL",                                /* license type */
  NULL,                                 /* for internal use */
  NULL,                                 /* no init function */
  NULL,                                 /* no deinit function */
  NULL,                                 /* no option-handling function */
  auth_simple_client                    /* main function */
mysql_end_client_plugin;

The descriptor members from the plugin name through the option-handling function are common to all client plugin types. (For descriptions, see Section 4.4.2.3, “Client Plugin Descriptors”.) Following the common members, the descriptor has an additional member specific to authentication plugins. This is the “main” function, which handles communication with the server. The function takes two arguments representing an I/O structure and a connection handler. For our simple any-password plugin, the main function does nothing but write to the server the password provided by the user:

static int auth_simple_client (MYSQL_PLUGIN_VIO *vio, MYSQL *mysql)
{
  int res;

  /* send password as null-terminated string as cleartext */
  res= vio->write_packet(vio, (const unsigned char *) mysql->passwd,
                         strlen(mysql->passwd) + 1);

  return res ? CR_ERROR : CR_OK;
}

The main function should return one of the error codes shown in the following table.

CR_OK_HANDSHAKE_COMPLETE indicates that the client has done its part successfully and has read the last packet. A client plugin may return CR_OK_HANDSHAKE_COMPLETE if the number of round trips in the authentication protocol is not known in advance and the plugin must read another packet to determine whether authentication is finished.