6   Plugins

Plugins are the “glue” that binds the underlying system to Clixon and where application semantics is added by an application developer.

The backend, CLI, Restconf and Netconf applications may use plugins, although the main use of plugins are the Backend section plugins.

Plugins are written in C as dynamically loaded modules (.so files). At startup, the application looks in the assigned directory and loads all files with .so suffixes from that dir in alphabetical order.

Plugins are located as follows:

  • Backend plugins are are in CLICON_BACKEND_DIR
  • CLI plugins are are in CLICON_CLI_DIR
  • RESTCONF plugins are in CLICON_CLI_DIR;

For example, to load all backend plugins from: /usr/local/lib/example/backend:

Callbacks are registered in two ways:
  • clixon_plugin_init : Return an API struct containing a fixed set of callbacks
  • register functions : Register callback functions for more complex interaction

Further, CLI callbacks as described in CLI section are special in the way that they are invoked from CLIgen specifications, not from the application itself.

6.1   Clixon_plugin_init

On startup, the application loads each plugin and calls clixon_plugin_init in that plugin. The function is expected to return an API struct clixon_plugin_api defining a set of static callbacks. The init function may return NULL in which case it is logged and ignored.

Once the plugin is loaded, it awaits callbacks from the application.

An example of a minimal plugin is as follows:

static clixon_plugin_api api = {
  "example",                              /* Plugin name */
  clixon_plugin_init,                     /* init - must be called clixon_plugin_init */
  example_start,                          /* start */
  example_exit,                           /* exit */
  example_extension,                      /* yang extensions */
  .ca_daemon=example_daemon,              /* Plugin daemonized () (backend only) */

clixon_plugin_api *
clixon_plugin_init(clicon_handle h)
   return &api;

First the clixon_plugin_api struct defines the callbacks. There are two blockx of callbacks:

  • Common callbacks for backend, cli, netconf, and restconf
  • Application-specific callbacks. In the example, only ca_daemon is specific to backends. By far, the backend have most specific callbacks.

Second, the clixon_plugin_init() function is defined and (at a minimum) returns the struct.

The following callbacks are common plugins for all Clixon applications:

Clixon plugin init function, called immediately after plugin is loaded into the application. The name of the function must be called clixon_plugin_init. It returns a struct with the name of the plugin, and all other callback names.
Called when application is started and initialization is complete, but before the application is placed in the background and privileges are dropped (if applicable)
Called just before plugin is unloaded at exit
Called at parsing of yang modules containing an extension statement. A plugin may identify the extension by its name, and perform actions on the yang statement, such as transforming the yang in-memory. A callback is made for every statement, which means that several calls per extension can be made. See Misc section and the main example on how to use extensions.

See Backend section for backend specific callbacks, as well as corresponding manual sections for netconf/restconf/cli callbacks.

6.2   Registered callbacks

A second group of callbacks use register functions. This is a more detailed mechanism than the fixed callbacks described previously, but are only defined to a limited sets of functions:

  • rpc_callback_register() - for user-defined RPC callbacks. Applicable for NETCONF, RESTCONF and backend.
  • action_callback_register() - for user-defined Action callbacks. Applicable for backend.
  • upgrade_callback_register() - for upgrading, see Upgrade section. Applicable only for backend.
  • clixon_pagination_cb_register() - for pagination, as described in Pagination section. Applicable only for backend.

A user may register may register a callback for an incoming RPC, and that function will be called.

There may be several callbacks for the same RPC. The order the callbacks are registered are as follows:

  1. plugin_init
  2. backend_rpc_init (where system callbacks are registered)
  3. plugin_start

Which means if you register a copy-config callback in (1), it will be called before the system copy-config callback registered from (2) backend_rpc_init. If you register a copy-config in (3) plugin-start it will be called after the system copy-config.

Second, if there are more than one reply (eg <rpc-reply/><rpc-reply/>) only the first reply will be parsed and used by the cli/netconf/restconf clients.

If you want to take the original and modify it, you should therefore register the callback in plugin_start (3) so that your callback will be called after the system RPC. Then you should modify the original reply (not add a new reply).

6.2.1   Example: RPC callback

This example shows how to define a new RPC in YANG for the backend, register a callback function in C, read and write a parameter. It is revised slightly from the main example.


module clixon-example {
  namespace "urn:example:clixon";
  rpc example {
      input {
          leaf x {
            type string;
      output {
          leaf y {
            type string;

Register RPC in clixon_plugin_init():

clixon_plugin_api *clixon_plugin_init(clicon_handle h)
   rpc_callback_register(h, example_rpc, NULL, "urn:example:clixon", "example");

Callback function reading value input x, modifying value and writing it as output value y:

static int
example_rpc(clicon_handle h,            /* Clicon handle */
            cxobj        *xe,           /* Request: <rpc><xn></rpc> */
            cbuf         *cbret,        /* Reply eg <rpc-reply>... */
            void         *arg,          /* client_entry */
            void         *regarg)       /* Argument given at register */
    char *val;
    val = xml_find_body(xe, "x");       /* Read x value of incoming rpc */
    cprintf(cbret, "<rpc-reply xmlns=\"%s\">", NETCONF_BASE_NAMESPACE);
    val[0]++;                           /* Increment first char */
    /* Construct reply */
    cprintf(cbret, "<y xmlns=\"urn:example:clixon\">%s</y>", val);
    cprintf(cbret, "</rpc-reply>");

Result netconf session:

<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="42">
   <example xmlns="urn:example:clixon">
<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="42">
   <y xmlns="urn:example:clixon">52</y>

6.2.2   Example: Action callback

This example follows RFC 7950 7.15.3.

An action is associated with a YANG node and can therefore not be registered at init, instead the start callback can be used, for example.

Register Action in example_reset():

example_start(clicon_handle h)
   if (action_callback_register(h, ya, example_action_reset, NULL) < 0)
     goto done;

6.3   Plugin callback guidelines


This information is important to understand for the stability of clixon

The Clixon programs run as non-blocking single-threaded applications. It calls functions from within dynamically loaded modules. The callback code must be written with this programming model in mind. The behavior of the callback directly impacts the behavior of the caller and the whole system.

The most serious effect is when crash within a callback happens. This will cause the whole program to crash.

A more subtle problem is the environment of the program. Clixon will configure the environment, and it expects that the callback will return with the exact same environment intact. If you change a signal handler, a terminal configuration, etc. you must restore the state as it was on entry. Failure to do this can cause problems that are difficult to isolate and fix.

A list of things to watch out for (but not complete!):

  • a crash in the plugin
  • change of signal behaviour, such as blocking or assigning signal handlers
  • change of terminal settings (for CLI callbacks)
  • change of process privileges
  • asynchronous calls
  • If you fork or create threads, ensure the main program continues uninterrupted

The following config option is related to checking callbacks:

Enable check of resources before and after each callback. Checks are currently limited to signal and terminal settings