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+.. _modules:
+
+Modules
+=======
+
+FRR has facilities to load DSOs at startup via ``dlopen()``. These are used to
+implement modules, such as SNMP and FPM.
+
+Limitations
+-----------
+
+-  can't load, unload, or reload during runtime. This just needs some
+   work and can probably be done in the future.
+-  doesn't fix any of the "things need to be changed in the code in the
+   library" issues. Most prominently, you can't add a CLI node because
+   CLI nodes are listed in the library...
+-  if your module crashes, the daemon crashes. Should be obvious.
+-  **does not provide a stable API or ABI**. Your module must match a
+   version of FRR and you may have to update it frequently to match
+   changes.
+-  **does not create a license boundary**. Your module will need to link
+   libzebra and include header files from the daemons, meaning it will
+   be GPL-encumbered.
+
+Installation
+------------
+
+Look for ``moduledir`` in ``configure.ac``, default is normally
+``/usr/lib64/frr/modules`` but depends on ``--libdir`` / ``--prefix``.
+
+The daemon's name is prepended when looking for a module, e.g. "snmp"
+tries to find "zebra\_snmp" first when used in zebra. This is just to
+make it nicer for the user, with the snmp module having the same name
+everywhere.
+
+Modules can be packaged separately from FRR. The SNMP and FPM modules
+are good candidates for this because they have dependencies (net-snmp /
+protobuf) that are not FRR dependencies. However, any distro packages
+should have an "exact-match" dependency onto the FRR package. Using a
+module from a different FRR version will probably blow up nicely.
+
+For snapcraft (and during development), modules can be loaded with full
+path (e.g. -M ``$SNAP/lib/frr/modules/zebra_snmp.so``). Note that
+libtool puts output files in the .libs directory, so during development
+you have to use ``./zebra -M .libs/zebra_snmp.so``.
+
+Creating a module
+-----------------
+
+... best to look at the existing SNMP or FPM modules.
+
+Basic boilerplate:
+
+::
+
+    #include "hook.h"
+    #include "module.h"
+    #include "libfrr.h"
+    #include "frrevent.h"
+
+    static int module_late_init(struct event_loop *master)
+    {
+        /* Do initialization stuff here */
+        return 0;
+    }
+
+    static int
+    module_init (void)
+    {
+      hook_register(frr_late_init, module_late_init);
+      return 0;
+    }
+
+    FRR_MODULE_SETUP(
+        .name = "my module",
+        .version = "0.0",
+        .description = "my module",
+        .init = module_init,
+    );
+
+The ``frr_late_init`` hook will be called after the daemon has finished
+its other startup and is about to enter the main event loop; this is the
+best place for most initialisation.
+
+Compiler & Linker magic
+-----------------------
+
+There's a ``THIS_MODULE`` (like in the Linux kernel), which uses
+``visibility`` attributes to restrict it to the current module. If you
+get a linker error with ``_frrmod_this_module``, there is some linker
+SNAFU. This shouldn't be possible, though one way to get it would be to
+not include libzebra (which provides a fallback definition for the
+symbol).
+
+libzebra and the daemons each have their own ``THIS_MODULE``, as do all
+loadable modules. In any other libraries (e.g. ``libfrrsnmp``),
+``THIS_MODULE`` will use the definition in libzebra; same applies if the
+main executable doesn't use ``FRR_DAEMON_INFO`` (e.g. all testcases).
+
+The deciding factor here is "what dynamic linker unit are you using the
+symbol from." If you're in a library function and want to know who
+called you, you can't use ``THIS_MODULE`` (because that'll just tell you
+you're in the library). Put a macro around your function that adds
+``THIS_MODULE`` in the *caller's code calling your function*.
+
+The idea is to use this in the future for module unloading. Hooks
+already remember which module they were installed by, as groundwork for
+a function that removes all of a module's installed hooks.
+
+There's also the ``frr_module`` symbol in modules, pretty much a
+standard entry point for loadable modules.
+
+Command line parameters
+-----------------------
+
+Command line parameters can be passed directly to a module by appending a
+colon to the module name when loading it, e.g. ``-M mymodule:myparameter``.
+The text after the colon will be accessible in the module's code through
+``THIS_MODULE->load_args``. For example, see how the format parameter is
+configured in the ``zfpm_init()`` function inside ``zebra_fpm.c``.
+
+Hooks
+-----
+
+Hooks are just points in the code where you can register your callback
+to be called. The parameter list is specific to the hook point. Since
+there is no stable API, the hook code has some extra type safety checks
+making sure you get a compiler warning when the hook parameter list
+doesn't match your callback. Don't ignore these warnings.
+
+Relation to MTYPE macros
+------------------------
+
+The MTYPE macros, while primarily designed to decouple MTYPEs from the
+library and beautify the code, also work very nicely with loadable
+modules -- both constructors and destructors are executed when
+loading/unloading modules.
+
+This means there is absolutely no change required to MTYPEs, you can
+just use them in a module and they will even clean up themselves when we
+implement module unloading and an unload happens. In fact, it's
+impossible to create a bug where unloading fails to de-register a MTYPE.