doc/developer/modules.rst

   1 Modules
   2 =======
   3
   4 FRR has facilities to load DSOs at startup via ``dlopen()``. These are used to
   5 implement modules, such as SNMP and FPM.
   6
   7 Limitations
   8 -----------
   9
  10 -  can't load, unload, or reload during runtime. This just needs some
  11    work and can probably be done in the future.
  12 -  doesn't fix any of the "things need to be changed in the code in the
  13    library" issues. Most prominently, you can't add a CLI node because
  14    CLI nodes are listed in the library...
  15 -  if your module crashes, the daemon crashes. Should be obvious.
  16 -  **does not provide a stable API or ABI**. Your module must match a
  17    version of FRR and you may have to update it frequently to match
  18    changes.
  19 -  **does not create a license boundary**. Your module will need to link
  20    libzebra and include header files from the daemons, meaning it will
  21    be GPL-encumbered.
  22
  23 Installation
  24 ------------
  25
  26 Look for ``moduledir`` in ``configure.ac``, default is normally
  27 ``/usr/lib64/frr/modules`` but depends on ``--libdir`` / ``--prefix``.
  28
  29 The daemon's name is prepended when looking for a module, e.g. "snmp"
  30 tries to find "zebra\_snmp" first when used in zebra. This is just to
  31 make it nicer for the user, with the snmp module having the same name
  32 everywhere.
  33
  34 Modules can be packaged separately from FRR. The SNMP and FPM modules
  35 are good candidates for this because they have dependencies (net-snmp /
  36 protobuf) that are not FRR dependencies. However, any distro packages
  37 should have an "exact-match" dependency onto the FRR package. Using a
  38 module from a different FRR version will probably blow up nicely.
  39
  40 For snapcraft (and during development), modules can be loaded with full
  41 path (e.g. -M ``$SNAP/lib/frr/modules/zebra_snmp.so``). Note that
  42 libtool puts output files in the .libs directory, so during development
  43 you have to use ``./zebra -M .libs/zebra_snmp.so``.
  44
  45 Creating a module
  46 -----------------
  47
  48 ... best to look at the existing SNMP or FPM modules.
  49
  50 Basic boilerplate:
  51
  52 ::
  53
  54     #include "hook.h"
  55     #include "module.h"
  56     #include "libfrr.h"
  57     #include "thread.h"
  58
  59     static int module_late_init(struct thread_master *master)
  60     {
  61         /* Do initialization stuff here */
  62         return 0;
  63     }
  64
  65     static int
  66     module_init (void)
  67     {
  68       hook_register(frr_late_init, module_late_init);
  69       return 0;
  70     }
  71
  72     FRR_MODULE_SETUP(
  73         .name = "my module",
  74         .version = "0.0",
  75         .description = "my module",
  76         .init = module_init,
  77     )
  78
  79 The ``frr_late_init`` hook will be called after the daemon has finished
  80 its other startup and is about to enter the main event loop; this is the
  81 best place for most initialisation.
  82
  83 Compiler & Linker magic
  84 -----------------------
  85
  86 There's a ``THIS_MODULE`` (like in the Linux kernel), which uses
  87 ``visibility`` attributes to restrict it to the current module. If you
  88 get a linker error with ``_frrmod_this_module``, there is some linker
  89 SNAFU. This shouldn't be possible, though one way to get it would be to
  90 not include libzebra (which provides a fallback definition for the
  91 symbol).
  92
  93 libzebra and the daemons each have their own ``THIS_MODULE``, as do all
  94 loadable modules. In any other libraries (e.g. ``libfrrsnmp``),
  95 ``THIS_MODULE`` will use the definition in libzebra; same applies if the
  96 main executable doesn't use ``FRR_DAEMON_INFO`` (e.g. all testcases).
  97
  98 The deciding factor here is "what dynamic linker unit are you using the
  99 symbol from." If you're in a library function and want to know who
 100 called you, you can't use ``THIS_MODULE`` (because that'll just tell you
 101 you're in the library). Put a macro around your function that adds
 102 ``THIS_MODULE`` in the *caller's code calling your function*.
 103
 104 The idea is to use this in the future for module unloading. Hooks
 105 already remember which module they were installed by, as groundwork for
 106 a function that removes all of a module's installed hooks.
 107
 108 There's also the ``frr_module`` symbol in modules, pretty much a
 109 standard entry point for loadable modules.
 110
 111 Command line parameters
 112 -----------------------
 113
 114 Command line parameters can be passed directly to a module by appending a
 115 colon to the module name when loading it, e.g. ``-M mymodule:myparameter``.
 116 The text after the colon will be accessible in the module's code through
 117 ``THIS_MODULE->load_args``. For example, see how the format parameter is
 118 configured in the ``zfpm_init()`` function inside ``zebra_fpm.c``.
 119
 120 Hooks
 121 -----
 122
 123 Hooks are just points in the code where you can register your callback
 124 to be called. The parameter list is specific to the hook point. Since
 125 there is no stable API, the hook code has some extra type safety checks
 126 making sure you get a compiler warning when the hook parameter list
 127 doesn't match your callback. Don't ignore these warnings.
 128
 129 Relation to MTYPE macros
 130 ------------------------
 131
 132 The MTYPE macros, while primarily designed to decouple MTYPEs from the
 133 library and beautify the code, also work very nicely with loadable
 134 modules -- both constructors and destructors are executed when
 135 loading/unloading modules.
 136
 137 This means there is absolutely no change required to MTYPEs, you can
 138 just use them in a module and they will even clean up themselves when we
 139 implement module unloading and an unload happens. In fact, it's
 140 impossible to create a bug where unloading fails to de-register a MTYPE.