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
  57     static int
  58     module_init (void)
  59     {
  60       hook_register(frr_late_init, module_late_init);
  61       return 0;
  62     }
  63
  64     FRR_MODULE_SETUP(
  65         .name = "my module",
  66         .version = "0.0",
  67         .description = "my module",
  68         .init = module_init,
  69     )
  70
  71 The ``frr_late_init`` hook will be called after the daemon has finished
  72 its other startup and is about to enter the main event loop; this is the
  73 best place for most initialisation.
  74
  75 Compiler & Linker magic
  76 -----------------------
  77
  78 There's a ``THIS_MODULE`` (like in the Linux kernel), which uses
  79 ``visibility`` attributes to restrict it to the current module. If you
  80 get a linker error with ``_frrmod_this_module``, there is some linker
  81 SNAFU. This shouldn't be possible, though one way to get it would be to
  82 not include libzebra (which provides a fallback definition for the
  83 symbol).
  84
  85 libzebra and the daemons each have their own ``THIS_MODULE``, as do all
  86 loadable modules. In any other libraries (e.g. ``libfrrsnmp``),
  87 ``THIS_MODULE`` will use the definition in libzebra; same applies if the
  88 main executable doesn't use ``FRR_DAEMON_INFO`` (e.g. all testcases).
  89
  90 The deciding factor here is "what dynamic linker unit are you using the
  91 symbol from." If you're in a library function and want to know who
  92 called you, you can't use ``THIS_MODULE`` (because that'll just tell you
  93 you're in the library). Put a macro around your function that adds
  94 ``THIS_MODULE`` in the *caller's code calling your function*.
  95
  96 The idea is to use this in the future for module unloading. Hooks
  97 already remember which module they were installed by, as groundwork for
  98 a function that removes all of a module's installed hooks.
  99
 100 There's also the ``frr_module`` symbol in modules, pretty much a
 101 standard entry point for loadable modules.
 102
 103 Hooks
 104 -----
 105
 106 Hooks are just points in the code where you can register your callback
 107 to be called. The parameter list is specific to the hook point. Since
 108 there is no stable API, the hook code has some extra type safety checks
 109 making sure you get a compiler warning when the hook parameter list
 110 doesn't match your callback. Don't ignore these warnings.
 111
 112 Relation to MTYPE macros
 113 ------------------------
 114
 115 The MTYPE macros, while primarily designed to decouple MTYPEs from the
 116 library and beautify the code, also work very nicely with loadable
 117 modules -- both constructors and destructors are executed when
 118 loading/unloading modules.
 119
 120 This means there is absolutely no change required to MTYPEs, you can
 121 just use them in a module and they will even clean up themselves when we
 122 implement module unloading and an unload happens. In fact, it's
 123 impossible to create a bug where unloading fails to de-register a MTYPE.