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     DECLARE_GEOM_CLASS — GEOM class declaration macro


     #include <geom/geom.h>

     DECLARE_GEOM_CLASS(class, mod_name);


     The DECLARE_GEOM_CLASS() macro registers a GEOM class in GEOM.  A GEOM class itself
     implements one particular kind of transformation.  Typical examples are: MBR disk partition,
     BSD disklabel and RAID5 classes.  DECLARE_GEOM_CLASS() can be used both for compiled in and
     loaded as kld(4) modules GEOM classes and it is the only official way for class

     The arguments to DECLARE_GEOM_CLASS() are:

           class         The g_class structure which describes a GEOM class.

           mod_name      A kernel module name (not a class name!).

     Structure g_class contains data describing the class.  They are:

           const char * name
                   Class name.

           g_taste_t * taste
                   Pointer to function used for taste event handling.  If it is non-NULL it is
                   called in three situations:

                   -   On class activation, all existing providers are offered for tasting.
                   -   When new provider is created it is offered for tasting.
                   -   After last write access to a provider is closed it is offered for
                       retasting (on first write open event “spoil” was sent).

           g_config_t * config
                   This field is not used anymore, its functionality was replaced by the ctlreq

           g_ctl_req_t * ctlreq
                   Pointer to function used for handling events from userland applications.

           g_init_t * init
                   Pointer to function which is called right after class registration.

           g_fini_t * fini
                   Pointer to function which is called right before class deregistration.

           g_ctl_destroy_geom_t * destroy_geom
                   Pointer to a function which is called for every geom on class unload.  If this
                   field is not set, the class can not be unloaded.

     Only a name field is required; the rest are optional.


     The fields of g_class should always be initialized using C99-style field naming (see the
     initialization of example_class below).


     Example class declaration.

           static struct g_geom *
           g_example_taste(struct g_class *mp, struct g_provider *pp,
               int flags __unused)


           static void
           g_example_ctlreq(struct gctl_req *req, struct g_class *cp,
               char const *verb)


           static int
           g_example_destroy_geom(struct gctl_req *req, struct g_class *cp,
               struct g_geom *gp)



           static void
           g_example_init(struct g_class *mp)


           static void
           g_example_fini(struct g_class *mp)


           struct g_class example_class = {
                   .name = "EXAMPLE",
                   .taste = g_example_taste,
                   .ctlreq = g_example_ctlreq,
                   .init = g_example_init,
                   .fini = g_example_fini,
                   .destroy_geom = g_example_destroy_geom

           DECLARE_GEOM_CLASS(example_class, g_example);


     geom(4), g_attach(9), g_bio(9), g_consumer(9), g_data(9), g_event(9), g_geom(9),
     g_provider(9), g_provider_by_name(9), g_wither_geom(9)


     This manual page was written by Pawel Jakub Dawidek <>.