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DECLARE_GEOM_CLASSGEOM class declaration macro


#include <geom/geom.h>

DECLARE_GEOM_CLASS(class, mod_name);


The () 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 registration.

The arguments to () are:

The g_class structure which describes a GEOM class.
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 field.
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 <>.

August 13, 2007 Debian