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.Dd January 16, 2004
.Dt G_ACCESS 9
.Os
.Sh NAME
.Nm g_access
.Nd "control access to GEOM consumers and their providers"
.Sh SYNOPSIS
.In geom/geom.h
.Ft int
.Fn g_access "struct g_consumer *cp" "int dcr" "int dcw" "int dce"
.Sh DESCRIPTION
The
.Fn g_access
function allows to open, close, and generally change access to the provider
which is attached to the given consumer
.Fa cp .
The arguments
.Fa dcr ,
.Fa dcw ,
and
.Fa dce
represent relative read, write, and exclusive access count changes.
Read and write access counts are self explanatory, and
exclusive access counts deny write access to other interested parties.
A provider's access count is the sum of the access counts of all
attached consumers.
.Pp
After attaching a consumer to a provider with
.Xr g_attach 9 ,
the
.Fn g_access
function has to be called on the consumer before starting I/O requests.
.Sh RESTRICTIONS/CONDITIONS
The consumer has to be attached to a provider.
.Pp
The intended change must not result in a negative access count.
.Pp
No-operation is not permitted
.Fa ( dcr
=
.Fa dcw
=
.Fa dce
=
.Li 0 ) .
.Pp
The provider's geom must have an access method defined (e.g.,
.Va gp->access ) .
.Pp
The topology lock has to be held.
.Sh RETURN VALUES
The
.Fn g_access
function returns 0 if successful; otherwise an error code is returned.
Note that
.Fn g_access
cannot fail when the arguments
.Fa dcr ,
.Fa dcw ,
and
.Fa dce
are less than or equal to 0.
.Sh EXAMPLES
Create a consumer, attach it to a given provider, gain read access and
read first sector.
.Bd -literal -offset indent
void
some_function(struct g_geom *mygeom, struct g_provider *pp)
{
	struct g_consumer *cp;
	void *ptr;
	int error;

	g_topology_assert();

	/* Create new consumer on 'mygeom' geom. */
	cp = g_new_consumer(mygeom);
	/* Attach newly created consumer to given provider. */
	if (g_attach(cp, pp) != 0) {
		g_destroy_consumer(cp);
		return;
	}
	/* Open provider for reading through our consumer. */
	error = g_access(cp, 1, 0, 0);
	if (error != 0) {
		printf("Cannot access provider: %s\\n", error);
		g_detach(cp);
		g_destroy_consumer(cp);
		return;
	}

	/*
	 * Don't hold topology lock while reading.
	 */
	g_topology_unlock();
	ptr = g_read_data(cp, 0, pp->sectorsize, &error);
	if (ptr == NULL)
		printf("Error while reading: %d\\n", error);
	/*
	 * Do something useful with data.
	 */
	g_topology_lock();

	/* Disconnect from provider (release access count). */
	g_access(cp, -1, 0, 0);
	/* Detach from provider. */
	g_detach(cp);
	/* Destroy consumer. */
	g_destroy_consumer(cp);
}
.Ed
.Sh ERRORS
Possible errors:
.Bl -tag -width Er
.It Bq Er EPERM
The function is trying to open a provider with an exclusive access count, but
it is already open for writing.
.It Bq Er EPERM
The function is trying to open a provider for writing, but it is already
exclusively open.
.El
.Pp
Any other error that can be returned by the provider's access method.
.Sh SEE ALSO
.Xr geom 4 ,
.Xr DECLARE_GEOM_CLASS 9 ,
.Xr g_attach 9 ,
.Xr g_bio 9 ,
.Xr g_consumer 9 ,
.Xr g_data 9 ,
.Xr g_event 9 ,
.Xr g_geom 9 ,
.Xr g_provider 9 ,
.Xr g_provider_by_name 9 ,
.Xr g_wither_geom 9
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Pawel Jakub Dawidek Aq pjd@FreeBSD.org .