g_access(9)

NAME

g_access - control access to GEOM consumers and their

providers

SYNOPSIS

#include <geom/geom.h>
int
g_access(struct g_consumer *cp, int dcr, int dcw, int dce);

DESCRIPTION

The g_access() function allows to open, close, and generally

change
access to the provider which is attached to the given con

sumer cp. The
arguments dcr, dcw, and 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 in

terested parties. A provider's access count is the sum of the access

counts of all
attached consumers.

After attaching a consumer to a provider with g_attach(9),

the g_access()
function has to be called on the consumer before starting

I/O requests.

RESTRICTIONS/CONDITIONS

The consumer has to be attached to a provider.

The intended change must not result in a negative access

count.

No-operation is not permitted (dcr = dcw = dce = 0).

The provider's geom must have an access method defined

(e.g.,
gp->access).

The topology lock has to be held.

RETURN VALUES

The g_access() function returns 0 if successful; otherwise

an error code
is returned. Note that g_access() cannot fail when the ar

guments dcr,
dcw, and dce are less than or equal to 0.

EXAMPLES

Create a consumer, attach it to a given provider, gain read

access and
read first sector.

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 con

sumer. */
error = g_access(cp, 1, 0, 0);
if (error != 0) {

printf("Cannot access provider: %s0,

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, &er

ror);
if (ptr == NULL)

printf("Error while reading: %d0, er

ror);

/*

* 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);

}

ERRORS

Possible errors:

[EPERM] The function is trying to open a provider

with an

exclusive access count, but it is already

open for
writing.

[EPERM] The function is trying to open a provider

for writing,

but it is already exclusively open.

Any other error that can be returned by the provider's ac

cess method.

SEE ALSO

geom(4), DECLARE_GEOM_CLASS(9), 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)

AUTHORS

This manual page was written by Pawel Jakub Dawidek

<pjd@FreeBSD.org>.

BSD January 16, 2004

BSD