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 consumer
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
interested 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 arguments
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 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);
}
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 access 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⟩.