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>.

Debian                                          January 16, 2004                                     G_ACCESS(9)