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G_ACCESS(9)		    Kernel Developer's Manual		     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 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  ex-
     clusive  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 exclu-
			sive 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>.

FreeBSD ports 15.quarterly	January 16, 2004		     G_ACCESS(9)

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<https://man.freebsd.org/cgi/man.cgi?query=g_access&sektion=9&manpath=FreeBSD+15.1-RELEASE+and+Ports.quarterly>

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