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Man Pages
G_ACCESS(9) FreeBSD Kernel Developer's Manual G_ACCESS(9)

g_access
control access to GEOM consumers and their providers

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

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.

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.

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.

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); 
}

Possible errors:
[]
The function is trying to open a provider with an exclusive access count, but it is already open for writing.
[]
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.

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)

This manual page was written by Pawel Jakub Dawidek <pjd@FreeBSD.org>.
January 16, 2004 FreeBSD 12.0-RELEASE

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