g_access (9)
Leading comments
Copyright (c) 2004 Pawel Jakub Dawidek <pjd@FreeBSD.org> All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the d...
NAME
g_access - control access to GEOM consumers and their providersSYNOPSIS
In geom/geom.h Ft int Fn g_access struct g_consumer *cp int dcr int dcw int dceDESCRIPTION
The Fn g_access function allows to open, close, and generally change access to the provider which is attached to the given consumer Fa cp . The arguments Fa dcr , Fa dcw , and Fa 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_attach9, the Fn 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 Fa ( dcr = Fa dcw = Fa 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 Fn g_access function returns 0 if successful; otherwise an error code is returned. Note that Fn g_access cannot fail when the arguments Fa dcr , Fa dcw , and Fa 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:- Bq Er EPERM
- The function is trying to open a provider with an exclusive access count, but it is already open for writing.
- Bq Er 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.