166 lines
4.5 KiB
Groff
166 lines
4.5 KiB
Groff
.\"
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.\" Copyright (c) 2004 Pawel Jakub Dawidek <pjd@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 16, 2004
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.Dt G_ACCESS 9
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.Os
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.Sh NAME
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.Nm g_access
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.Nd "control access to GEOM consumers and their providers"
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.Sh SYNOPSIS
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.In geom/geom.h
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.Ft int
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.Fn g_access "struct g_consumer *cp" "int dcr" "int dcw" "int dce"
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.Sh DESCRIPTION
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The
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.Fn g_access
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function allows to open, close, and generally change access to the provider
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which is attached to the given consumer
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.Fa cp .
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The arguments
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.Fa dcr ,
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.Fa dcw ,
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and
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.Fa dce
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represent relative read, write, and exclusive access count changes.
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Read and write access counts are self explanatory, and
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exclusive access counts deny write access to other interested parties.
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A provider's access count is the sum of the access counts of all
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attached consumers.
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.Pp
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After attaching a consumer to a provider with
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.Xr g_attach 9 ,
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the
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.Fn g_access
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function has to be called on the consumer before starting I/O requests.
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.Sh RESTRICTIONS/CONDITIONS
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The consumer has to be attached to a provider.
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.Pp
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The intended change must not result in a negative access count.
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.Pp
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No-operation is not permitted
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.Fa ( dcr
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=
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.Fa dcw
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=
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.Fa dce
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=
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.Li 0 ) .
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.Pp
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The provider's geom must have an access method defined (e.g.,
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.Va gp->access ) .
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.Pp
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The topology lock has to be held.
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.Sh RETURN VALUES
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The
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.Fn g_access
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function returns 0 if successful; otherwise an error code is returned.
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Note that
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.Fn g_access
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cannot fail when the arguments
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.Fa dcr ,
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.Fa dcw ,
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and
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.Fa dce
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are less than or equal to 0.
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.Sh EXAMPLES
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Create a consumer, attach it to a given provider, gain read access and
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read first sector.
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.Bd -literal -offset indent
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void
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some_function(struct g_geom *mygeom, struct g_provider *pp)
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{
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struct g_consumer *cp;
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void *ptr;
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int error;
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g_topology_assert();
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/* Create new consumer on 'mygeom' geom. */
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cp = g_new_consumer(mygeom);
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/* Attach newly created consumer to given provider. */
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if (g_attach(cp, pp) != 0) {
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g_destroy_consumer(cp);
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return;
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}
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/* Open provider for reading through our consumer. */
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error = g_access(cp, 1, 0, 0);
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if (error != 0) {
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printf("Cannot access provider: %s\\n", error);
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g_detach(cp);
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g_destroy_consumer(cp);
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return;
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}
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/*
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* Don't hold topology lock while reading.
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*/
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g_topology_unlock();
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ptr = g_read_data(cp, 0, pp->sectorsize, &error);
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if (ptr == NULL)
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printf("Error while reading: %d\\n", error);
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/*
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* Do something useful with data.
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*/
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g_topology_lock();
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/* Disconnect from provider (release access count). */
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g_access(cp, -1, 0, 0);
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/* Detach from provider. */
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g_detach(cp);
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/* Destroy consumer. */
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g_destroy_consumer(cp);
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}
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.Ed
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.Sh ERRORS
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Possible errors:
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.Bl -tag -width Er
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.It Bq Er EPERM
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The function is trying to open a provider with an exclusive access count, but
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it is already open for writing.
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.It Bq Er EPERM
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The function is trying to open a provider for writing, but it is already
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exclusively open.
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.El
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.Pp
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Any other error that can be returned by the provider's access method.
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.Sh SEE ALSO
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.Xr geom 4 ,
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.Xr DECLARE_GEOM_CLASS 9 ,
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.Xr g_attach 9 ,
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.Xr g_bio 9 ,
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.Xr g_consumer 9 ,
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.Xr g_data 9 ,
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.Xr g_event 9 ,
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.Xr g_geom 9 ,
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.Xr g_provider 9 ,
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.Xr g_provider_by_name 9 ,
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.Xr g_wither_geom 9
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.Sh AUTHORS
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.An -nosplit
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This manual page was written by
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.An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org .
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