G_ACCESS(9)

💅 G_ACCESS(9) 💅 — control access to GEOM consumers and their providers

okay bestie, welcome to the Kernel Developer's Manual page for g_access — the function that opens, closes, and generally changes access to the provider sitting behind a consumer. This is kernel/GEOM territory, so hydrate first. 🧚

🌸 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 🦋

okay so g_access() lets you open, close, and generally change access to the provider that's attached to the given consumer cp. The arguments dcr, dcw, and dce are relative read, write, and exclusive access count changes. Read and write counts are exactly what they sound like; 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 you attach a consumer to a provider with g_attach(9), you have to call g_access() on that consumer before starting I/O requests. no cap, that's the order. 🫶

🚧 RESTRICTIONS/CONDITIONS 🚧

bestie, these are not optional vibes — check them before you call:

• 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 🎀

g_access() 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>. 💌

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mwah 💋 this page is from Debian, dated January 16, 2004 — G_ACCESS(9) out. 💅✨