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G_BIO(9)               FreeBSD Kernel Developer's Manual              G_BIO(9)

     g_new_bio, g_clone_bio, g_destroy_bio, g_print_bio, g_reset_bio - GEOM
     bio controlling functions

     #include <sys/bio.h>
     #include <geom/geom.h>

     struct bio *

     struct bio *

     struct bio *
     g_clone_bio(struct bio *bp);

     struct bio *
     g_duplicate_bio(struct bio *bp);

     g_destroy_bio(struct bio *bp);

     g_print_bio(struct bio *bp);

     g_reset_bio(struct bio *bp);

     A struct bio is used by GEOM to describe I/O requests, its most important
     fields are described below:

     bio_cmd            I/O request command.  There are four I/O requests
                        available in GEOM:

                        BIO_READ         A read request.

                        BIO_WRITE        A write request.

                        BIO_DELETE       Indicates that a certain range of
                                         data is no longer used and that it
                                         can be erased or freed as the
                                         underlying technology supports.
                                         Technologies like flash adaptation
                                         layers can arrange to erase the
                                         relevant blocks before they will
                                         become reassigned and cryptographic
                                         devices may want to fill random bits
                                         into the range to reduce the amount
                                         of data available for attack.

                        BIO_GETATTR      Inspect and manipulate out-of-band
                                         attributes on a particular provider
                                         or path.  Attributes are named by
                                         ascii strings and are stored in the
                                         bio_attribute field.

                        BIO_FLUSH        Tells underlying providers to flush
                                         their write caches.

     bio_flags          Available flags:

                        BIO_ERROR      Request failed (error value is stored
                                       in bio_error field).

                        BIO_DONE       Request finished.

     bio_cflags         Private use by the consumer.

     bio_pflags         Private use by the provider.

     bio_offset         Offset into provider.

     bio_data           Pointer to data buffer.

     bio_error          Error value when BIO_ERROR is set.

     bio_done           Pointer to function which will be called when the
                        request is finished.

     bio_driver1        Private use by the provider.

     bio_driver2        Private use by the provider.

     bio_caller1        Private use by the consumer.

     bio_caller2        Private use by the consumer.

     bio_attribute      Attribute string for BIO_GETATTR request.

     bio_from           Consumer to use for request (attached to provider
                        stored in bio_to field) (typically read-only for a

     bio_to             Destination provider (typically read-only for a

     bio_length         Request length in bytes.

     bio_completed      Number of bytes completed, but they may not be
                        completed from the front of the request.

     bio_children       Number of bio clones (typically read-only for a

     bio_inbed          Number of finished bio clones.

     bio_parent         Pointer to parent bio.

     The g_new_bio() function allocates a new, empty bio structure.

     g_alloc_bio() - same as g_new_bio(), but always succeeds (allocates bio
     with the M_WAITOK malloc flag).

     The g_clone_bio() function allocates a new bio structure and copies the
     following fields from the bio given as an argument to clone: bio_cmd,
     bio_length, bio_offset, bio_data, bio_attribute.  The field bio_parent in
     the clone points to the passed bio and the field bio_children in the
     passed bio is incremented.

     This function should be used for every request which enters through the
     provider of a particular geom and needs to be scheduled down.  Proper
     order is:

     1.   Clone the received struct bio.
     2.   Modify the clone.
     3.   Schedule the clone on its own consumer.

     g_duplicate_bio() - same as g_clone_bio(), but always succeeds (allocates
     bio with the M_WAITOK malloc flag).

     The g_destroy_bio() function deallocates and destroys the given bio

     The g_print_bio() function prints information about the given bio
     structure (for debugging purposes).

     The g_reset_bio() function resets the given bio structure back to its
     initial state.  g_reset_bio() preserves internal data structures, while
     setting all user visible fields to their initial values.  When reusing a
     bio obtained from g_new_bio(), g_alloc_bio(), g_clone_bio(), or
     g_duplicate_bio() for multiple transactions, g_reset_bio() must be called
     between the transactions in lieu of bzero().  While not strictly required
     for a bio structure created by other means, g_reset_bio() should be used
     to initialize it and between transactions.

     The g_new_bio() and g_clone_bio() functions return a pointer to the
     allocated bio, or NULL if an error occurred.

     Implementation of ``NULL-transformation'', meaning that an I/O request is
     cloned and scheduled down without any modifications.  Let us assume that
     field ex_consumer in structure example_softc contains a consumer attached
     to the provider we want to operate on.

           example_start(struct bio *bp)
                   struct example_softc *sc;
                   struct bio *cbp;

                   printf("Request received: ");

                   sc = bp->bio_to->geom->softc;
                   if (sc == NULL) {
                           g_io_deliver(bp, ENXIO);

                   /* Let's clone our bio request. */
                   cbp = g_clone_bio(bp);
                   if (cbp == NULL) {
                           g_io_deliver(bp, ENOMEM);
                   cbp->bio_done = g_std_done;     /* Standard 'done' function. */

                   /* Ok, schedule it down. */
                    * The consumer can be obtained from
                    * LIST_FIRST(&bp->bio_to->geom->consumer) as well,
                    * if there is only one in our geom.
                   g_io_request(cbp, sc->ex_consumer);

     geom(4), DECLARE_GEOM_CLASS(9), g_access(9), g_attach(9), g_consumer(9),
     g_data(9), g_event(9), g_geom(9), g_provider(9), g_provider_by_name(9),

     This manual page was written by Pawel Jakub Dawidek <[email protected]>.

FreeBSD 11.1-RELEASE-p4          May 17, 2016          FreeBSD 11.1-RELEASE-p4
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