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NAME

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

SYNOPSIS

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

     struct bio *
     g_new_bio(void);

     struct bio *
     g_alloc_bio(void);

     struct bio *
     g_clone_bio(struct bio *bp);

     struct bio *
     g_duplicate_bio(struct bio *bp);

     void
     g_destroy_bio(struct bio *bp);

     void
     g_print_bio(struct bio *bp);

DESCRIPTION

     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 class).

     bio_to         Destination provider (typically read-only for a class).

     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 class).

     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
     structure.

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

RETURN VALUES

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

EXAMPLES

     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.

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

                   printf("Request received: ");
                   g_print_bio(bp);
                   printf("\n");

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

                   /* Let’s clone our bio request. */
                   cbp = g_clone_bio(bp);
                   if (cbp == NULL) {
                           g_io_deliver(bp, ENOMEM);
                           return;
                   }
                   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->consumers) as well,
                    * if there is only one in our geom.
                    */
                   g_io_request(cbp, sc->ex_consumer);
           }

SEE ALSO

     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),
     g_wither_geom(9)

AUTHORS

     This manual page was written by Pawel Jakub Dawidek 〈pjd@FreeBSD.org〉.