Provided by: freebsd-manpages_12.2-1_all 
NAME
g_new_bio, g_clone_bio, g_destroy_bio, g_print_bio, g_reset_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);
void
g_reset_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 five 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).
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.
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->consumer) 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>.
Debian Mar 7, 2018 G_BIO(9)