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

g_new_bio, g_clone_bio, g_destroy_bio, g_format_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_format_bio(struct sbuf *sb, const struct bio *bp);

g_print_bio(struct sbuf *sb, const char *prefix, const struct bio *bp, const char *fmtsuffix, ...);

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:
I/O request command. There are five I/O requests available in GEOM:
A read request.
A write request.
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.
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.
Tells underlying providers to flush their write caches.
Available flags:
Request failed (error value is stored in bio_error field).
Request finished.
Private use by the consumer.
Private use by the provider.
Offset into provider.
Pointer to data buffer.
Error value when BIO_ERROR is set.
Pointer to function which will be called when the request is finished.
Private use by the provider.
Private use by the provider.
Private use by the consumer.
Private use by the consumer.
Attribute string for BIO_GETATTR request.
Consumer to use for request (attached to provider stored in bio_to field) (typically read-only for a class).
Destination provider (typically read-only for a class).
Request length in bytes.
Number of bytes completed, but they may not be completed from the front of the request.
Number of bio clones (typically read-only for a class).
Number of finished bio clones.
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_format_bio() function prints information about the given bio structure into the provided sbuf.

The g_print_bio() function is a convenience wrapper around g_format_bio() that can be used for debugging purposes. It prints a provided prefix string, followed by the formatted bio, followed by a fmtsuffix in the style of 9(printf). Any of the prefix or suffix strings may be the empty string. g_print_bio() always prints a newline character at the end of the line.

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;

	g_print_bio("Request received: ", bp, "");

	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), g_wither_geom(9)

This manual page was written by Pawel Jakub Dawidek <>.
August 7, 2019 FreeBSD 13.1-RELEASE

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