buf(9S)
NAME
buf - block I/O data transfer structure
SYNOPSIS
#include <sys/ddi.h>
#include <sys/sunddi.h>
INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI)
DESCRIPTION
The buf structure is the basic data structure for block I/O
transfers. Each block I/O transfer has an associated buffer
header. The header contains all the buffer control and
status information. For drivers, the buffer header pointer
is the sole argument to a block driver strategy(9E) routine.
Do not depend on the size of the buf structure when writing
a driver.
A buffer header can be linked in multiple lists simultane-
ously. Because of this, most of the members in the buffer
header cannot be changed by the driver, even when the buffer
header is in one of the driver's work lists.
Buffer headers are also used by the system for unbuffered or
physical I/O for block drivers. In this case, the buffer
describes a portion of user data space that is locked into
memory.
Block drivers often chain block requests so that overall
throughput for the device is maximized. The av_forw and the
av_back members of the buf structure can serve as link
pointers for chaining block requests.
STRUCTURE MEMBERS
int b_flags; /* Buffer status */
struct buf *av_forw; /* Driver work list link */
struct buf *av_back; /* Driver work list link */
size_t b_bcount; /* # of bytes to transfer */
union {
caddr_t b_addr; /* Buffer's virtual address */
} b_un;
daddr_t b_blkno; /* Block number on device */
diskaddr_t b_lblkno; /* Expanded block number on device */
size_t b_resid; /* # of bytes not transferred */
size_t b_bufsize; /* size of allocated buffer */
int (*b_iodone)(struct buf *); /* function called */
/* by biodone */
int b_error; /* expanded error field */
void *b_private; /* "opaque" driver private area */
dev_t b_edev; /* expanded dev field */
The members of the buffer header available to test or set by
a driver are as follows:
b_flags stores the buffer status and indicates to the driver
whether to read or write to the device. The driver must
never clear the b_flags member. If this is done, unpredict-
able results can occur including loss of disk sanity and the
possible failure of other kernel processes.
All b_flags bit values not otherwise specified above are
reserved by the kernel and may not be used.
Valid flags are as follows:
B_BUSY
Indicates the buffer is in use. The driver must
not change this flag unless it allocated the
buffer with getrbuf(9F) and no I/O operation is
in progress.
B_DONE
Indicates the data transfer has completed. This
flag is read-only.
B_ERROR
Indicates an I/O transfer error. It is set in
conjunction with the b_error field. bioerror(9F)
should be used in preference to setting the
B_ERROR bit.
B_PAGEIO
Indicates the buffer is being used in a paged I/O
request. See the description of the b_un.b_addr
field for more information. This flag is read-
only.
B_PHYS
indicates the buffer header is being used for
physical (direct) I/O to a user data area. See
the description of the b_un.b_addr field for more
information. This flag is read-only.
B_READ
Indicates that data is to be read from the peri-
pheral device into main memory.
B_WRITE
Indicates that the data is to be transferred from
main memory to the peripheral device. B_WRITE is
a pseudo flag and cannot be directly tested; it
is only detected as the NOT form of B_READ.
av_forw and av_back can be used by the driver to link the
buffer into driver work lists.
b_bcount specifies the number of bytes to be transferred in
both a paged and a non-paged I/O request.
b_un.b_addr is the virtual address of the I/O request,
unless B_PAGEIO is set. The address is a kernel virtual
address, unless B_PHYS is set, in which case it is a user
virtual address. If B_PAGEIO is set, b_un.b_addr contains
kernel private data. Note that either one of B_PHYS and
B_PAGEIO, or neither, can be set, but not both.
b_blkno identifies which logical block on the device (the
device is defined by the device number) is to be accessed.
The driver might have to convert this logical block number
to a physical location such as a cylinder, track, and sector
of a disk. This is a 32-bit value. The driver should use
b_blkno or b_lblkno, but not both.
b_lblkno identifies which logical block on the device (the
device is defined by the device number) is to be accessed.
The driver might have to convert this logical block number
to a physical location such as a cylinder, track, and sector
of a disk. This is a 64-bit value. The driver should use
b_lblkno or b_blkno, but not both.
b_resid should be set to the number of bytes not transferred
because of an error.
b_bufsize contains the size of the allocated buffer.
b_iodone identifies a specific biodone routine to be called
by the driver when the I/O is complete.
b_error can hold an error code that should be passed as a
return code from the driver. b_error is set in conjunction
with the B_ERROR bit set in the b_flags member. bioerror(9F)
should be used in preference to setting the b_error field.
b_private is for the private use of the device driver.
b_edev contains the major and minor device numbers of the
device accessed.
SEE ALSO
strategy(9E), aphysio(9F), bioclone(9F), biodone(9F),
bioerror(9F), bioinit(9F), clrbuf(9F), getrbuf(9F),
physio(9F), iovec(9S), uio(9S)
Writing Device Drivers
WARNINGS
Buffers are a shared resource within the kernel. Drivers
should read or write only the members listed in this sec-
tion. Drivers that attempt to use undocumented members of
the buf structure risk corrupting data in the kernel or on
the device.
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