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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 */
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The members of the buffer header available to test or set by a driver
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 peripheral 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.
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, unpredictable
results can occur including loss of disk sanity and the possible failure of
other kernel processes.
Valid flags are as follows:
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.
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