xref: /illumos-gate/usr/src/man/man9s/buf.9s (revision 427723622c44f91d933c66b35e3de35f4ccd4669)

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.Dd July 9, 2018
.Dt BUF 9S
.Os
.Sh NAME
.Nm buf
.Nd block I/O data transfer structure
.Sh SYNOPSIS
.In sys/ddi.h
.In sys/sunddi.h
.Sh INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI)
.Sh DESCRIPTION
The
.Vt buf
structure is the basic data structure for block
.Sy I/O
transfers.
Each block
.Sy 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
.Xr strategy 9E
routine.
Do not depend on the size of the
.Vt buf
structure when writing a driver.
.Pp
A buffer header can be linked in multiple lists simultaneously.
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.
.Pp
Buffer headers are also used by the system for unbuffered or physical
.Sy I/O
for block drivers.
In this case, the buffer describes a portion of user data
space that is locked into memory.
.Pp
Block drivers often chain block requests so that overall throughput for the
device is maximized.
The
.Fa av_forw
and the
.Fa av_back
members of the
.Vt buf
structure can serve as link pointers for chaining block requests.
.Sh STRUCTURE MEMBERS
.Bd -literal -offset 2n
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 dev.  */
size_t        b_resid;           /* # of bytes not xferred */
size_t        b_bufsize;         /* size of alloc. 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 */
.Ed
.Pp
The members of the buffer header available to test or set by a driver are as
follows:
.Pp
.Fa 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
.Fa b_flags
member.
If this is done, unpredictable results can occur including loss of disk
sanity and the possible failure of other kernel processes.
.Pp
All
.Fa b_flags
bit values not otherwise specified above are reserved by the
kernel and may not be used.
.Pp
Valid flags are as follows:
.Bl -tag -width "B_PAGEIO"
.It Dv B_BUSY
Indicates the buffer is in use.
The driver must not change this flag unless it allocated the buffer with
.Xr getrbuf 9F
and no
.Sy I/O
operation is in progress.
.It Dv B_DONE
Indicates the data transfer has completed.
This flag is read-only.
.It Dv B_ERROR
Indicates an
.Sy I/O
transfer error.
It is set in conjunction with the
.Fa b_error
field.
.Xr bioerror 9F
should be used in preference to setting the
.Dv B_ERROR
bit.
.It Dv B_PAGEIO
Indicates the buffer is being used in a paged
.Sy I/O
request.
See the description of the
.Fa b_un.b_addr
field for more information.
This flag is read-only.
.It Dv B_PHYS
indicates the buffer header is being used for physical (direct)
.Sy I/O
to a user data area.
See the description of the
.Fa b_un.b_addr
field for more information.
This flag is read-only.
.It Dv B_READ
Indicates that data is to be read from the peripheral device into main memory.
.It Dv B_WRITE
Indicates that the data is to be transferred from main memory to the peripheral
device.
.Dv B_WRITE
is a pseudo flag and cannot be directly tested; it is
only detected as the NOT form of
.Dv B_READ .
.El
.Pp
.Fa av_forw
and
.Fa av_back
can be used by the driver to link the buffer into driver work lists.
.Pp
.Fa b_bcount
specifies the number of bytes to be transferred in both a paged and a non-paged
.Sy I/O
request.
.Pp
.Fa b_un.b_addr
is the virtual address of the
.Sy I/O
request, unless
.Dv B_PAGEIO
is set.
The address is a kernel virtual address, unless
.Dv B_PHYS
is set, in which case it is a user virtual address.
If
.Dv B_PAGEIO
is set,
.Fa b_un.b_addr
contains kernel private data.
Note that either one of
.Dv B_PHYS
and
.Dv B_PAGEIO ,
or neither, can be set, but
not both.
.Pp
.Fa 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
.Fa b_blkno
or
.Fa b_lblkno ,
but not both.
.Pp
.Fa 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
.Fa b_lblkno
or
.Fa b_blkno ,
but not both.
.Pp
.Fa b_resid
should be set to the number of bytes not transferred because of an error.
.Pp
.Fa b_bufsize
contains the size of the allocated buffer.
.Pp
.Fa b_iodone
identifies a specific
.Xr biodone 9F
routine to be called by the driver when the
.Sy I/O
is complete.
.Pp
.Fa b_error
can hold an error code that should be passed as a return code
from the driver.
.Fa b_error
is set in conjunction with the
.Dv B_ERROR
bit
set in the
.Fa b_flags
member.
.Xr bioerror 9F
should be used in
preference to setting the
.Fa b_error
field.
.Pp
.Fa b_private
is for the private use of the device driver.
.Pp
.Fa b_edev
contains the major and minor device numbers of the device
accessed.
.Sh SEE ALSO
.Xr strategy 9E ,
.Xr aphysio 9F ,
.Xr bioclone 9F ,
.Xr biodone 9F ,
.Xr bioerror 9F ,
.Xr bioinit 9F ,
.Xr clrbuf 9F ,
.Xr getrbuf 9F ,
.Xr physio 9F ,
.Xr iovec 9S ,
.Xr uio 9S
.Rs
.%T Writing Device Drivers
.Re
.Sh WARNINGS
Buffers are a shared resource within the kernel.
Drivers should read or write only the members listed in this section.
Drivers that attempt to use undocumented members of the
.Fa buf
structure risk corrupting data in the kernel or on the device.