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17 .Dd July 9, 2018
18 .Dt BUF 9S
19 .Os
20 .Sh NAME
21 .Nm buf
22 .Nd block I/O data transfer structure
23 .Sh SYNOPSIS
24 .In sys/ddi.h
25 .In sys/sunddi.h
26 .Sh INTERFACE LEVEL
27 Architecture independent level 1 (DDI/DKI)
28 .Sh DESCRIPTION
29 The
30 .Vt buf
31 structure is the basic data structure for block
32 .Sy I/O
33 transfers.
34 Each block
35 .Sy I/O
36 transfer has an associated buffer header.
37 The header contains all the buffer control and status information.
38 For drivers, the buffer header pointer is the sole argument to a block driver
39 .Xr strategy 9E
40 routine.
41 Do not depend on the size of the
42 .Vt buf
43 structure when writing a driver.
44 .Pp
45 A buffer header can be linked in multiple lists simultaneously.
46 Because of
47 this, most of the members in the buffer header cannot be changed by the driver,
48 even when the buffer header is in one of the driver's work lists.
49 .Pp
50 Buffer headers are also used by the system for unbuffered or physical
51 .Sy I/O
52 for block drivers.
53 In this case, the buffer describes a portion of user data
54 space that is locked into memory.
55 .Pp
56 Block drivers often chain block requests so that overall throughput for the
57 device is maximized.
58 The
59 .Fa av_forw
60 and the
61 .Fa av_back
62 members of the
63 .Vt buf
64 structure can serve as link pointers for chaining block requests.
65 .Sh STRUCTURE MEMBERS
66 .Bd -literal -offset 2n
67 int b_flags; /* Buffer status */
68 struct buf *av_forw; /* Driver work list link */
69 struct buf *av_back; /* Driver work list link */
70 size_t b_bcount; /* # of bytes to transfer */
71 union {
72 caddr_t b_addr; /* Buffer's virtual address */
73 } b_un;
74 daddr_t b_blkno; /* Block number on device */
75 diskaddr_t b_lblkno; /* Expanded block number on dev. */
76 size_t b_resid; /* # of bytes not xferred */
77 size_t b_bufsize; /* size of alloc. buffer */
78 int (*b_iodone)(struct buf *); /* function called */
79 /* by biodone */
80 int b_error; /* expanded error field */
81 void *b_private; /* "opaque" driver private area */
82 dev_t b_edev; /* expanded dev field */
83 .Ed
84 .Pp
85 The members of the buffer header available to test or set by a driver are as
86 follows:
87 .Pp
88 .Fa b_flags
89 stores the buffer status and indicates to the driver whether to
90 read or write to the device.
91 The driver must never clear the
92 .Fa b_flags
93 member.
94 If this is done, unpredictable results can occur including loss of disk
95 sanity and the possible failure of other kernel processes.
96 .Pp
97 All
98 .Fa b_flags
99 bit values not otherwise specified above are reserved by the
100 kernel and may not be used.
101 .Pp
102 Valid flags are as follows:
103 .Bl -tag -width "B_PAGEIO"
104 .It Dv B_BUSY
105 Indicates the buffer is in use.
106 The driver must not change this flag unless it allocated the buffer with
107 .Xr getrbuf 9F
108 and no
109 .Sy I/O
110 operation is in progress.
111 .It Dv B_DONE
112 Indicates the data transfer has completed.
113 This flag is read-only.
114 .It Dv B_ERROR
115 Indicates an
116 .Sy I/O
117 transfer error.
118 It is set in conjunction with the
119 .Fa b_error
120 field.
121 .Xr bioerror 9F
122 should be used in preference to setting the
123 .Dv B_ERROR
124 bit.
125 .It Dv B_PAGEIO
126 Indicates the buffer is being used in a paged
127 .Sy I/O
128 request.
129 See the description of the
130 .Fa b_un.b_addr
131 field for more information.
132 This flag is read-only.
133 .It Dv B_PHYS
134 indicates the buffer header is being used for physical (direct)
135 .Sy I/O
136 to a user data area.
137 See the description of the
138 .Fa b_un.b_addr
139 field for more information.
140 This flag is read-only.
141 .It Dv B_READ
142 Indicates that data is to be read from the peripheral device into main memory.
143 .It Dv B_WRITE
144 Indicates that the data is to be transferred from main memory to the peripheral
145 device.
146 .Dv B_WRITE
147 is a pseudo flag and cannot be directly tested; it is
148 only detected as the NOT form of
149 .Dv B_READ .
150 .El
151 .Pp
152 .Fa av_forw
153 and
154 .Fa av_back
155 can be used by the driver to link the buffer into driver work lists.
156 .Pp
157 .Fa b_bcount
158 specifies the number of bytes to be transferred in both a paged and a non-paged
159 .Sy I/O
160 request.
161 .Pp
162 .Fa b_un.b_addr
163 is the virtual address of the
164 .Sy I/O
165 request, unless
166 .Dv B_PAGEIO
167 is set.
168 The address is a kernel virtual address, unless
169 .Dv B_PHYS
170 is set, in which case it is a user virtual address.
171 If
172 .Dv B_PAGEIO
173 is set,
174 .Fa b_un.b_addr
175 contains kernel private data.
176 Note that either one of
177 .Dv B_PHYS
178 and
179 .Dv B_PAGEIO ,
180 or neither, can be set, but
181 not both.
182 .Pp
183 .Fa b_blkno
184 identifies which logical block on the device (the device is
185 defined by the device number) is to be accessed.
186 The driver might have to
187 convert this logical block number to a physical location such as a cylinder,
188 track, and sector of a disk.
189 This is a 32-bit value.
190 The driver should use
191 .Fa b_blkno
192 or
193 .Fa b_lblkno ,
194 but not both.
195 .Pp
196 .Fa b_lblkno
197 identifies which logical block on the device (the device is
198 defined by the device number) is to be accessed.
199 The driver might have to
200 convert this logical block number to a physical location such as a cylinder,
201 track, and sector of a disk.
202 This is a 64-bit value.
203 The driver should use
204 .Fa b_lblkno
205 or
206 .Fa b_blkno ,
207 but not both.
208 .Pp
209 .Fa b_resid
210 should be set to the number of bytes not transferred because of an error.
211 .Pp
212 .Fa b_bufsize
213 contains the size of the allocated buffer.
214 .Pp
215 .Fa b_iodone
216 identifies a specific
217 .Xr biodone 9F
218 routine to be called by the driver when the
219 .Sy I/O
220 is complete.
221 .Pp
222 .Fa b_error
223 can hold an error code that should be passed as a return code
224 from the driver.
225 .Fa b_error
226 is set in conjunction with the
227 .Dv B_ERROR
228 bit
229 set in the
230 .Fa b_flags
231 member.
232 .Xr bioerror 9F
233 should be used in
234 preference to setting the
235 .Fa b_error
236 field.
237 .Pp
238 .Fa b_private
239 is for the private use of the device driver.
240 .Pp
241 .Fa b_edev
242 contains the major and minor device numbers of the device
243 accessed.
244 .Sh SEE ALSO
245 .Xr strategy 9E ,
246 .Xr aphysio 9F ,
247 .Xr bioclone 9F ,
248 .Xr biodone 9F ,
249 .Xr bioerror 9F ,
250 .Xr bioinit 9F ,
251 .Xr clrbuf 9F ,
252 .Xr getrbuf 9F ,
253 .Xr physio 9F ,
254 .Xr iovec 9S ,
255 .Xr uio 9S
256 .Rs
257 .%T Writing Device Drivers
258 .Re
259 .Sh WARNINGS
260 Buffers are a shared resource within the kernel.
261 Drivers should read or write only the members listed in this section.
262 Drivers that attempt to use undocumented members of the
263 .Fa buf
264 structure risk corrupting data in the kernel or on the device.