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14 .Dd August 2, 2018
15 .Dt BYTEORDER 5
16 .Os
17 .Sh NAME
18 .Nm byteorder ,
19 .Nm endian
20 .Nd byte order and endianness
21 .Sh DESCRIPTION
22 Integer values which occupy more than 1 byte in memory can be laid out
23 in different ways on different platforms.
24 In particular, there is a major split between those which place the least
25 significant byte of an integer at the lowest address, and those which place the
26 most significant byte there instead.
27 As this difference relates to which end of the integer is found in memory first,
28 the term
29 .Em endian
30 is used to refer to a particular byte order.
31 .Pp
32 A platform is referred to as using a
33 .Em big-endian
34 byte order when it places the most significant byte at the lowest
35 address, and
36 .Em little-endian
37 when it places the least significant byte first.
38 Some platforms may also switch between big- and little-endian mode and run code
39 compiled for either.
40 .Pp
41 Historically, there have also been some systems that utilized
42 .Em middle-endian
43 byte orders for integers larger than 2 bytes.
44 Such orderings are not in common use today.
45 .Pp
46 Endianness is also of particular importance when dealing with values
47 that are being read into memory from an external source.
48 For example, network protocols such as IP conventionally define the fields in a
49 packet as being always stored in big-endian byte order.
50 This means that a little-endian machine will have to perform transformations on
51 these fields in order to process them.
52 .Ss Examples
53 To illustrate endianness in memory, let us consider the decimal integer
54 2864434397.
55 This number fits in 32 bits of storage (4 bytes).
56 .Pp
57 On a big-endian system, this integer would be written into memory as
58 the bytes 0xAA, 0xBB, 0xCC, 0xDD, in order from lowest memory address to
59 highest.
60 .Pp
61 On a little-endian system, it would be written instead as the bytes
62 0xDD, 0xCC, 0xBB, 0xAA, in that order.
63 .Pp
64 If both the big- and little-endian systems were asked to store this
65 integer at address 0x100, we would see the following in each of their
66 memory:
67 .Bd -literal
68
69 Big-Endian
70
71 ++------++------++------++------++
72 || 0xAA || 0xBB || 0xCC || 0xDD ||
73 ++------++------++------++------++
74 ^^ ^^ ^^ ^^
75 0x100 0x101 0x102 0x103
76 vv vv vv vv
77 ++------++------++------++------++
78 || 0xDD || 0xCC || 0xBB || 0xAA ||
79 ++------++------++------++------++
80
81 Little-Endian
82 .Ed
83 .Pp
84 It is particularly important to note that even though the byte order is
85 different between these two machines, the bit ordering within each byte,
86 by convention, is still the same.
87 .Pp
88 For example, take the decimal integer 4660, which occupies in 16 bits (2
89 bytes).
90 .Pp
91 On a big-endian system, this would be written into memory as 0x12, then
92 0x34.
93 .Pp
94 On a little-endian system, it would be written as 0x34, then 0x12.
95 Note that this is not at all the same as seeing 0x43 then 0x21 in memory --
96 only the bytes are re-ordered, not any bits (or nybbles) within them.
97 .Pp
98 As before, storing this at address 0x100:
99 .Bd -literal
100 Big-Endian
101
102 ++------++------++
103 || 0x12 || 0x34 ||
104 ++------++------++
105 ^^ ^^
106 0x100 0x101
107 vv vv
108 ++------++------++
109 || 0x34 || 0x12 ||
110 ++------++------++
111
112 Little-Endian
113 .Ed
114 .Pp
115 This example shows how an eight byte number, 0xBADCAFEDEADBEEF is stored
116 in both big and little-endian:
117 .Bd -literal
118 Big-Endian
119
120 +------+------+------+------+------+------+------+------+
121 | 0xBA | 0xDC | 0xAF | 0xFE | 0xDE | 0xAD | 0xBE | 0xEF |
122 +------+------+------+------+------+------+------+------+
123 ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^
124 0x100 0x101 0x102 0x103 0x104 0x105 0x106 0x107
125 vv vv vv vv vv vv vv vv
126 +------+------+------+------+------+------+------+------+
127 | 0xEF | 0xBE | 0xAD | 0xDE | 0xFE | 0xAF | 0xDC | 0xBA |
128 +------+------+------+------+------+------+------+------+
129
130 Little-Endian
131
132 .Ed
133 .Pp
134 The treatment of different endian values would not be complete without
135 discussing
136 .Em PDP-endian ,
137 which is also known as
138 .Em middle-endian .
139 While the PDP-11 was a 16-bit little-endian system, it laid out 32-bit
140 values in a different way from current little-endian systems.
141 First, it would divide a 32-bit number into two 16-bit numbers.
142 Each 16-bit number would be stored in little-endian; however, the two 16-bit
143 words would be stored with the larger 16-bit word appearing first in memory,
144 followed by the latter.
145 .Pp
146 The following image illustrates PDP-endian and compares it against
147 little-endian values.
148 Here, we'll start with the value 0xAABBCCDD and show how the four bytes for it
149 will be laid out, starting at 0x100.
150 .Bd -literal
151 PDP-Endian
152
153 ++------++------++------++------++
154 || 0xBB || 0xAA || 0xDD || 0xCC ||
155 ++------++------++------++------++
156 ^^ ^^ ^^ ^^
157 0x100 0x101 0x102 0x103
158 vv vv vv vv
159 ++------++------++------++------++
160 || 0xDD || 0xCC || 0xBB || 0xAA ||
161 ++------++------++------++------++
162
163 Little-Endian
164
165 .Ed
166 .Ss Network Byte Order
167 The term 'network byte order' refers to big-endian ordering, and
168 originates from the IEEE.
169 Early disagreements over which byte ordering to use for network traffic prompted
170 RFC1700 to define that all IETF-specified network protocols use big-endian
171 ordering unless noted explicitly otherwise.
172 The Internet protocol family (IP, and thus TCP and UDP etc) particularly adhere
173 to this convention.
174 .Ss Determining the System's Byte Order
175 The operating system supports both big-endian and little-endian CPUs.
176 To make it easier for programs to determine the endianness of the platform they
177 are being compiled for, functions and macro constants are provided in the system
178 header files.
179 .Pp
180 The endianness of the system can be obtained by including the header
181 .In sys/types.h
182 and using the pre-processor macros
183 .Sy _LITTLE_ENDIAN
184 and
185 .Sy _BIG_ENDIAN .
186 See
187 .Xr types.h 3HEAD
188 for more information.
189 .Pp
190 Additionally, the header
191 .In endian.h
192 defines an alternative means for determining the endianness of the
193 current system.
194 See
195 .Xr endian.h 3HEAD
196 for more information.
197 .Pp
198 illumos runs on both big- and little-endian systems.
199 When writing software for which the endianness is important, one must always
200 check the byte order and convert it appropriately.
201 .Ss Converting Between Byte Orders
202 The system provides two different sets of functions to convert values
203 between big-endian and little-endian.
204 They are defined in
205 .Xr byteorder 3C
206 and
207 .Xr endian 3C .
208 .Pp
209 The
210 .Xr byteorder 3C
211 family of functions convert data between the host's native byte order
212 and big- or little-endian.
213 The functions operate on either 16-bit, 32-bit, or 64-bit values.
214 Functions that convert from network byte order to the host's byte order
215 start with the string
216 .Sy ntoh ,
217 while functions which convert from the host's byte order to network byte
218 order, begin with
219 .Sy hton .
220 For example, to convert a 32-bit value, a long, from network byte order
221 to the host's, one would use the function
222 .Xr ntohl 3C .
223 .Pp
224 These functions have been standardized by POSIX.
225 However, the 64-bit variants,
226 .Xr ntohll 3C
227 and
228 .Xr htonll 3C
229 are not standardized and may not be found on other systems.
230 For more information on these functions, see
231 .Xr byteorder 3C .
232 .Pp
233 The second family of functions,
234 .Xr endian 3C ,
235 provide a means to convert between the host's byte order
236 and big-endian and little-endian specifically.
237 While these functions are similar to those in
238 .Xr byteorder 3C ,
239 they more explicitly cover different data conversions.
240 Like them, these functions operate on either 16-bit, 32-bit, or 64-bit values.
241 When converting from big-endian, to the host's endianness, the functions
242 begin with
243 .Sy betoh .
244 If instead, one is converting data from the host's native endianness to
245 another, then it starts with
246 .Sy htobe .
247 When working with little-endian data, the prefixes
248 .Sy letoh
249 and
250 .Sy htole
251 convert little-endian data to the host's endianness and from the host's
252 to little-endian respectively.
253 .Pp
254 These functions are not standardized and the header they appear in varies
255 between the BSDs and GNU/Linux.
256 Applications that wish to be portable, should instead use the
257 .Xr byteorder 3C
258 functions.
259 .Pp
260 All of these functions in both families simply return their input when
261 the host's native byte order is the same as the desired order.
262 For example, when calling
263 .Xr htonl 3C
264 on a big-endian system the original data is returned with no conversion
265 or modification.
266 .Sh SEE ALSO
267 .Xr byteorder 3C ,
268 .Xr endian 3C ,
269 .Xr endian.h 3HEAD ,
270 .Xr inet 3HEAD