1INET(3) Linux Programmer's Manual INET(3)
2
6 inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr,
7 inet_lnaof, inet_netof - Internet address manipulation routines
8
10 #include <sys/socket.h>
11 #include <netinet/in.h>
12 #include <arpa/inet.h>
13 int inet_aton(const char *cp, struct in_addr *inp);
15 in_addr_t inet_addr(const char *cp);
17 in_addr_t inet_network(const char *cp);
19 char *inet_ntoa(struct in_addr in);
21 struct in_addr inet_makeaddr(in_addr_t net, in_addr_t host);
23 in_addr_t inet_lnaof(struct in_addr in);
25 in_addr_t inet_netof(struct in_addr in);
27 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
29 inet_aton(), inet_ntoa():
31 Since glibc 2.19:
32 _DEFAULT_SOURCE
33 In glibc up to and including 2.19:
34 _BSD_SOURCE || _BSD_SOURCE
35
37 inet_aton() converts the Internet host address cp from the IPv4 num‐
38 bers-and-dots notation into binary form (in network byte order) and
39 stores it in the structure that inp points to. inet_aton() returns
40 nonzero if the address is valid, zero if not. The address supplied in
41 cp can have one of the following forms:
42 a.b.c.d Each of the four numeric parts specifies a byte of the
44 address; the bytes are assigned in left-to-right order to
45 produce the binary address.
46 a.b.c Parts a and b specify the first two bytes of the binary
48 address. Part c is interpreted as a 16-bit value that
49 defines the rightmost two bytes of the binary address. This
50 notation is suitable for specifying (outmoded) Class B net‐
51 work addresses.
52 a.b Part a specifies the first byte of the binary address. Part
54 b is interpreted as a 24-bit value that defines the rightmost
55 three bytes of the binary address. This notation is suitable
56 for specifying (outmoded) Class A network addresses.
57 a The value a is interpreted as a 32-bit value that is stored
59 directly into the binary address without any byte rearrange‐
60 ment.
61 In all of the above forms, components of the dotted address can be
63 specified in decimal, octal (with a leading 0), or hexadecimal, with a
64 leading 0X). Addresses in any of these forms are collectively termed
65 IPV4 numbers-and-dots notation. The form that uses exactly four deci‐
66 mal numbers is referred to as IPv4 dotted-decimal notation (or some‐
67 times: IPv4 dotted-quad notation).
68 inet_aton() returns 1 if the supplied string was successfully inter‐
70 preted, or 0 if the string is invalid (errno is not set on error).
71 The inet_addr() function converts the Internet host address cp from
73 IPv4 numbers-and-dots notation into binary data in network byte order.
74 If the input is invalid, INADDR_NONE (usually -1) is returned. Use of
75 this function is problematic because -1 is a valid address
76 (255.255.255.255). Avoid its use in favor of inet_aton(),
77 inet_pton(3), or getaddrinfo(3), which provide a cleaner way to indi‐
78 cate error return.
79 The inet_network() function converts cp, a string in IPv4 numbers-and-
81 dots notation, into a number in host byte order suitable for use as an
82 Internet network address. On success, the converted address is
83 returned. If the input is invalid, -1 is returned.
84 The inet_ntoa() function converts the Internet host address in, given
86 in network byte order, to a string in IPv4 dotted-decimal notation.
87 The string is returned in a statically allocated buffer, which subse‐
88 quent calls will overwrite.
89 The inet_lnaof() function returns the local network address part of the
91 Internet address in. The returned value is in host byte order.
92 The inet_netof() function returns the network number part of the Inter‐
94 net address in. The returned value is in host byte order.
95 The inet_makeaddr() function is the converse of inet_netof() and
97 inet_lnaof(). It returns an Internet host address in network byte
98 order, created by combining the network number net with the local
99 address host, both in host byte order.
100 The structure in_addr as used in inet_ntoa(), inet_makeaddr(),
102 inet_lnaof() and inet_netof() is defined in <netinet/in.h> as:
103 typedef uint32_t in_addr_t;
105 struct in_addr {
107 in_addr_t s_addr;
108 };
109
111 For an explanation of the terms used in this section, see
112 attributes(7).
113 ┌───────────────────────────────┬───────────────┬────────────────┐
115 │Interface │ Attribute │ Value │
116 ├───────────────────────────────┼───────────────┼────────────────┤
117 │inet_aton(), inet_addr(), │ Thread safety │ MT-Safe locale │
118 │inet_network(), inet_ntoa() │ │ │
119 ├───────────────────────────────┼───────────────┼────────────────┤
120 │inet_makeaddr(), inet_lnaof(), │ Thread safety │ MT-Safe │
121 │inet_netof() │ │ │
122 └───────────────────────────────┴───────────────┴────────────────┘
124 inet_addr(), inet_ntoa(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.
125 inet_aton() is not specified in POSIX.1, but is available on most sys‐
127 tems.
128
130 On x86 architectures, the host byte order is Least Significant Byte
131 first (little endian), whereas the network byte order, as used on the
132 Internet, is Most Significant Byte first (big endian).
133 inet_lnaof(), inet_netof(), and inet_makeaddr() are legacy functions
135 that assume they are dealing with classful network addresses. Classful
136 networking divides IPv4 network addresses into host and network compo‐
137 nents at byte boundaries, as follows:
138 Class A This address type is indicated by the value 0 in the most
140 significant bit of the (network byte ordered) address. The
141 network address is contained in the most significant byte,
142 and the host address occupies the remaining three bytes.
143 Class B This address type is indicated by the binary value 10 in the
145 most significant two bits of the address. The network
146 address is contained in the two most significant bytes, and
147 the host address occupies the remaining two bytes.
148 Class C This address type is indicated by the binary value 110 in the
150 most significant three bits of the address. The network
151 address is contained in the three most significant bytes, and
152 the host address occupies the remaining byte.
153 Classful network addresses are now obsolete, having been superseded by
155 Classless Inter-Domain Routing (CIDR), which divides addresses into
156 network and host components at arbitrary bit (rather than byte) bound‐
157 aries.
158
160 An example of the use of inet_aton() and inet_ntoa() is shown below.
161 Here are some example runs:
162 $ ./a.out 226.000.000.037 # Last byte is in octal
164 226.0.0.31
165 $ ./a.out 0x7f.1 # First byte is in hex
166 127.0.0.1
167 Program source
169 #define _BSD_SOURCE
171 #include <arpa/inet.h>
172 #include <stdio.h>
173 #include <stdlib.h>
174 int
176 main(int argc, char *argv[])
177 {
178 struct in_addr addr;
179 if (argc != 2) {
181 fprintf(stderr, "%s <dotted-address>\n", argv[0]);
182 exit(EXIT_FAILURE);
183 }
184 if (inet_aton(argv[1], &addr) == 0) {
186 fprintf(stderr, "Invalid address\n");
187 exit(EXIT_FAILURE);
188 }
189 printf("%s\n", inet_ntoa(addr));
191 exit(EXIT_SUCCESS);
192 }
193
195 byteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3), getne‐
196 tent(3), inet_net_pton(3), inet_ntop(3), inet_pton(3), hosts(5), net‐
197 works(5)
198
200 This page is part of release 5.07 of the Linux man-pages project. A
201 description of the project, information about reporting bugs, and the
202 latest version of this page, can be found at
203 https://www.kernel.org/doc/man-pages/.
204GNU 2020-06-09 INET(3)