1INET(3)                    Linux Programmer's Manual                   INET(3)
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NAME

6       inet_aton,    inet_addr,    inet_network,   inet_ntoa,   inet_makeaddr,
7       inet_lnaof, inet_netof - Internet address manipulation routines
8

SYNOPSIS

10       #include <sys/socket.h>
11       #include <netinet/in.h>
12       #include <arpa/inet.h>
13
14       int inet_aton(const char *cp, struct in_addr *inp);
15
16       in_addr_t inet_addr(const char *cp);
17       in_addr_t inet_network(const char *cp);
18
19       char *inet_ntoa(struct in_addr in);
20
21       struct in_addr inet_makeaddr(in_addr_t net, in_addr_t host);
22
23       in_addr_t inet_lnaof(struct in_addr in);
24       in_addr_t inet_netof(struct in_addr in);
25
26   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
27
28       inet_aton(), inet_ntoa():
29           Since glibc 2.19:
30               _DEFAULT_SOURCE
31           In glibc up to and including 2.19:
32               _BSD_SOURCE || _BSD_SOURCE
33

DESCRIPTION

35       inet_aton() converts the Internet host address cp from  the  IPv4  num‐
36       bers-and-dots  notation  into  binary  form (in network byte order) and
37       stores it in the structure that inp  points  to.   inet_aton()  returns
38       nonzero  if the address is valid, zero if not.  The address supplied in
39       cp can have one of the following forms:
40
41       a.b.c.d   Each of the four numeric parts specifies a byte  of  the  ad‐
42                 dress;  the bytes are assigned in left-to-right order to pro‐
43                 duce the binary address.
44
45       a.b.c     Parts a and b specify the first two bytes of the  binary  ad‐
46                 dress.   Part c is interpreted as a 16-bit value that defines
47                 the rightmost two bytes of the binary address.  This notation
48                 is  suitable  for  specifying  (outmoded) Class B network ad‐
49                 dresses.
50
51       a.b       Part a specifies the first byte of the binary address.   Part
52                 b is interpreted as a 24-bit value that defines the rightmost
53                 three bytes of the binary address.  This notation is suitable
54                 for specifying (outmoded) Class A network addresses.
55
56       a         The  value  a is interpreted as a 32-bit value that is stored
57                 directly into the binary address without any byte  rearrange‐
58                 ment.
59
60       In  all  of  the  above  forms, components of the dotted address can be
61       specified in decimal, octal (with a leading 0), or hexadecimal, with  a
62       leading  0X).   Addresses in any of these forms are collectively termed
63       IPV4 numbers-and-dots notation.  The form that uses exactly four  deci‐
64       mal  numbers  is  referred to as IPv4 dotted-decimal notation (or some‐
65       times: IPv4 dotted-quad notation).
66
67       inet_aton() returns 1 if the supplied string  was  successfully  inter‐
68       preted, or 0 if the string is invalid (errno is not set on error).
69
70       The  inet_addr()  function  converts  the Internet host address cp from
71       IPv4 numbers-and-dots notation into binary data in network byte  order.
72       If  the input is invalid, INADDR_NONE (usually -1) is returned.  Use of
73       this  function  is  problematic  because  -1   is   a   valid   address
74       (255.255.255.255).    Avoid   its   use   in   favor   of  inet_aton(),
75       inet_pton(3), or getaddrinfo(3), which provide a cleaner way  to  indi‐
76       cate error return.
77
78       The  inet_network() function converts cp, a string in IPv4 numbers-and-
79       dots notation, into a number in host byte order suitable for use as  an
80       Internet  network  address.   On  success, the converted address is re‐
81       turned.  If the input is invalid, -1 is returned.
82
83       The inet_ntoa() function converts the Internet host address  in,  given
84       in  network  byte  order,  to a string in IPv4 dotted-decimal notation.
85       The string is returned in a statically allocated buffer,  which  subse‐
86       quent calls will overwrite.
87
88       The inet_lnaof() function returns the local network address part of the
89       Internet address in.  The returned value is in host byte order.
90
91       The inet_netof() function returns the network number part of the Inter‐
92       net address in.  The returned value is in host byte order.
93
94       The  inet_makeaddr()  function  is  the  converse  of  inet_netof() and
95       inet_lnaof().  It returns an Internet host address in network byte  or‐
96       der, created by combining the network number net with the local address
97       host, both in host byte order.
98
99       The  structure  in_addr  as  used  in   inet_ntoa(),   inet_makeaddr(),
100       inet_lnaof(), and inet_netof() is defined in <netinet/in.h> as:
101
102           typedef uint32_t in_addr_t;
103
104           struct in_addr {
105               in_addr_t s_addr;
106           };
107

ATTRIBUTES

109       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
110       tributes(7).
111
112       ┌─────────────────────────────────────┬───────────────┬────────────────┐
113Interface                            Attribute     Value          
114       ├─────────────────────────────────────┼───────────────┼────────────────┤
115inet_aton(), inet_addr(),            │ Thread safety │ MT-Safe locale │
116inet_network(), inet_ntoa()          │               │                │
117       ├─────────────────────────────────────┼───────────────┼────────────────┤
118inet_makeaddr(), inet_lnaof(),       │ Thread safety │ MT-Safe        │
119inet_netof()                         │               │                │
120       └─────────────────────────────────────┴───────────────┴────────────────┘
121

CONFORMING TO

123       inet_addr(), inet_ntoa(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.
124
125       inet_aton() is not specified in POSIX.1, but is available on most  sys‐
126       tems.
127

NOTES

129       On  x86  architectures,  the  host byte order is Least Significant Byte
130       first (little endian), whereas the network byte order, as used  on  the
131       Internet, is Most Significant Byte first (big endian).
132
133       inet_lnaof(),  inet_netof(),  and  inet_makeaddr() are legacy functions
134       that assume they are dealing with classful network addresses.  Classful
135       networking  divides IPv4 network addresses into host and network compo‐
136       nents at byte boundaries, as follows:
137
138       Class A   This address type is indicated by the value  0  in  the  most
139                 significant  bit  of the (network byte ordered) address.  The
140                 network address is contained in the  most  significant  byte,
141                 and the host address occupies the remaining three bytes.
142
143       Class B   This  address type is indicated by the binary value 10 in the
144                 most significant two bits of the address.   The  network  ad‐
145                 dress is contained in the two most significant bytes, and the
146                 host address occupies the remaining two bytes.
147
148       Class C   This address type is indicated by the binary value 110 in the
149                 most  significant three bits of the address.  The network ad‐
150                 dress is contained in the three most significant  bytes,  and
151                 the host address occupies the remaining byte.
152
153       Classful  network addresses are now obsolete, having been superseded by
154       Classless Inter-Domain Routing (CIDR),  which  divides  addresses  into
155       network  and host components at arbitrary bit (rather than byte) bound‐
156       aries.
157

EXAMPLES

159       An example of the use of inet_aton() and inet_ntoa()  is  shown  below.
160       Here are some example runs:
161
162           $ ./a.out 226.000.000.037      # Last byte is in octal
163           226.0.0.31
164           $ ./a.out 0x7f.1               # First byte is in hex
165           127.0.0.1
166
167   Program source
168
169       #define _BSD_SOURCE
170       #include <arpa/inet.h>
171       #include <stdio.h>
172       #include <stdlib.h>
173
174       int
175       main(int argc, char *argv[])
176       {
177           struct in_addr addr;
178
179           if (argc != 2) {
180               fprintf(stderr, "%s <dotted-address>\n", argv[0]);
181               exit(EXIT_FAILURE);
182           }
183
184           if (inet_aton(argv[1], &addr) == 0) {
185               fprintf(stderr, "Invalid address\n");
186               exit(EXIT_FAILURE);
187           }
188
189           printf("%s\n", inet_ntoa(addr));
190           exit(EXIT_SUCCESS);
191       }
192

SEE ALSO

194       byteorder(3),  getaddrinfo(3), gethostbyname(3), getnameinfo(3), getne‐
195       tent(3), inet_net_pton(3), inet_ntop(3), inet_pton(3),  hosts(5),  net‐
196       works(5)
197

COLOPHON

199       This  page  is  part of release 5.13 of the Linux man-pages project.  A
200       description of the project, information about reporting bugs,  and  the
201       latest     version     of     this    page,    can    be    found    at
202       https://www.kernel.org/doc/man-pages/.
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206GNU                               2021-03-22                           INET(3)
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