man3/inet.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\" and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
   3 .\"     <mtk.manpages@gmail.com>
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  26 .\"
  27 .\" References consulted:
  28 .\"     Linux libc source code
  29 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  30 .\"     386BSD man pages
  31 .\"     libc.info (from glibc distribution)
  32 .\" Modified Sat Jul 24 19:12:00 1993 by Rik Faith <faith@cs.unc.edu>
  33 .\" Modified Sun Sep  3 20:29:36 1995 by Jim Van Zandt <jrv@vanzandt.mv.com>
  34 .\" Changed network into host byte order (for inet_network),
  35 .\"     Andreas Jaeger <aj@arthur.rhein-neckar.de>, 980130.
  36 .\" 2008-06-19, mtk
  37 .\"     Describe the various address forms supported by inet_aton().
  38 .\"     Clarify discussion of inet_lnaof(), inet_netof(), and inet_makeaddr().
  39 .\"     Add discussion of Classful Addressing, noting that it is obsolete.
  40 .\"     Added an EXAMPLE program.
  41 .\"
  42 .TH INET 3  2016-03-15 "GNU" "Linux Programmer's Manual"
  43 .SH NAME
  44 inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof,
  45 inet_netof \- Internet address manipulation routines
  46 .SH SYNOPSIS
  47 .nf
  48 .B #include <sys/socket.h>
  49 .B #include <netinet/in.h>
  50 .B #include <arpa/inet.h>
  51 .PP
  52 .BI "int inet_aton(const char *" cp ", struct in_addr *" inp );
  53 .PP
  54 .BI "in_addr_t inet_addr(const char *" cp );
  55 .PP
  56 .BI "in_addr_t inet_network(const char *" cp );
  57 .PP
  58 .BI "char *inet_ntoa(struct in_addr " in );
  59 .PP
  60 .BI "struct in_addr inet_makeaddr(in_addr_t " net ", in_addr_t " host );
  61 .PP
  62 .BI "in_addr_t inet_lnaof(struct in_addr " in );
  63 .PP
  64 .BI "in_addr_t inet_netof(struct in_addr " in );
  65 .fi
  66 .PP
  67 .in -4n
  68 Feature Test Macro Requirements for glibc (see
  69 .BR feature_test_macros (7)):
  70 .in
  71 .PP
  72 .BR inet_aton (),
  73 .BR inet_ntoa ():
  74 .nf
  75     Since glibc 2.19:
  76         _DEFAULT_SOURCE
  77     In glibc up to and including 2.19:
  78         _BSD_SOURCE || _BSD_SOURCE
  79 .fi
  80 .SH DESCRIPTION
  81 .BR inet_aton ()
  82 converts the Internet host address \fIcp\fP from the
  83 IPv4 numbers-and-dots notation into binary form (in network byte order)
  84 and stores it in the structure that \fIinp\fP points to.
  85 .BR inet_aton ()
  86 returns nonzero if the address is valid, zero if not.
  87 The address supplied in
  88 .I cp
  89 can have one of the following forms:
  90 .TP 10
  91 .I a.b.c.d
  92 Each of the four numeric parts specifies a byte of the address;
  93 the bytes are assigned in left-to-right order to produce the binary address.
  94 .TP
  95 .I a.b.c
  96 Parts
  97 .I a
  98 and
  99 .I b
 100 specify the first two bytes of the binary address.
 101 Part
 102 .I c
 103 is interpreted as a 16-bit value that defines the rightmost two bytes
 104 of the binary address.
 105 This notation is suitable for specifying (outmoded) Class B
 106 network addresses.
 107 .TP
 108 .I a.b
 109 Part
 110 .I a
 111 specifies the first byte of the binary address.
 112 Part
 113 .I b
 114 is interpreted as a 24-bit value that defines the rightmost three bytes
 115 of the binary address.
 116 This notation is suitable for specifying (outmoded) Class A
 117 network addresses.
 118 .TP
 119 .I a
 120 The value
 121 .I a
 122 is interpreted as a 32-bit value that is stored directly
 123 into the binary address without any byte rearrangement.
 124 .PP
 125 In all of the above forms,
 126 components of the dotted address can be specified in decimal,
 127 octal (with a leading
 128 .IR 0 ),
 129 or hexadecimal, with a leading
 130 .IR 0X ).
 131 Addresses in any of these forms are collectively termed
 132 .IR "IPV4 numbers-and-dots notation" .
 133 The form that uses exactly four decimal numbers is referred to as
 134 .IR "IPv4 dotted-decimal notation"
 135 (or sometimes:
 136 .IR "IPv4 dotted-quad notation" ).
 137 .PP
 138 .BR inet_aton ()
 139 returns 1 if the supplied string was successfully interpreted,
 140 or 0 if the string is invalid
 141 .RB ( errno
 142 is
 143 .I not
 144 set on error).
 145 .PP
 146 The
 147 .BR inet_addr ()
 148 function converts the Internet host address
 149 \fIcp\fP from IPv4 numbers-and-dots notation into binary data in network
 150 byte order.
 151 If the input is invalid,
 152 .B INADDR_NONE
 153 (usually \-1) is returned.
 154 Use of this function is problematic because \-1 is a valid address
 155 (255.255.255.255).
 156 Avoid its use in favor of
 157 .BR inet_aton (),
 158 .BR inet_pton (3),
 159 or
 160 .BR getaddrinfo (3),
 161 which provide a cleaner way to indicate error return.
 162 .PP
 163 The
 164 .BR inet_network ()
 165 function converts
 166 .IR cp ,
 167 a string in IPv4 numbers-and-dots notation,
 168 into a number in host byte order suitable for use as an
 169 Internet network address.
 170 On success, the converted address is returned.
 171 If the input is invalid, \-1 is returned.
 172 .PP
 173 The
 174 .BR inet_ntoa ()
 175 function converts the Internet host address
 176 \fIin\fP, given in network byte order, to a string in IPv4
 177 dotted-decimal notation.
 178 The string is returned in a statically
 179 allocated buffer, which subsequent calls will overwrite.
 180 .PP
 181 The
 182 .BR inet_lnaof ()
 183 function returns the local network address part
 184 of the Internet address \fIin\fP.
 185 The returned value is in host byte order.
 186 .PP
 187 The
 188 .BR inet_netof ()
 189 function returns the network number part of
 190 the Internet address \fIin\fP.
 191 The returned value is in host byte order.
 192 .PP
 193 The
 194 .BR inet_makeaddr ()
 195 function is the converse of
 196 .BR inet_netof ()
 197 and
 198 .BR inet_lnaof ().
 199 It returns an Internet host address in network byte order,
 200 created by combining the network number \fInet\fP
 201 with the local address \fIhost\fP, both in
 202 host byte order.
 203 .PP
 204 The structure \fIin_addr\fP as used in
 205 .BR inet_ntoa (),
 206 .BR inet_makeaddr (),
 207 .BR inet_lnaof ()
 208 and
 209 .BR inet_netof ()
 210 is defined in
 211 .I <netinet/in.h>
 212 as:
 213 .PP
 214 .in +4n
 215 .EX
 216 typedef uint32_t in_addr_t;
 217
 218 struct in_addr {
 219     in_addr_t s_addr;
 220 };
 221 .EE
 222 .in
 223 .SH ATTRIBUTES
 224 For an explanation of the terms used in this section, see
 225 .BR attributes (7).
 226 .TS
 227 allbox;
 228 lbw30 lb lb
 229 l l l.
 230 Interface       Attribute       Value
 231 T{
 232 .BR inet_aton (),
 233 .BR inet_addr (),
 234 .br
 235 .BR inet_network (),
 236 .BR inet_ntoa ()
 237 T}      Thread safety   MT-Safe locale
 238 T{
 239 .BR inet_makeaddr (),
 240 .BR inet_lnaof (),
 241 .br
 242 .BR inet_netof ()
 243 T}      Thread safety   MT-Safe
 244 .TE
 245 .SH CONFORMING TO
 246 .BR inet_addr (),
 247 .BR inet_ntoa ():
 248 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
 249 .PP
 250 .BR inet_aton ()
 251 is not specified in POSIX.1, but is available on most systems.
 252 .SH NOTES
 253 On x86 architectures, the host byte order is Least Significant Byte
 254 first (little endian), whereas the network byte order, as used on the
 255 Internet, is Most Significant Byte first (big endian).
 256 .PP
 257 .BR inet_lnaof (),
 258 .BR inet_netof (),
 259 and
 260 .BR inet_makeaddr ()
 261 are legacy functions that assume they are dealing with
 262 .IR "classful network addresses" .
 263 Classful networking divides IPv4 network addresses into host and network
 264 components at byte boundaries, as follows:
 265 .TP 10
 266 Class A
 267 This address type is indicated by the value 0 in the
 268 most significant bit of the (network byte ordered) address.
 269 The network address is contained in the most significant byte,
 270 and the host address occupies the remaining three bytes.
 271 .TP
 272 Class B
 273 This address type is indicated by the binary value 10 in the
 274 most significant two bits of the address.
 275 The network address is contained in the two most significant bytes,
 276 and the host address occupies the remaining two bytes.
 277 .TP
 278 Class C
 279 This address type is indicated by the binary value 110 in the
 280 most significant three bits of the address.
 281 The network address is contained in the three most significant bytes,
 282 and the host address occupies the remaining byte.
 283 .PP
 284 Classful network addresses are now obsolete,
 285 having been superseded by Classless Inter-Domain Routing (CIDR),
 286 which divides addresses into network and host components at
 287 arbitrary bit (rather than byte) boundaries.
 288 .SH EXAMPLE
 289 An example of the use of
 290 .BR inet_aton ()
 291 and
 292 .BR inet_ntoa ()
 293 is shown below.
 294 Here are some example runs:
 295 .PP
 296 .in +4n
 297 .EX
 298 .RB "$" " ./a.out 226.000.000.037" "      # Last byte is in octal"
 299 226.0.0.31
 300 .RB "$" " ./a.out 0x7f.1         " "      # First byte is in hex"
 301 127.0.0.1
 302 .EE
 303 .in
 304 .SS Program source
 305 \&
 306 .EX
 307 #define _BSD_SOURCE
 308 #include <arpa/inet.h>
 309 #include <stdio.h>
 310 #include <stdlib.h>
 311
 312 int
 313 main(int argc, char *argv[])
 314 {
 315     struct in_addr addr;
 316
 317     if (argc != 2) {
 318         fprintf(stderr, "%s <dotted\-address>\\n", argv[0]);
 319         exit(EXIT_FAILURE);
 320     }
 321
 322     if (inet_aton(argv[1], &addr) == 0) {
 323         fprintf(stderr, "Invalid address\\n");
 324         exit(EXIT_FAILURE);
 325     }
 326
 327     printf("%s\\n", inet_ntoa(addr));
 328     exit(EXIT_SUCCESS);
 329 }
 330 .EE
 331 .SH SEE ALSO
 332 .BR byteorder (3),
 333 .BR getaddrinfo (3),
 334 .BR gethostbyname (3),
 335 .BR getnameinfo (3),
 336 .BR getnetent (3),
 337 .BR inet_net_pton (3),
 338 .BR inet_ntop (3),
 339 .BR inet_pton (3),
 340 .BR hosts (5),
 341 .BR networks (5)