man3/inet.3

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   3 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   4 .\" and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk <mtk.manpages@gmail.com>
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  23 .\" Formatted or processed versions of this manual, if unaccompanied by
  24 .\" the source, must acknowledge the copyright and authors of this work.
  25 .\"
  26 .\" References consulted:
  27 .\"     Linux libc source code
  28 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  29 .\"     386BSD man pages
  30 .\"     libc.info (from glibc distribution)
  31 .\" Modified Sat Jul 24 19:12:00 1993 by Rik Faith <faith@cs.unc.edu>
  32 .\" Modified Sun Sep  3 20:29:36 1995 by Jim Van Zandt <jrv@vanzandt.mv.com>
  33 .\" Changed network into host byte order (for inet_network),
  34 .\"     Andreas Jaeger <aj@arthur.rhein-neckar.de>, 980130.
  35 .\" 2008-06-19, mtk
  36 .\"     Describe the various address forms supported by inet_aton().
  37 .\"     Clarify discussion of inet_lnaof(), inet_netof(), and inet_makeaddr().
  38 .\"     Add discussion of Classful Addressing, noting that it is obsolete.
  39 .\"     Added an EXAMPLE program.
  40 .\"
  41 .TH INET 3  2008-06-19 "GNU" "Linux Programmer's Manual"
  42 .SH NAME
  43 inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof,
  44 inet_netof \- Internet address manipulation routines
  45 .SH SYNOPSIS
  46 .nf
  47 .B #include <sys/socket.h>
  48 .B #include <netinet/in.h>
  49 .B #include <arpa/inet.h>
  50 .sp
  51 .BI "int inet_aton(const char *" cp ", struct in_addr *" inp );
  52 .sp
  53 .BI "in_addr_t inet_addr(const char *" cp );
  54 .sp
  55 .BI "in_addr_t inet_network(const char *" cp );
  56 .sp
  57 .BI "char *inet_ntoa(struct in_addr " in );
  58 .sp
  59 .BI "struct in_addr inet_makeaddr(int " net ", int " host );
  60 .sp
  61 .BI "in_addr_t inet_lnaof(struct in_addr " in );
  62 .sp
  63 .BI "in_addr_t inet_netof(struct in_addr " in );
  64 .fi
  65 .sp
  66 .in -4n
  67 Feature Test Macro Requirements for glibc (see
  68 .BR feature_test_macros (7)):
  69 .in
  70 .sp
  71 .BR inet_aton (),
  72 .BR inet_ntoa ():
  73 _BSD_SOURCE || _SVID_SOURCE
  74 .SH DESCRIPTION
  75 .BR inet_aton ()
  76 converts the Internet host address \fIcp\fP from the
  77 IPv4 numbers-and-dots notation into binary form (in network byte order)
  78 and stores it in the structure that \fIinp\fP points to.
  79 .BR inet_aton ()
  80 returns non-zero if the address is valid, zero if not.
  81 The address supplied in
  82 .I cp
  83 can have one of the following forms:
  84 .TP 10
  85 .I a.b.c.d
  86 Each of the for numeric parts specifies a byte of the address;
  87 the bytes are assigned in left-to-right order to produce the binary address.
  88 .TP
  89 .I a.b.c
  90 Parts
  91 .I a
  92 and
  93 .I b
  94 specify the first two bytes of the binary address.
  95 Part
  96 .I c
  97 is interpreted as a 16-bit value that defines the rightmost two bytes
  98 of the binary address.
  99 This notation is suitable for specifying (outmoded) Class B
 100 network addresses.
 101 .TP
 102 .I a.b
 103 Part
 104 .I a
 105 specifies the first byte of the binary address.
 106 Part
 107 .I b
 108 is interpreted as a 24-bit value that defines the rightmost three bytes
 109 of the binary address.
 110 This notation is suitable for specifying (outmoded) Class C
 111 network addresses.
 112 .TP
 113 .I a
 114 The value
 115 .I a
 116 is interpreted as a 32-bit value that is stored directly
 117 into the binary address without any byte rearrangement.
 118 .PP
 119 In all of the above forms,
 120 components of the dotted address can be specified in decimal,
 121 octal (with a leading
 122 .IR 0 ),
 123 or hexadecimal, with a leading
 124 .IR 0X ).
 125 Addresses in any of these forms are collectively termed
 126 .IR "IPV4 numbers-and-dots notation" .
 127 The form that uses exactly four decimal numbers is referred to as
 128 .IR "IPv4 dotted-decimal notation"
 129 (or sometimes:
 130 .IR "IPv4 dotted-quad notation" ).
 131 .PP
 132 The
 133 .BR inet_addr ()
 134 function converts the Internet host address
 135 \fIcp\fP from IPv4 numbers-and-dots notation into binary data in network
 136 byte order.
 137 If the input is invalid,
 138 .B INADDR_NONE
 139 (usually \-1) is returned.
 140 Use of this function is problematic because \-1 is a valid address
 141 (255.255.255.255).
 142 Avoid its use in favor of
 143 .BR inet_aton (),
 144 .BR inet_pton (3),
 145 or
 146 .BR getaddrinfo (3)
 147 which provide a cleaner way to indicate error return.
 148 .PP
 149 The
 150 .BR inet_network ()
 151 function converts
 152 .IR cp ,
 153 a string in IPv4 numbers-and-dots notation,
 154 into a number in host byte order suitable for use as an
 155 Internet network address.
 156 On success, the converted address is returned.
 157 If the input is invalid, \-1 is returned.
 158 .PP
 159 The
 160 .BR inet_ntoa ()
 161 function converts the Internet host address
 162 \fIin\fP, given in network byte order, to a string in IPv4
 163 dotted-decimal notation.
 164 The string is returned in a statically
 165 allocated buffer, which subsequent calls will overwrite.
 166 .PP
 167 The
 168 .BR inet_lnaof ()
 169 function returns the local network address part
 170 of the Internet address \fIin\fP.
 171 The returned value is in host byte order.
 172 .PP
 173 The
 174 .BR inet_netof ()
 175 function returns the network number part of
 176 the Internet address \fIin\fP.
 177 The returned value is in host byte order.
 178 .PP
 179 The
 180 .BR inet_makeaddr ()
 181 function is the converse of
 182 .BR inet_netof ()
 183 and
 184 .BR inet_lnaof ().
 185 It returns an Internet host address in network byte order,
 186 created by combining the network number \fInet\fP
 187 with the local address \fIhost\fP, both in
 188 host byte order.
 189 .PP
 190 The structure \fIin_addr\fP as used in
 191 .BR inet_ntoa (),
 192 .BR inet_makeaddr (),
 193 .BR inet_lnaof ()
 194 and
 195 .BR inet_netof ()
 196 is defined in
 197 .I <netinet/in.h>
 198 as:
 199 .sp
 200 .in +4n
 201 .nf
 202 typedef uint32_t in_addr_t;
 203
 204 struct in_addr {
 205     in_addr_t s_addr;
 206 };
 207 .fi
 208 .in
 209 .SH "CONFORMING TO"
 210 4.3BSD.
 211 .BR inet_addr ()
 212 and
 213 .BR inet_ntoa ()
 214 are specified in POSIX.1-2001.
 215 .BR inet_aton ()
 216 is not specified in POSIX.1.-2001, but is available on most systems.
 217 .SH NOTES
 218 On the i386 the host byte order is Least Significant Byte
 219 first (little endian), whereas the network byte order, as used on the
 220 Internet, is Most Significant Byte first (big endian).
 221
 222 .BR inet_lnaof (),
 223 .BR inet_netof (),
 224 and
 225 .BR inet_makeaddr ()
 226 are legacy functions that assume they are dealing with
 227 .IR "classful network addresses" .
 228 Classful networking divides IPv4 network addresses into host and network
 229 components at byte boundaries, as follows:
 230 .TP 10
 231 Class A
 232 This address type is indicated by the value 0 in the
 233 most significant bit of the (network byte ordered) address.
 234 The network address is contained in the most significant byte,
 235 and the host address occupies the remaining three bytes.
 236 .TP
 237 Class B
 238 This address type is indicated by the binary value 10 in the
 239 most significant two bits of the address.
 240 The network address is contained in the two most significant bytes,
 241 and the host address occupies the remaining two bytes.
 242 .TP
 243 Class C
 244 This address type is indicated by the binary value 110 in the
 245 most significant three bits of the address.
 246 The network address is contained in the three most significant bytes,
 247 and the host address occupies the remaining byte.
 248 .PP
 249 Classful network addresses are now obolete,
 250 having been superseded by Classless Inter-Domain Routing (CIDR),
 251 which divides addresses into network and host components at
 252 arbitrary bit (rather than byte) boundaries.
 253 .SH EXAMPLE
 254 An example of the use of
 255 .BR inet_aton ()
 256 and
 257 .BR inet_ntoa ()
 258 is shown below.
 259 Here are some example runs:
 260 .in +4n
 261 .nf
 262
 263 $ ./a.out 226.000.000.037   # Last byte is in octal
 264 226.0.0.31
 265 $ ./a.out 0x7f.1            # First byte is in hex
 266 127.0.0.1
 267
 268 .fi
 269 .in
 270 .nf
 271 #define _BSD_SOURCE
 272 #include <arpa/inet.h>
 273 #include <stdio.h>
 274 #include <stdlib.h>
 275
 276 int
 277 main(int argc, char *argv[])
 278 {
 279     struct in_addr addr;
 280
 281     if (argc != 2) {
 282         fprintf(stderr, "%s <dotted\-address>\\n", argv[0]);
 283         exit(EXIT_FAILURE);
 284     }
 285
 286     if (inet_aton(argv[1], &addr) == 0) {
 287         perror("inet_aton");
 288         exit(EXIT_FAILURE);
 289     }
 290
 291     printf("%s\\n", inet_ntoa(addr));
 292     exit(EXIT_SUCCESS);
 293 }
 294 .fi
 295 .SH "SEE ALSO"
 296 .BR byteorder (3),
 297 .BR getaddrinfo (3),
 298 .BR gethostbyname (3),
 299 .BR getnameinfo (3),
 300 .BR getnetent (3),
 301 .BR inet_ntop (3),
 302 .BR inet_pton (3),
 303 .BR hosts (5),
 304 .BR networks (5)