1inet_net_pton(3) Library Functions Manual inet_net_pton(3)
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6 inet_net_pton, inet_net_ntop - Internet network number conversion
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9 Resolver library (libresolv, -lresolv)
10
12 #include <arpa/inet.h>
13 int inet_net_pton(int af, const char *pres,
15 void netp[.nsize], size_t nsize);
16 char *inet_net_ntop(int af,
17 const void netp[(.bits - CHAR_BIT + 1) / CHAR_BIT],
18 int bits,
19 char pres[.psize], size_t psize);
20 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
22 inet_net_pton(), inet_net_ntop():
24 Since glibc 2.20:
25 _DEFAULT_SOURCE
26 Before glibc 2.20:
27 _BSD_SOURCE || _SVID_SOURCE
28
30 These functions convert network numbers between presentation (i.e.,
31 printable) format and network (i.e., binary) format.
32 For both functions, af specifies the address family for the conversion;
34 the only supported value is AF_INET.
35 inet_net_pton()
37 The inet_net_pton() function converts pres, a null-terminated string
38 containing an Internet network number in presentation format to network
39 format. The result of the conversion, which is in network byte order,
40 is placed in the buffer pointed to by netp. (The netp argument typi‐
41 cally points to an in_addr structure.) The nsize argument specifies
42 the number of bytes available in netp.
43 On success, inet_net_pton() returns the number of bits in the network
45 number field of the result placed in netp. For a discussion of the in‐
46 put presentation format and the return value, see NOTES.
47 Note: the buffer pointed to by netp should be zeroed out before calling
49 inet_net_pton(), since the call writes only as many bytes as are re‐
50 quired for the network number (or as are explicitly specified by pres),
51 which may be less than the number of bytes in a complete network ad‐
52 dress.
53 inet_net_ntop()
55 The inet_net_ntop() function converts the network number in the buffer
56 pointed to by netp to presentation format; *netp is interpreted as a
57 value in network byte order. The bits argument specifies the number of
58 bits in the network number in *netp.
59 The null-terminated presentation-format string is placed in the buffer
61 pointed to by pres. The psize argument specifies the number of bytes
62 available in pres. The presentation string is in CIDR format: a dot‐
63 ted-decimal number representing the network address, followed by a
64 slash, and the size of the network number in bits.
65
67 On success, inet_net_pton() returns the number of bits in the network
68 number. On error, it returns -1, and errno is set to indicate the er‐
69 ror.
70 On success, inet_net_ntop() returns pres. On error, it returns NULL,
72 and errno is set to indicate the error.
73
75 EAFNOSUPPORT
76 af specified a value other than AF_INET.
77 EMSGSIZE
79 The size of the output buffer was insufficient.
80 ENOENT (inet_net_pton()) pres was not in correct presentation format.
82
84 None.
85
87 Input presentation format for inet_net_pton()
88 The network number may be specified either as a hexadecimal value or in
89 dotted-decimal notation.
90 Hexadecimal values are indicated by an initial "0x" or "0X". The hexa‐
92 decimal digits populate the nibbles (half octets) of the network number
93 from left to right in network byte order.
94 In dotted-decimal notation, up to four octets are specified, as decimal
96 numbers separated by dots. Thus, any of the following forms are ac‐
97 cepted:
98 a.b.c.d
100 a.b.c
101 a.b
102 a
103 Each part is a number in the range 0 to 255 that populates one byte of
105 the resulting network number, going from left to right, in network-byte
106 (big endian) order. Where a part is omitted, the resulting byte in the
107 network number is zero.
108 For either hexadecimal or dotted-decimal format, the network number can
110 optionally be followed by a slash and a number in the range 0 to 32,
111 which specifies the size of the network number in bits.
112 Return value of inet_net_pton()
114 The return value of inet_net_pton() is the number of bits in the net‐
115 work number field. If the input presentation string terminates with a
116 slash and an explicit size value, then that size becomes the return
117 value of inet_net_pton(). Otherwise, the return value, bits, is in‐
118 ferred as follows:
119 • If the most significant byte of the network number is greater than
121 or equal to 240, then bits is 32.
122 • Otherwise, if the most significant byte of the network number is
124 greater than or equal to 224, then bits is 4.
125 • Otherwise, if the most significant byte of the network number is
127 greater than or equal to 192, then bits is 24.
128 • Otherwise, if the most significant byte of the network number is
130 greater than or equal to 128, then bits is 16.
131 • Otherwise, bits is 8.
133 If the resulting bits value from the above steps is greater than or
135 equal to 8, but the number of octets specified in the network number
136 exceed bits/8, then bits is set to 8 times the number of octets actu‐
137 ally specified.
138
140 The program below demonstrates the use of inet_net_pton() and
141 inet_net_ntop(). It uses inet_net_pton() to convert the presentation
142 format network address provided in its first command-line argument to
143 binary form, displays the return value from inet_net_pton(). It then
144 uses inet_net_ntop() to convert the binary form back to presentation
145 format, and displays the resulting string.
146 In order to demonstrate that inet_net_pton() may not write to all bytes
148 of its netp argument, the program allows an optional second command-
149 line argument, a number used to initialize the buffer before
150 inet_net_pton() is called. As its final line of output, the program
151 displays all of the bytes of the buffer returned by inet_net_pton() al‐
152 lowing the user to see which bytes have not been touched by
153 inet_net_pton().
154 An example run, showing that inet_net_pton() infers the number of bits
156 in the network number:
157 $ ./a.out 193.168
159 inet_net_pton() returned: 24
160 inet_net_ntop() yielded: 193.168.0/24
161 Raw address: c1a80000
162 Demonstrate that inet_net_pton() does not zero out unused bytes in its
164 result buffer:
165 $ ./a.out 193.168 0xffffffff
167 inet_net_pton() returned: 24
168 inet_net_ntop() yielded: 193.168.0/24
169 Raw address: c1a800ff
170 Demonstrate that inet_net_pton() will widen the inferred size of the
172 network number, if the supplied number of bytes in the presentation
173 string exceeds the inferred value:
174 $ ./a.out 193.168.1.128
176 inet_net_pton() returned: 32
177 inet_net_ntop() yielded: 193.168.1.128/32
178 Raw address: c1a80180
179 Explicitly specifying the size of the network number overrides any in‐
181 ference about its size (but any extra bytes that are explicitly speci‐
182 fied will still be used by inet_net_pton(): to populate the result buf‐
183 fer):
184 $ ./a.out 193.168.1.128/24
186 inet_net_pton() returned: 24
187 inet_net_ntop() yielded: 193.168.1/24
188 Raw address: c1a80180
189 Program source
191 /* Link with "-lresolv" */
192 #include <arpa/inet.h>
194 #include <stdio.h>
195 #include <stdlib.h>
196 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
198 } while (0)
199 int
201 main(int argc, char *argv[])
202 {
203 char buf[100];
204 struct in_addr addr;
205 int bits;
206 if (argc < 2) {
208 fprintf(stderr,
209 "Usage: %s presentation-form [addr-init-value]\n",
210 argv[0]);
211 exit(EXIT_FAILURE);
212 }
213 /* If argv[2] is supplied (a numeric value), use it to initialize
215 the output buffer given to inet_net_pton(), so that we can see
216 that inet_net_pton() initializes only those bytes needed for
217 the network number. If argv[2] is not supplied, then initialize
218 the buffer to zero (as is recommended practice). */
219 addr.s_addr = (argc > 2) ? strtod(argv[2], NULL) : 0;
221 /* Convert presentation network number in argv[1] to binary. */
223 bits = inet_net_pton(AF_INET, argv[1], &addr, sizeof(addr));
225 if (bits == -1)
226 errExit("inet_net_ntop");
227 printf("inet_net_pton() returned: %d\n", bits);
229 /* Convert binary format back to presentation, using 'bits'
231 returned by inet_net_pton(). */
232 if (inet_net_ntop(AF_INET, &addr, bits, buf, sizeof(buf)) == NULL)
234 errExit("inet_net_ntop");
235 printf("inet_net_ntop() yielded: %s\n", buf);
237 /* Display 'addr' in raw form (in network byte order), so we can
239 see bytes not displayed by inet_net_ntop(); some of those bytes
240 may not have been touched by inet_net_ntop(), and so will still
241 have any initial value that was specified in argv[2]. */
242 printf("Raw address: %x\n", htonl(addr.s_addr));
244 exit(EXIT_SUCCESS);
246 }
247
249 inet(3), networks(5)
250Linux man-pages 6.04 2023-03-30 inet_net_pton(3)