1INET_NET_PTON(3) Linux Programmer's Manual INET_NET_PTON(3)
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6 inet_net_pton, inet_net_ntop - Internet network number conversion
7
9 #include <arpa/inet.h>
10 int inet_net_pton(int af, const char *pres,
12 void *netp, size_t nsize);
13 char *inet_net_ntop(int af, const void *netp, int bits,
15 char *pres, size_t psize);
16 Link with -lresolv.
18 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20 inet_net_pton(), inet_net_ntop():
22 Since glibc 2.20:
23 _DEFAULT_SOURCE
24 Before glibc 2.20:
25 _BSD_SOURCE || _SVID_SOURCE
26
28 These functions convert network numbers between presentation (i.e.,
29 printable) format and network (i.e., binary) format.
30 For both functions, af specifies the address family for the conversion;
32 the only supported value is AF_INET.
33 inet_net_pton()
35 The inet_net_pton() function converts pres, a null-terminated string
36 containing an Internet network number in presentation format to network
37 format. The result of the conversion, which is in network byte order,
38 is placed in the buffer pointed to by net. (The netp argument typi‐
39 cally points to an in_addr structure.) The nsize argument specifies
40 the number of bytes available in netp.
41 On success, inet_net_pton() returns the number of bits in the network
43 number field of the result placed in netp. For a discussion of the
44 input presentation format and the return value, see NOTES.
45 Note: the buffer pointed to by netp should be zeroed out before calling
47 inet_net_pton(), since the call writes only as many bytes as are
48 required for the network number (or as are explicitly specified by
49 pres), which may be less than the number of bytes in a complete network
50 address.
51 inet_net_ntop()
53 The inet_net_ntop() function converts the network number in the buffer
54 pointed to by netp to presentation format; *netp is interpreted as a
55 value in network byte order. The bits argument specifies the number of
56 bits in the network number in *netp.
57 The null-terminated presentation-format string is placed in the buffer
59 pointed to by pres. The psize argument specifies the number of bytes
60 available in pres. The presentation string is in CIDR format: a dot‐
61 ted-decimal number representing the network address, followed by a
62 slash, and the size of the network number in bits.
63
65 On success, inet_net_pton() returns the number of bits in the network
66 number. On error, it returns -1, and errno is set to indicate the
67 cause of the error.
68 On success, inet_net_ntop() returns pres. On error, it returns NULL,
70 and errno is set to indicate the cause of the error.
71
73 EAFNOSUPPORT
74 af specified a value other than AF_INET.
75 EMSGSIZE
77 The size of the output buffer was insufficient.
78 ENOENT (inet_net_pton()) pres was not in correct presentation format.
80
82 The inet_net_pton() and inet_net_ntop() functions are nonstandard, but
83 widely available.
84
86 Input presentation format for inet_net_pton()
87 The network number may be specified either as a hexadecimal value or in
88 dotted-decimal notation.
89 Hexadecimal values are indicated by an initial "0x" or "0X". The hexa‐
91 decimal digits populate the nibbles (half octets) of the network number
92 from left to right in network byte order.
93 In dotted-decimal notation, up to four octets are specified, as decimal
95 numbers separated by dots. Thus, any of the following forms are
96 accepted:
97 a.b.c.d
99 a.b.c
100 a.b
101 a
102 Each part is a number in the range 0 to 255 that populates one byte of
104 the resulting network number, going from left to right, in network-byte
105 (big endian) order. Where a part is omitted, the resulting byte in the
106 network number is zero.
107 For either hexadecimal or dotted-decimal format, the network number can
109 optionally be followed by a slash and a number in the range 0 to 32,
110 which specifies the size of the network number in bits.
111 Return value of inet_net_pton()
113 The return value of inet_net_pton() is the number of bits in the net‐
114 work number field. If the input presentation string terminates with a
115 slash and an explicit size value, then that size becomes the return
116 value of inet_net_pton(). Otherwise, the return value, bits, is
117 inferred as follows:
118 * If the most significant byte of the network number is greater than
120 or equal to 240, then bits is 32.
121 * Otherwise, if the most significant byte of the network number is
123 greater than or equal to 224, then bits is 4.
124 * Otherwise, if the most significant byte of the network number is
126 greater than or equal to 192, then bits is 24.
127 * Otherwise, if the most significant byte of the network number is
129 greater than or equal to 128, then bits is 16.
130 * Otherwise, bits is 8.
132 If the resulting bits value from the above steps is greater than or
134 equal to 8, but the number of octets specified in the network number
135 exceed bits/8, then bits is set to 8 times the number of octets actu‐
136 ally specified.
137
139 The program below demonstrates the use of inet_net_pton() and
140 inet_net_ntop(). It uses inet_net_pton() to convert the presentation
141 format network address provided in its first command-line argument to
142 binary form, displays the return value from inet_net_pton(). It then
143 uses inet_net_ntop() to convert the binary form back to presentation
144 format, and displays the resulting string.
145 In order to demonstrate that inet_net_pton() may not write to all bytes
147 of its netp argument, the program allows an optional second command-
148 line argument, a number used to initialize the buffer before
149 inet_net_pton() is called. As its final line of output, the program
150 displays all of the bytes of the buffer returned by inet_net_pton()
151 allowing the user to see which bytes have not been touched by
152 inet_net_pton().
153 An example run, showing that inet_net_pton() infers the number of bits
155 in the network number:
156 $ ./a.out 193.168
158 inet_net_pton() returned: 24
159 inet_net_ntop() yielded: 193.168.0/24
160 Raw address: c1a80000
161 Demonstrate that inet_net_pton() does not zero out unused bytes in its
163 result buffer:
164 $ ./a.out 193.168 0xffffffff
166 inet_net_pton() returned: 24
167 inet_net_ntop() yielded: 193.168.0/24
168 Raw address: c1a800ff
169 Demonstrate that inet_net_pton() will widen the inferred size of the
171 network number, if the supplied number of bytes in the presentation
172 string exceeds the inferred value:
173 $ ./a.out 193.168.1.128
175 inet_net_pton() returned: 32
176 inet_net_ntop() yielded: 193.168.1.128/32
177 Raw address: c1a80180
178 Explicitly specifying the size of the network number overrides any
180 inference about its size (but any extra bytes that are explicitly spec‐
181 ified will still be used by inet_net_pton(): to populate the result
182 buffer):
183 $ ./a.out 193.168.1.128/24
185 inet_net_pton() returned: 24
186 inet_net_ntop() yielded: 193.168.1/24
187 Raw address: c1a80180
188 Program source
190 /* Link with "-lresolv" */
191 #include <arpa/inet.h>
193 #include <stdio.h>
194 #include <stdlib.h>
195 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
197 } while (0)
198 int
200 main(int argc, char *argv[])
201 {
202 char buf[100];
203 struct in_addr addr;
204 int bits;
205 if (argc < 2) {
207 fprintf(stderr,
208 "Usage: %s presentation-form [addr-init-value]\n",
209 argv[0]);
210 exit(EXIT_FAILURE);
211 }
212 /* If argv[2] is supplied (a numeric value), use it to initialize
214 the output buffer given to inet_net_pton(), so that we can see
215 that inet_net_pton() initializes only those bytes needed for
216 the network number. If argv[2] is not supplied, then initialize
217 the buffer to zero (as is recommended practice). */
218 addr.s_addr = (argc > 2) ? strtod(argv[2], NULL) : 0;
220 /* Convert presentation network number in argv[1] to binary */
222 bits = inet_net_pton(AF_INET, argv[1], &addr, sizeof(addr));
224 if (bits == -1)
225 errExit("inet_net_ntop");
226 printf("inet_net_pton() returned: %d\n", bits);
228 /* Convert binary format back to presentation, using 'bits'
230 returned by inet_net_pton() */
231 if (inet_net_ntop(AF_INET, &addr, bits, buf, sizeof(buf)) == NULL)
233 errExit("inet_net_ntop");
234 printf("inet_net_ntop() yielded: %s\n", buf);
236 /* Display 'addr' in raw form (in network byte order), so we can
238 see bytes not displayed by inet_net_ntop(); some of those bytes
239 may not have been touched by inet_net_ntop(), and so will still
240 have any initial value that was specified in argv[2]. */
241 printf("Raw address: %x\n", htonl(addr.s_addr));
243 exit(EXIT_SUCCESS);
245 }
246
248 inet(3), networks(5)
249
251 This page is part of release 5.07 of the Linux man-pages project. A
252 description of the project, information about reporting bugs, and the
253 latest version of this page, can be found at
254 https://www.kernel.org/doc/man-pages/.
255Linux 2020-06-09 INET_NET_PTON(3)