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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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inet_net_pton(3) Library Functions Manual inet_net_pton(3)
inet_net_pton, inet_net_ntop - Internet network number conversion
Resolver library (libresolv, -lresolv)
#include <arpa/inet.h>
int inet_net_pton(int af, const char *pres,
void netp[.nsize], size_t nsize);
char *inet_net_ntop(int af,
const void netp[(.bits - CHAR_BIT + 1) / CHAR_BIT],
int bits,
char pres[.psize], size_t psize);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
inet_net_pton(), inet_net_ntop():
Since glibc 2.20:
_DEFAULT_SOURCE
Before glibc 2.20:
_BSD_SOURCE || _SVID_SOURCE
These functions convert network numbers between presentation
(i.e., printable) format and network (i.e., binary) format.
For both functions, af specifies the address family for the
conversion; the only supported value is AF_INET.
inet_net_pton()
The inet_net_pton() function converts pres, a null-terminated
string containing an Internet network number in presentation
format to network format. The result of the conversion, which is
in network byte order, is placed in the buffer pointed to by
netp. (The netp argument typically points to an in_addr
structure.) The nsize argument specifies the number of bytes
available in netp.
On success, inet_net_pton() returns the number of bits in the
network number field of the result placed in netp. For a
discussion of the input presentation format and the return value,
see NOTES.
Note: the buffer pointed to by netp should be zeroed out before
calling inet_net_pton(), since the call writes only as many bytes
as are required for the network number (or as are explicitly
specified by pres), which may be less than the number of bytes in
a complete network address.
inet_net_ntop()
The inet_net_ntop() function converts the network number in the
buffer pointed to by netp to presentation format; *netp is
interpreted as a value in network byte order. The bits argument
specifies the number of bits in the network number in *netp.
The null-terminated presentation-format string is placed in the
buffer pointed to by pres. The psize argument specifies the
number of bytes available in pres. The presentation string is in
CIDR format: a dotted-decimal number representing the network
address, followed by a slash, and the size of the network number
in bits.
On success, inet_net_pton() returns the number of bits in the
network number. On error, it returns -1, and errno is set to
indicate the error.
On success, inet_net_ntop() returns pres. On error, it returns
NULL, and errno is set to indicate the error.
EAFNOSUPPORT
af specified a value other than AF_INET.
EMSGSIZE
The size of the output buffer was insufficient.
ENOENT (inet_net_pton()) pres was not in correct presentation
format.
None.
Input presentation format for inet_net_pton()
The network number may be specified either as a hexadecimal value
or in dotted-decimal notation.
Hexadecimal values are indicated by an initial "0x" or "0X". The
hexadecimal digits populate the nibbles (half octets) of the
network number from left to right in network byte order.
In dotted-decimal notation, up to four octets are specified, as
decimal numbers separated by dots. Thus, any of the following
forms are accepted:
a.b.c.d
a.b.c
a.b
a
Each part is a number in the range 0 to 255 that populates one
byte of the resulting network number, going from left to right,
in network-byte (big endian) order. Where a part is omitted, the
resulting byte in the network number is zero.
For either hexadecimal or dotted-decimal format, the network
number can optionally be followed by a slash and a number in the
range 0 to 32, which specifies the size of the network number in
bits.
Return value of inet_net_pton()
The return value of inet_net_pton() is the number of bits in the
network number field. If the input presentation string
terminates with a slash and an explicit size value, then that
size becomes the return value of inet_net_pton(). Otherwise, the
return value, bits, is inferred as follows:
• If the most significant byte of the network number is greater
than or equal to 240, then bits is 32.
• Otherwise, if the most significant byte of the network number
is greater than or equal to 224, then bits is 4.
• Otherwise, if the most significant byte of the network number
is greater than or equal to 192, then bits is 24.
• Otherwise, if the most significant byte of the network number
is greater than or equal to 128, then bits is 16.
• Otherwise, bits is 8.
If the resulting bits value from the above steps is greater than
or equal to 8, but the number of octets specified in the network
number exceed bits/8, then bits is set to 8 times the number of
octets actually specified.
The program below demonstrates the use of inet_net_pton() and
inet_net_ntop(). It uses inet_net_pton() to convert the
presentation format network address provided in its first
command-line argument to binary form, displays the return value
from inet_net_pton(). It then uses inet_net_ntop() to convert
the binary form back to presentation format, and displays the
resulting string.
In order to demonstrate that inet_net_pton() may not write to all
bytes of its netp argument, the program allows an optional second
command-line argument, a number used to initialize the buffer
before inet_net_pton() is called. As its final line of output,
the program displays all of the bytes of the buffer returned by
inet_net_pton() allowing the user to see which bytes have not
been touched by inet_net_pton().
An example run, showing that inet_net_pton() infers the number of
bits in the network number:
$ ./a.out 193.168
inet_net_pton() returned: 24
inet_net_ntop() yielded: 193.168.0/24
Raw address: c1a80000
Demonstrate that inet_net_pton() does not zero out unused bytes
in its result buffer:
$ ./a.out 193.168 0xffffffff
inet_net_pton() returned: 24
inet_net_ntop() yielded: 193.168.0/24
Raw address: c1a800ff
Demonstrate that inet_net_pton() will widen the inferred size of
the network number, if the supplied number of bytes in the
presentation string exceeds the inferred value:
$ ./a.out 193.168.1.128
inet_net_pton() returned: 32
inet_net_ntop() yielded: 193.168.1.128/32
Raw address: c1a80180
Explicitly specifying the size of the network number overrides
any inference about its size (but any extra bytes that are
explicitly specified will still be used by inet_net_pton(): to
populate the result buffer):
$ ./a.out 193.168.1.128/24
inet_net_pton() returned: 24
inet_net_ntop() yielded: 193.168.1/24
Raw address: c1a80180
Program source
/* Link with "-lresolv" */
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
int
main(int argc, char *argv[])
{
char buf[100];
struct in_addr addr;
int bits;
if (argc < 2) {
fprintf(stderr,
"Usage: %s presentation-form [addr-init-value]\n",
argv[0]);
exit(EXIT_FAILURE);
}
/* If argv[2] is supplied (a numeric value), use it to initialize
the output buffer given to inet_net_pton(), so that we can see
that inet_net_pton() initializes only those bytes needed for
the network number. If argv[2] is not supplied, then initialize
the buffer to zero (as is recommended practice). */
addr.s_addr = (argc > 2) ? strtod(argv[2], NULL) : 0;
/* Convert presentation network number in argv[1] to binary. */
bits = inet_net_pton(AF_INET, argv[1], &addr, sizeof(addr));
if (bits == -1)
errExit("inet_net_ntop");
printf("inet_net_pton() returned: %d\n", bits);
/* Convert binary format back to presentation, using 'bits'
returned by inet_net_pton(). */
if (inet_net_ntop(AF_INET, &addr, bits, buf, sizeof(buf)) == NULL)
errExit("inet_net_ntop");
printf("inet_net_ntop() yielded: %s\n", buf);
/* Display 'addr' in raw form (in network byte order), so we can
see bytes not displayed by inet_net_ntop(); some of those bytes
may not have been touched by inet_net_ntop(), and so will still
have any initial value that was specified in argv[2]. */
printf("Raw address: %x\n", htonl(addr.s_addr));
exit(EXIT_SUCCESS);
}
inet(3), networks(5)
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Linux man-pages 6.9.1 2024-06-15 inet_net_pton(3)
Pages that refer to this page: inet(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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