INET(3C) Standard C Library Functions INET(3C)

NAME

inet, inet6, inet_ntop, inet_pton, inet_aton, inet_addr, inet_network,
inet_makeaddr, inet_lnaof, inet_netof, inet_ntoa - Internet address
manipulation

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

const char *
inet_ntop(int af, const void *addr, char *cp, size_t size);

int
inet_pton(int af, const char *cp, void *addr);

int
inet_aton(const char *cp, struct in_addr *addr);

in_addr_t
inet_addr(const char *cp);

in_addr_t
inet_network(const char *cp);

struct in_addr
inet_makeaddr(const int net, const int lna);

in_addr_t
inet_lnaof(const struct in_addr in);

in_addr_t
inet_netof(const struct in_addr in);

char *
inet_ntoa(const struct in_addr in);

DESCRIPTION

The inet_ntop() and inet_pton() functions can manipulate both IPv4 and
IPv6 addresses. The inet_aton(), inet_addr(), inet_network(),
inet_makeaddr(), inet_lnaof(), inet_netof(), and inet_ntoa() functions
can only manipulate IPv4 addresses.

The inet_ntop() function converts a numeric address into a string
suitable for presentation. The af argument specifies the family of the
address which can be AF_INET or AF_INET6. The addr argument points to
a buffer that holds an IPv4 address if the af argument is AF_INET. The
addr argument points to a buffer that holds an IPv6 address if the af
argument is AF_INET6. The address must be in network byte order. The
cp argument points to a buffer where the function stores the resulting
string. The application must specify a non-NULL cp argument. The size
argument specifies the size of this buffer. For IPv6 addresses, the
buffer must be at least 46-octets. For IPv4 addresses, the buffer must
be at least 16-octets. To allow applications to easily declare buffers
of the proper size to store IPv4 and IPv6 addresses in string form, the
following two constants are defined in <netinet/in.h>:

#define INET_ADDRSTRLEN 16
#define INET6_ADDRSTRLEN 46

The inet_pton() function converts the standard text presentation form
of a function to the numeric binary form. The af argument specifies
the family of the address. Currently, the AF_INET and AF_INET6 address
families are supported. The cp argument points to the string being
passed in. The addr argument points to a buffer where the function
stores the numeric address. The calling application must ensure that
the buffer referred to by addr is large enough to hold the numeric
address, at least 4 bytes for AF_INET or 16 bytes for AF_INET6.

The inet_aton(), inet_addr(), and inet_network() functions interpret
character strings that represent numbers expressed in the IPv4 standard
`.' notation, returning numbers suitable for use as IPv4 addresses and
IPv4 network numbers, respectively. The inet_makeaddr() function uses
an IPv4 network number and a local network address to construct an IPv4
address. The inet_netof() and inet_lnaof() functions break apart IPv4
host addresses, then return the network number and local network
address, respectively.

The inet_ntoa() function returns a pointer to a string in the base 256
notation `d.d.d.d'. See the following section on IPv4 addresses.

Internet addresses are returned in network order, bytes ordered from
left to right. Network numbers and local address parts are returned as
machine format integer values.

IPv6 Addresses
There are three conventional forms for representing IPv6 addresses as
strings:

1. The preferred form is `x:x:x:x:x:x:x:x', where the x 's are the
hexadecimal values of the eight 16-bit pieces of the address. For
example, `1080:0:0:0:8:800:200C:417A'.

It is not necessary to write the leading zeros in an individual
field. There must be at least one numeral in every field, except
when the special syntax described in the following is used.

2. It is common for addresses to contain long strings of zero bits in
some methods used to allocate certain IPv6 address styles. A
special syntax is available to compress the zeros. The use of
`::' indicates multiple groups of 16 bits of zeros. The `::' may
only appear once in an address. The `::' can also be used to
compress the leading and trailing zeros in an address. For
example, `1080::8:800:200C:417A'.

3. The alternative form `x:x:x:x:x:x:d.d.d.d' is sometimes more
convenient when dealing with a mixed environment of IPv4 and IPv6
nodes. The x 's in this form represent the hexadecimal values of
the six high-order 16-bit pieces of the address. The d 's
represent the decimal values of the four low-order 8-bit pieces of
the standard IPv4 address. For example:

::FFFF:129.144.52.38 .
::129.144.52.38

The `::FFFF:d.d.d.d' and `::d.d.d.d' pieces are the general forms
of an IPv4-mapped IPv6 address and an IPv4-compatible IPv6
address.

The IPv4 portion must be in the `d.d.d.d' form. The following
forms are invalid:

::FFFF:d.d.d
::FFFF:d.d
::d.d.d
::d.d

The `::FFFF:d' form is a valid but unconventional representation
of the IPv4-compatible IPv6 address `::255.255.0.d'.

The `::d' form corresponds to the general IPv6 address
`0:0:0:0:0:0:0:d'.

IPv4 Addresses
Values specified using `.' notation take one of the following forms:

d.d.d.d
d.d.d
d.d
d

When four parts are specified, each part is interpreted as a byte of
data and assigned from left to right to the four bytes of an IPv4
address.

When a three-part address is specified, the last part is interpreted as
a 16-bit quantity and placed in the right most two bytes of the network
address. The three part address format is convenient for specifying
Class B network addresses such as `128.net.host'.

When a two-part address is supplied, the last part is interpreted as a
24-bit quantity and placed in the right most three bytes of the network
address. The two part address format is convenient for specifying
Class A network addresses such as `net.host'.

When only one part is given, the value is stored directly in the
network address without any byte rearrangement.

With the exception of inet_pton(), numbers supplied as parts in `.'
notation may be decimal, octal, or hexadecimal, as specified in C
language. For example, a leading `0x' or `0X' implies hexadecimal. A
leading `0' implies octal. Otherwise, the number is interpreted as
decimal.

For IPv4 addresses, inet_pton() accepts only a string in standard IPv4
dot notation `d.d.d.d'.

Each number has one to three digits with a decimal value between 0 and
255.

The inet_addr() function has been obsoleted by inet_aton().

RETURN VALUES

The inet_aton() function returns nonzero if the address is valid, 0 if
the address is invalid.

The inet_ntop() function returns a pointer to the buffer that contains
a string if the conversion succeeds. Otherwise, NULL is returned.
Upon failure, errno is set to EAFNOSUPPORT if the af argument is
invalid or ENOSPC if the size of the result buffer is inadequate.

The inet_pton() function returns 1 if the conversion succeeds, 0 if the
input is not a valid IPv4 dotted-decimal string or a valid IPv6 address
string. The function returns -1 with errno set to EAFNOSUPPORT if the
af argument is unknown.

The value INADDR_NONE, which is equivalent to (in_addr_t)(-1), is
returned by inet_addr() and inet_network() for malformed requests.

The functions inet_netof() and inet_lnaof() break apart IPv4 host
addresses, returning the network number and local network address part,
respectively.

The function inet_ntoa() returns a pointer to a string in the base 256
notation `d.d.d.d', described in the section on IPv4 addresses.

MT-LEVEL
Safe

INTERFACE STABILITY

The inet_ntop(), inet_pton(), inet_aton(), inet_addr(), and
inet_network() functions are Committed. The inet_lnaof(),
inet_makeaddr(), inet_netof(), and inet_network() functions are
Obsolete Committed.

SEE ALSO

inet.h(3HEAD), gethostbyname(3NSL), getipnodebyname(3SOCKET),
getnetbyname(3SOCKET), hosts(5), networks(5), attributes(7)

NOTES

The return value from inet_ntoa() points to a buffer which is
overwritten on each call. This buffer is implemented as thread-
specific data in multithreaded applications.

IPv4-mapped addresses are not recommended.

BUGS

The problem of host byte ordering versus network byte ordering is
confusing.

A simple way to specify Class C network addresses in a manner similar
to that for Class B and Class A is needed.

illumos July 22, 2018 illumos