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