inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof,
inet_netof - Internet address manipulation routines
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int inet_aton(const char *cp, struct in_addr *inp);
in_addr_t inet_addr(const char *cp);
in_addr_t inet_network(const char *cp);
char *inet_ntoa(struct in_addr in);
struct in_addr inet_makeaddr(in_addr_t net, in_addr_t host);
in_addr_t inet_lnaof(struct in_addr in);
in_addr_t inet_netof(struct in_addr in);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
inet_aton(),
inet_ntoa():
Since glibc 2.19:
_DEFAULT_SOURCE
In glibc up to and including 2.19:
_BSD_SOURCE || _BSD_SOURCE
inet_aton() converts the Internet host address
cp from the IPv4
numbers-and-dots notation into binary form (in network byte order) and stores
it in the structure that
inp points to.
inet_aton() returns
nonzero if the address is valid, zero if not. The address supplied in
cp can have one of the following forms:
- a.b.c.d
- Each of the four numeric parts specifies a byte of the address; the bytes
are assigned in left-to-right order to produce the binary address.
- a.b.c
- Parts a and b specify the first two bytes of the binary
address. Part c is interpreted as a 16-bit value that defines the
rightmost two bytes of the binary address. This notation is suitable for
specifying (outmoded) Class B network addresses.
- a.b
- Part a specifies the first byte of the binary address. Part
b is interpreted as a 24-bit value that defines the rightmost three
bytes of the binary address. This notation is suitable for specifying
(outmoded) Class A network addresses.
- a
- The value a is interpreted as a 32-bit value that is stored
directly into the binary address without any byte rearrangement.
In all of the above forms, components of the dotted address can be specified in
decimal, octal (with a leading
0), or hexadecimal, with a leading
0X). Addresses in any of these forms are collectively termed
IPV4
numbers-and-dots notation. The form that uses exactly four decimal numbers
is referred to as
IPv4 dotted-decimal notation (or sometimes:
IPv4
dotted-quad notation).
inet_aton() returns 1 if the supplied string was successfully
interpreted, or 0 if the string is invalid (
errno is
not set on
error).
The
inet_addr() function converts the Internet host address
cp
from IPv4 numbers-and-dots notation into binary data in network byte order. If
the input is invalid,
INADDR_NONE (usually -1) is returned. Use of this
function is problematic because -1 is a valid address (255.255.255.255). Avoid
its use in favor of
inet_aton(),
inet_pton(3), or
getaddrinfo(3), which provide a cleaner way to indicate error return.
The
inet_network() function converts
cp, a string in IPv4
numbers-and-dots notation, into a number in host byte order suitable for use
as an Internet network address. On success, the converted address is returned.
If the input is invalid, -1 is returned.
The
inet_ntoa() function converts the Internet host address
in,
given in network byte order, to a string in IPv4 dotted-decimal notation. The
string is returned in a statically allocated buffer, which subsequent calls
will overwrite.
The
inet_lnaof() function returns the local network address part of the
Internet address
in. The returned value is in host byte order.
The
inet_netof() function returns the network number part of the Internet
address
in. The returned value is in host byte order.
The
inet_makeaddr() function is the converse of
inet_netof() and
inet_lnaof(). It returns an Internet host address in network byte
order, created by combining the network number
net with the local
address
host, both in host byte order.
The structure
in_addr as used in
inet_ntoa(),
inet_makeaddr(),
inet_lnaof() and
inet_netof() is defined
in
<netinet/in.h> as:
typedef uint32_t in_addr_t;
struct in_addr {
in_addr_t s_addr;
};
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
inet_aton (), inet_addr (), inet_network (), inet_ntoa () |
Thread safety |
MT-Safe locale |
inet_makeaddr (), inet_lnaof (), inet_netof () |
Thread safety |
MT-Safe |
inet_addr(),
inet_ntoa(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.
inet_aton() is not specified in POSIX.1, but is available on most
systems.
On x86 architectures, the host byte order is Least Significant Byte first
(little endian), whereas the network byte order, as used on the Internet, is
Most Significant Byte first (big endian).
inet_lnaof(),
inet_netof(), and
inet_makeaddr() are legacy
functions that assume they are dealing with
classful network addresses.
Classful networking divides IPv4 network addresses into host and network
components at byte boundaries, as follows:
- Class A
- This address type is indicated by the value 0 in the most significant bit
of the (network byte ordered) address. The network address is contained in
the most significant byte, and the host address occupies the remaining
three bytes.
- Class B
- This address type is indicated by the binary value 10 in the most
significant two bits of the address. The network address is contained in
the two most significant bytes, and the host address occupies the
remaining two bytes.
- Class C
- This address type is indicated by the binary value 110 in the most
significant three bits of the address. The network address is contained in
the three most significant bytes, and the host address occupies the
remaining byte.
Classful network addresses are now obsolete, having been superseded by Classless
Inter-Domain Routing (CIDR), which divides addresses into network and host
components at arbitrary bit (rather than byte) boundaries.
An example of the use of
inet_aton() and
inet_ntoa() is shown
below. Here are some example runs:
$ ./a.out 226.000.000.037 # Last byte is in octal
226.0.0.31
$ ./a.out 0x7f.1 # First byte is in hex
127.0.0.1
#define _BSD_SOURCE
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
struct in_addr addr;
if (argc != 2) {
fprintf(stderr, "%s <dotted-address>\n", argv[0]);
exit(EXIT_FAILURE);
}
if (inet_aton(argv[1], &addr) == 0) {
fprintf(stderr, "Invalid address\n");
exit(EXIT_FAILURE);
}
printf("%s\n", inet_ntoa(addr));
exit(EXIT_SUCCESS);
}
byteorder(3),
getaddrinfo(3),
gethostbyname(3),
getnameinfo(3),
getnetent(3),
inet_net_pton(3),
inet_ntop(3),
inet_pton(3),
hosts(5),
networks(5)