NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON
INET(3) Linux Programmer's Manual INET(3)
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(int net, int 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(): _BSD_SOURCE || _SVID_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 non-zero 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 C 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).
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;
};
4.3BSD. inet_addr() and inet_ntoa() are specified in POSIX.1-2001.
inet_aton() is not specified in POSIX.1-2001, but is available on most
systems.
On the i386 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) {
perror("inet_aton");
exit(EXIT_FAILURE);
}
printf("%s\n", inet_ntoa(addr));
exit(EXIT_SUCCESS);
}
byteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3), getnetent(3),
inet_ntop(3), inet_pton(3), hosts(5), networks(5)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2008-06-19 INET(3)