INET(3) Linux Programmer's Manual INET(3)
NAME
inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr,
inet_lnaof, inet_netof - Internet address manipulation routines
SYNOPSIS
#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
DESCRIPTION
inet_aton() converts the Internet host address cp from the IPv4 num-
bers-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 ad-
dress; the bytes are assigned in left-to-right order to pro-
duce the binary address.
a.b.c Parts a and b specify the first two bytes of the binary ad-
dress. 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 ad-
dresses.
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 rearrange-
ment.
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 deci-
mal numbers is referred to as IPv4 dotted-decimal notation (or some-
times: IPv4 dotted-quad notation).
inet_aton() returns 1 if the supplied string was successfully inter-
preted, 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 indi-
cate 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 re-
turned. 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 subse-
quent 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 Inter-
net 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 or-
der, 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;
};
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
+-------------------------------+---------------+----------------+
|Interface | Attribute | Value |
+-------------------------------+---------------+----------------+
|inet_aton(), inet_addr(), | Thread safety | MT-Safe locale |
|inet_network(), inet_ntoa() | | |
+-------------------------------+---------------+----------------+
|inet_makeaddr(), inet_lnaof(), | Thread safety | MT-Safe |
|inet_netof() | | |
+-------------------------------+---------------+----------------+
CONFORMING TO
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 sys-
tems.
NOTES
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 compo-
nents 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 ad-
dress 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 ad-
dress 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) bound-
aries.
EXAMPLES
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
Program source
#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);
}
SEE ALSO
byteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3), getne-
tent(3), inet_net_pton(3), inet_ntop(3), inet_pton(3), hosts(5), net-
works(5)
COLOPHON
This page is part of release 5.07 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2020-06-09 INET(3)