inet(3) - Linux manual page (original) (raw)
inet(3) Library Functions Manual inet(3)
NAME top
inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr,
inet_lnaof, inet_netof - Internet address manipulation routinesLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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_**);**
**[[deprecated]] char *inet_ntoa(struct in_addr** _in_**);**
**[[deprecated]] struct in_addr inet_makeaddr(in_addr_t** _net_**,**
**in_addr_t** _host_**);**
**[[deprecated]] in_addr_t inet_lnaof(struct in_addr** _in_**);**
**[[deprecated]] 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_SOURCEDESCRIPTION top
**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)](../man3/inet%5Fpton.3.html), or [getaddrinfo(3)](../man3/getaddrinfo.3.html), 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 _inaddr_ 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 top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌───────────────────────────────┬───────────────┬────────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├───────────────────────────────┼───────────────┼────────────────┤
│ **inet_aton**(), **inet_addr**(), │ Thread safety │ MT-Safe locale │
│ **inet_network**(), **inet_ntoa**() │ │ │
├───────────────────────────────┼───────────────┼────────────────┤
│ **inet_makeaddr**(), │ Thread safety │ MT-Safe │
│ **inet_lnaof**(), **inet_netof**() │ │ │
└───────────────────────────────┴───────────────┴────────────────┘STANDARDS top
**inet_addr**()
**inet_ntoa**()
POSIX.1-2008.
**inet_aton**()
None.STANDARDS top
**inet_addr**()
**inet_ntoa**()
POSIX.1-2001, 4.3BSD.
**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.NOTES top
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).EXAMPLES top
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
127.0.0.1Program source
#define _DEFAULT_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 top
[byteorder(3)](../man3/byteorder.3.html), [getaddrinfo(3)](../man3/getaddrinfo.3.html), [gethostbyname(3)](../man3/gethostbyname.3.html), [getnameinfo(3)](../man3/getnameinfo.3.html),
[getnetent(3)](../man3/getnetent.3.html), [inet_net_pton(3)](../man3/inet%5Fnet%5Fpton.3.html), [inet_ntop(3)](../man3/inet%5Fntop.3.html), [inet_pton(3)](../man3/inet%5Fpton.3.html),
[hosts(5)](../man5/hosts.5.html), [networks(5)](../man5/networks.5.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 inet(3)
Pages that refer to this page:getaddrinfo(3), getaddrinfo_a(3), gethostbyname(3), inet_net_pton(3), inet_ntop(3), inet_pton(3), networks(5), ip(7)