Ubuntu Manpage: inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr, inet_lnaof, inet

NAME

       inet_aton,  inet_addr,  inet_network,  inet_ntoa,  inet_makeaddr, inet_lnaof, inet_netof -
       Internet address manipulation routines

LIBRARY

       Standard C library (libc, -lc)

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);

       [[deprecated]] 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 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;
           };

ATTRIBUTES

       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        │
       └────────────────────────────────────────────────────────┴───────────────┴────────────────┘

STANDARDS

       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.

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 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.

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 _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);
       }