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
		 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 net-
		 work 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  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
       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	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
       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(),      | 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
		 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) bound-
       aries.

EXAMPLE
       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 4.10 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				  2016-03-15			       INET(3)