inet_addr man page on ElementaryOS

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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(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

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 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  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).

       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 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;
	   };

CONFORMING TO
       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.

NOTES
       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	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_ntop(3), inet_pton(3), hosts(5), networks(5)

COLOPHON
       This  page  is  part of release 3.54 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				  2013-02-10			       INET(3)
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