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GETADDRINFO(3)		   Linux Programmer's Manual		GETADDRINFO(3)

NAME
       getaddrinfo,  freeaddrinfo,  gai_strerror - network address and service
       translation

SYNOPSIS
       #include	<sys/types.h>
       #include	<sys/socket.h>
       #include	<netdb.h>

       int getaddrinfo(const char *node, const char *service,
		       const struct addrinfo *hints,
		       struct addrinfo **res);

       void freeaddrinfo(struct	addrinfo *res);

       const char *gai_strerror(int errcode);

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       getaddrinfo(), freeaddrinfo(), gai_strerror():
	   _POSIX_C_SOURCE >= 1	|| _XOPEN_SOURCE || _POSIX_SOURCE

DESCRIPTION
       Given node and service, which identify an Internet host and a  service,
       getaddrinfo()  returns  one  or more addrinfo structures, each of which
       contains	an Internet address that can be	specified in a call to bind(2)
       or  connect(2).	 The getaddrinfo() function combines the functionality
       provided	by the gethostbyname(3)	and getservbyname(3) functions into  a
       single  interface,  but	unlike	the latter functions, getaddrinfo() is
       reentrant and allows programs to	eliminate  IPv4-versus-IPv6  dependen-
       cies.

       The  addrinfo  structure	 used  by getaddrinfo()	contains the following
       fields:

	   struct addrinfo {
	       int		ai_flags;
	       int		ai_family;
	       int		ai_socktype;
	       int		ai_protocol;
	       socklen_t	ai_addrlen;
	       struct sockaddr *ai_addr;
	       char	       *ai_canonname;
	       struct addrinfo *ai_next;
	   };

       The hints argument points to an addrinfo	structure that specifies  cri-
       teria  for selecting the	socket address structures returned in the list
       pointed to by res.  If hints is not  NULL  it  points  to  an  addrinfo
       structure  whose	ai_family, ai_socktype,	and ai_protocol	specify	crite-
       ria that	limit the set of socket	addresses returned  by	getaddrinfo(),
       as follows:

       ai_family   This	field specifies	the desired address family for the re-
		   turned addresses.  Valid  values  for  this	field  include
		   AF_INET  and	 AF_INET6.  The	value AF_UNSPEC	indicates that
		   getaddrinfo() should	return socket addresses	 for  any  ad-
		   dress family	(either	IPv4 or	IPv6, for example) that	can be
		   used	with node and service.

       ai_socktype This	field specifies	the preferred socket type, for example
		   SOCK_STREAM	or SOCK_DGRAM.	Specifying 0 in	this field in-
		   dicates that	socket addresses of any	type can  be  returned
		   by getaddrinfo().

       ai_protocol This	 field	specifies the protocol for the returned	socket
		   addresses.  Specifying  0  in  this	field  indicates  that
		   socket  addresses  with  any	 protocol  can	be returned by
		   getaddrinfo().

       ai_flags	   This	field specifies	additional options,  described	below.
		   Multiple  flags  are	 specified  by bitwise OR-ing them to-
		   gether.

       All the other fields in the structure pointed to	by hints must  contain
       either 0	or a null pointer, as appropriate.

       Specifying  hints  as  NULL  is	equivalent  to setting ai_socktype and
       ai_protocol  to	0;   ai_family	 to   AF_UNSPEC;   and	 ai_flags   to
       (AI_V4MAPPED | AI_ADDRCONFIG).  (POSIX specifies	different defaults for
       ai_flags; see NOTES.)  node specifies either a  numerical  network  ad-
       dress   (for   IPv4,   numbers-and-dots	 notation   as	 supported  by
       inet_aton(3); for IPv6,	hexadecimal  string  format  as	 supported  by
       inet_pton(3)),  or  a  network  hostname,  whose	 network addresses are
       looked up and resolved.	If hints.ai_flags contains the	AI_NUMERICHOST
       flag,  then  node  must be a numerical network address.	The AI_NUMERI-
       CHOST flag suppresses any  potentially  lengthy	network	 host  address
       lookups.

       If  the	AI_PASSIVE  flag  is  specified	in hints.ai_flags, and node is
       NULL,  then  the	 returned  socket  addresses  will  be	suitable   for
       bind(2)ing  a  socket  that  will  accept(2) connections.  The returned
       socket address will contain the "wildcard address" (INADDR_ANY for IPv4
       addresses, IN6ADDR_ANY_INIT for IPv6 address).  The wildcard address is
       used by applications (typically servers)	that intend to accept  connec-
       tions  on  any  of the hosts's network addresses.  If node is not NULL,
       then the	AI_PASSIVE flag	is ignored.

       If the AI_PASSIVE flag is not set in hints.ai_flags, then the  returned
       socket  addresses  will be suitable for use with	connect(2), sendto(2),
       or sendmsg(2).  If node is NULL,	then the network address will  be  set
       to  the loopback	interface address (INADDR_LOOPBACK for IPv4 addresses,
       IN6ADDR_LOOPBACK_INIT for IPv6 address);	this is	used  by  applications
       that intend to communicate with peers running on	the same host.

       service	sets the port in each returned address structure.  If this ar-
       gument is a service name	(see services(5)), it  is  translated  to  the
       corresponding  port  number.   This argument can	also be	specified as a
       decimal number, which is	simply converted to  binary.   If  service  is
       NULL,  then  the	 port  number of the returned socket addresses will be
       left uninitialized.  If AI_NUMERICSERV is specified  in	hints.ai_flags
       and service is not NULL,	then service must point	to a string containing
       a numeric port number.  This flag is used to inhibit the	invocation  of
       a  name	resolution  service  in	 cases where it	is known not to	be re-
       quired.

       Either node or service, but not both, may be NULL.

       The getaddrinfo() function allocates and	initializes a linked  list  of
       addrinfo	structures, one	for each network address that matches node and
       service,	subject	to any restrictions imposed by hints,  and  returns  a
       pointer	to the start of	the list in res.  The items in the linked list
       are linked by the ai_next field.

       There are several reasons why the linked	list may have  more  than  one
       addrinfo	structure, including: the network host is multihomed, accessi-
       ble over	multiple protocols (e.g., both AF_INET and AF_INET6);  or  the
       same  service  is available from	multiple socket	types (one SOCK_STREAM
       address and another SOCK_DGRAM address, for  example).	Normally,  the
       application  should  try	using the addresses in the order in which they
       are returned.  The sorting function used	within	getaddrinfo()  is  de-
       fined  in RFC 3484; the order can be tweaked for	a particular system by
       editing /etc/gai.conf (available	since glibc 2.5).

       If hints.ai_flags includes the AI_CANONNAME flag, then the ai_canonname
       field  of  the first of the addrinfo structures in the returned list is
       set to point to the official name of the	host.

       The remaining fields of each returned addrinfo structure	 are  initial-
       ized as follows:

       * The  ai_family, ai_socktype, and ai_protocol fields return the	socket
	 creation parameters (i.e., these fields have the same meaning as  the
	 corresponding	arguments of socket(2)).  For example, ai_family might
	 return	AF_INET	or AF_INET6; ai_socktype might	return	SOCK_DGRAM  or
	 SOCK_STREAM; and ai_protocol returns the protocol for the socket.

       * A  pointer  to	the socket address is placed in	the ai_addr field, and
	 the length of the socket address, in bytes, is	placed in  the	ai_ad-
	 drlen field.

       If  hints.ai_flags includes the AI_ADDRCONFIG flag, then	IPv4 addresses
       are returned in the list	pointed	to by res only if the local system has
       at  least  one IPv4 address configured, and IPv6	addresses are returned
       only if the local system	has at least one IPv6 address configured.  The
       loopback	 address is not	considered for this case as valid as a config-
       ured address.  This flag	is useful on, for example, IPv4-only  systems,
       to ensure that getaddrinfo() does not return IPv6 socket	addresses that
       would always fail in connect(2) or bind(2).

       If hints.ai_flags specifies the AI_V4MAPPED flag,  and  hints.ai_family
       was  specified  as  AF_INET6,  and  no matching IPv6 addresses could be
       found, then return IPv4-mapped IPv6 addresses in	the list pointed to by
       res.   If  both AI_V4MAPPED and AI_ALL are specified in hints.ai_flags,
       then return both	IPv6  and  IPv4-mapped	IPv6  addresses	 in  the  list
       pointed to by res.  AI_ALL is ignored if	AI_V4MAPPED is not also	speci-
       fied.

       The freeaddrinfo() function frees the memory that was allocated for the
       dynamically allocated linked list res.

   Extensions to getaddrinfo() for Internationalized Domain Names
       Starting	 with  glibc  2.3.4, getaddrinfo() has been extended to	selec-
       tively allow the	incoming and outgoing hostnames	 to  be	 transparently
       converted  to  and  from	the Internationalized Domain Name (IDN)	format
       (see RFC	3490, Internationalizing Domain	Names in Applications (IDNA)).
       Four new	flags are defined:

       AI_IDN If  this	flag is	specified, then	the node name given in node is
	      converted	to IDN format if necessary.  The  source  encoding  is
	      that of the current locale.

	      If  the  input  name contains non-ASCII characters, then the IDN
	      encoding is used.	 Those parts of	the node  name	(delimited  by
	      dots)  that contain non-ASCII characters are encoded using ASCII
	      Compatible Encoding (ACE)	before being passed to the name	 reso-
	      lution functions.

       AI_CANONIDN
	      After a successful name lookup, and if the AI_CANONNAME flag was
	      specified, getaddrinfo() will return the canonical name  of  the
	      node  corresponding to the addrinfo structure value passed back.
	      The return value is an exact copy	of the value returned  by  the
	      name resolution function.

	      If  the name is encoded using ACE, then it will contain the xn--
	      prefix for one or	more components	of the name.  To convert these
	      components  into	a  readable  form  the AI_CANONIDN flag	can be
	      passed in	addition to AI_CANONNAME.  The resulting string	is en-
	      coded using the current locale's encoding.

       AI_IDN_ALLOW_UNASSIGNED,	AI_IDN_USE_STD3_ASCII_RULES
	      Setting these flags will enable the IDNA_ALLOW_UNASSIGNED	(allow
	      unassigned Unicode code  points)	and  IDNA_USE_STD3_ASCII_RULES
	      (check  output  to  make	sure it	is a STD3 conforming hostname)
	      flags respectively to be used in the IDNA	handling.

RETURN VALUE
       getaddrinfo() returns 0 if it succeeds, or one of the following nonzero
       error codes:

       EAI_ADDRFAMILY
	      The  specified  network host does	not have any network addresses
	      in the requested address family.

       EAI_AGAIN
	      The name server returned a temporary  failure  indication.   Try
	      again later.

       EAI_BADFLAGS
	      hints.ai_flags  contains	invalid	 flags;	or, hints.ai_flags in-
	      cluded AI_CANONNAME and name was NULL.

       EAI_FAIL
	      The name server returned a permanent failure indication.

       EAI_FAMILY
	      The requested address family is not supported.

       EAI_MEMORY
	      Out of memory.

       EAI_NODATA
	      The specified network host exists, but does not have any network
	      addresses	defined.

       EAI_NONAME
	      The  node	 or service is not known; or both node and service are
	      NULL; or AI_NUMERICSERV was specified in hints.ai_flags and ser-
	      vice was not a numeric port-number string.

       EAI_SERVICE
	      The  requested service is	not available for the requested	socket
	      type.  It	may be available through another socket	type.  For ex-
	      ample,  this error could occur if	service	was "shell" (a service
	      available	only on	stream sockets), and either  hints.ai_protocol
	      was IPPROTO_UDP, or hints.ai_socktype was	SOCK_DGRAM; or the er-
	      ror could	occur if service was not NULL,	and  hints.ai_socktype
	      was SOCK_RAW (a socket type that does not	support	the concept of
	      services).

       EAI_SOCKTYPE
	      The requested socket type	is not supported.  This	 could	occur,
	      for  example, if hints.ai_socktype and hints.ai_protocol are in-
	      consistent (e.g.,	SOCK_DGRAM and IPPROTO_TCP, respectively).

       EAI_SYSTEM
	      Other system error, check	errno for details.

       The gai_strerror() function translates these error  codes  to  a	 human
       readable	string,	suitable for error reporting.

FILES
       /etc/gai.conf

CONFORMING TO
       POSIX.1-2001.  The getaddrinfo()	function is documented in RFC 2553.

NOTES
       getaddrinfo() supports the address%scope-id notation for	specifying the
       IPv6 scope-ID.

       AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc	2.3.3.
       AI_NUMERICSERV is available since glibc 2.3.4.

       According  to  POSIX.1-2001,  specifying	 hints	as  NULL  should cause
       ai_flags	to be assumed as 0.  The GNU C library instead assumes a value
       of  (AI_V4MAPPED	| AI_ADDRCONFIG)  for  this  case, since this value is
       considered an improvement on the	specification.

EXAMPLE
       The following programs demonstrate the use of  getaddrinfo(),  gai_str-
       error(),	 freeaddrinfo(), and getnameinfo(3).  The programs are an echo
       server and client for UDP datagrams.

   Server program

       #include	<sys/types.h>
       #include	<stdio.h>
       #include	<stdlib.h>
       #include	<unistd.h>
       #include	<string.h>
       #include	<sys/socket.h>
       #include	<netdb.h>

       #define BUF_SIZE	500

       int
       main(int	argc, char *argv[])
       {
	   struct addrinfo hints;
	   struct addrinfo *result, *rp;
	   int sfd, s;
	   struct sockaddr_storage peer_addr;
	   socklen_t peer_addr_len;
	   ssize_t nread;
	   char	buf[BUF_SIZE];

	   if (argc != 2) {
	       fprintf(stderr, "Usage: %s port\n", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   memset(&hints, 0, sizeof(struct addrinfo));
	   hints.ai_family = AF_UNSPEC;	   /* Allow IPv4 or IPv6 */
	   hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
	   hints.ai_flags = AI_PASSIVE;	   /* For wildcard IP address */
	   hints.ai_protocol = 0;	   /* Any protocol */
	   hints.ai_canonname =	NULL;
	   hints.ai_addr = NULL;
	   hints.ai_next = NULL;

	   s = getaddrinfo(NULL, argv[1], &hints, &result);
	   if (s != 0) {
	       fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
	       exit(EXIT_FAILURE);
	   }

	   /* getaddrinfo() returns a list of address structures.
	      Try each address until we	successfully bind(2).
	      If socket(2) (or bind(2))	fails, we (close the socket
	      and) try the next	address. */

	   for (rp = result; rp	!= NULL; rp = rp->ai_next) {
	       sfd = socket(rp->ai_family, rp->ai_socktype,
		       rp->ai_protocol);
	       if (sfd == -1)
		   continue;

	       if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
		   break;		   /* Success */

	       close(sfd);
	   }

	   if (rp == NULL) {		   /* No address succeeded */
	       fprintf(stderr, "Could not bind\n");
	       exit(EXIT_FAILURE);
	   }

	   freeaddrinfo(result);	   /* No longer	needed */

	   /* Read datagrams and echo them back	to sender */

	   for (;;) {
	       peer_addr_len = sizeof(struct sockaddr_storage);
	       nread = recvfrom(sfd, buf, BUF_SIZE, 0,
		       (struct sockaddr	*) &peer_addr, &peer_addr_len);
	       if (nread == -1)
		   continue;		   /* Ignore failed request */

	       char host[NI_MAXHOST], service[NI_MAXSERV];

	       s = getnameinfo((struct sockaddr	*) &peer_addr,
			       peer_addr_len, host, NI_MAXHOST,
			       service,	NI_MAXSERV, NI_NUMERICSERV);
	      if (s == 0)
		   printf("Received %zd	bytes from %s:%s\n",
			   nread, host,	service);
	       else
		   fprintf(stderr, "getnameinfo: %s\n",	gai_strerror(s));

	       if (sendto(sfd, buf, nread, 0,
			   (struct sockaddr *) &peer_addr,
			   peer_addr_len) != nread)
		   fprintf(stderr, "Error sending response\n");
	   }
       }

   Client program

       #include	<sys/types.h>
       #include	<sys/socket.h>
       #include	<netdb.h>
       #include	<stdio.h>
       #include	<stdlib.h>
       #include	<unistd.h>
       #include	<string.h>

       #define BUF_SIZE	500

       int
       main(int	argc, char *argv[])
       {
	   struct addrinfo hints;
	   struct addrinfo *result, *rp;
	   int sfd, s, j;
	   size_t len;
	   ssize_t nread;
	   char	buf[BUF_SIZE];

	   if (argc < 3) {
	       fprintf(stderr, "Usage: %s host port msg...\n", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   /* Obtain address(es) matching host/port */

	   memset(&hints, 0, sizeof(struct addrinfo));
	   hints.ai_family = AF_UNSPEC;	   /* Allow IPv4 or IPv6 */
	   hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
	   hints.ai_flags = 0;
	   hints.ai_protocol = 0;	   /* Any protocol */

	   s = getaddrinfo(argv[1], argv[2], &hints, &result);
	   if (s != 0) {
	       fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
	       exit(EXIT_FAILURE);
	   }

	   /* getaddrinfo() returns a list of address structures.
	      Try each address until we	successfully connect(2).
	      If socket(2) (or connect(2)) fails, we (close the	socket
	      and) try the next	address. */

	   for (rp = result; rp	!= NULL; rp = rp->ai_next) {
	       sfd = socket(rp->ai_family, rp->ai_socktype,
			    rp->ai_protocol);
	       if (sfd == -1)
		   continue;

	       if (connect(sfd,	rp->ai_addr, rp->ai_addrlen) !=	-1)
		   break;		   /* Success */

	       close(sfd);
	   }

	   if (rp == NULL) {		   /* No address succeeded */
	       fprintf(stderr, "Could not connect\n");
	       exit(EXIT_FAILURE);
	   }

	   freeaddrinfo(result);	   /* No longer	needed */

	   /* Send remaining command-line arguments as separate
	      datagrams, and read responses from server	*/

	   for (j = 3; j < argc; j++) {
	       len = strlen(argv[j]) + 1;
		       /* +1 for terminating null byte */

	       if (len + 1 > BUF_SIZE) {
		   fprintf(stderr,
			   "Ignoring long message in argument %d\n", j);
		   continue;
	       }

	       if (write(sfd, argv[j], len) != len) {
		   fprintf(stderr, "partial/failed write\n");
		   exit(EXIT_FAILURE);
	       }

	       nread = read(sfd, buf, BUF_SIZE);
	       if (nread == -1)	{
		   perror("read");
		   exit(EXIT_FAILURE);
	       }

	       printf("Received	%zd bytes: %s\n", nread, buf);
	   }

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       getaddrinfo_a(3),    gethostbyname(3),	  getnameinfo(3),     inet(3),
       gai.conf(5), hostname(7), ip(7)

COLOPHON
       This  page  is  part of release 3.74 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
       http://www.kernel.org/doc/man-pages/.

GNU				  2014-04-06			GETADDRINFO(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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