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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
		   returned 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
		   address  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
		   indicates 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
		   together.

       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
       address	 (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_NUMERICHOST
       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
       argument	 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
       required.

       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
       defined 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_addrlen 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
	      encoded 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
	      included 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
	      example, this error could occur if service was "shell"  (a  ser‐
	      vice available only on stream sockets), and either hints.ai_pro‐
	      tocol was IPPROTO_UDP, or hints.ai_socktype was  SOCK_DGRAM;  or
	      the   error   could   occur   if	 service  was  not  NULL,  and
	      hints.ai_socktype was SOCK_RAW (a socket type that does not sup‐
	      port 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
	      inconsistent (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 %ld bytes from %s:%s\n",
			   (long) 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 %ld bytes: %s\n", (long) 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 3.58 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				  2013-11-08			GETADDRINFO(3)
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