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



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

SYNOPSIS
       #include 
       #include 
       #include 

       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 getservbyname(3) and getservbyport(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;
	       size_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  logically  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).

       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 multi-homed,	acces-
       sible  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 result only if the local system
       has at least one IPv4 address configured, and IPv6 addresses  are  only
       returned	 if the local system has at least one IPv6 address configured.

       If hint.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
       result.	 If both AI_V4MAPPED and AI_ALL are specified in hints.ai_fam-
       ily, then return both IPv6 and IPv4-mapped IPv6 addresses in  the  list
       pointed	to  by	result.	  AI_ALL is ignored if AI_V4MAPPED is not also
       specified.

       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  non-
       zero 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 only available 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, repectively).

       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.

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 
       #include 
       #include 
       #include 
       #include 
       #include 
       #include 

       #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 
       #include 
       #include 
       #include 
       #include 
       #include 
       #include 

       #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
       gethostbyname(3), getnameinfo(3), inet(3), hostname(7), ip(7)

COLOPHON
       This  page  is  part of release 3.22 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				  2008-12-04			GETADDRINFO(3)