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Linux 2.6 - man page for getaddrinfo (linux section 3)

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 dependencies.

       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 criteria  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 criteria that limit the set of socket addresses returned by getaddrinfo(), as fol-
       lows:

       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 ser-
		   vice.

       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.  Specify-
		   ing	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, hexa-
       decimal 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  applica-
       tions  (typically servers) that intend to accept connections 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  ser-
       vice  is  NULL,	then the port number of the returned socket addresses will be left unini-
       tialized.  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, accessible 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 applica-
       tion 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 initialized 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 configured
       address.  This flag is useful on, for example, IPv4-only systems, to  ensure  that  getad-
       drinfo()  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 specified.

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

   Extensions to getaddrinfo() for Internationalized Domain Names
       Starting with glibc 2.3.4, getaddrinfo() has been extended to selectively allow the incom-
       ing 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
	      resolution functions.

       AI_CANONIDN
	      After a successful name lookup, and if the AI_CANONNAME flag was specified,  getad-
	      drinfo()	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_NUMERIC-
	      SERV  was  specified  in	hints.ai_flags	and service 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 ser-
	      vice 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
	      error 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  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, suit-
       able 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_strerror(), freead-
       drinfo(), and getnameinfo(3).  The programs are an echo server and client  for  UDP  data-
       grams.

   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 3.55 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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