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GETADDRINFO(3)		 BSD Library Functions Manual		GETADDRINFO(3)

     getaddrinfo, freeaddrinfo — socket address structure to host and service

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

     getaddrinfo(const char *hostname, const char *servname,
	 const struct addrinfo *hints, struct addrinfo **res);

     freeaddrinfo(struct addrinfo *ai);

     The getaddrinfo() function is used to get a list of IP addresses and port
     numbers for host hostname and service servname.  It is a replacement for
     and provides more flexibility than the gethostbyname(3) and
     getservbyname(3) functions.

     The hostname and servname arguments are either pointers to NUL-terminated
     strings or the null pointer.  An acceptable value for hostname is either
     a valid host name or a numeric host address string consisting of a dotted
     decimal IPv4 address or an IPv6 address.  The servname is either a deci‐
     mal port number or a service name listed in services(5).  At least one of
     hostname and servname must be non-null.

     hints is an optional pointer to a struct addrinfo, as defined by

     struct addrinfo {
	     int ai_flags;	     /* input flags */
	     int ai_family;	     /* protocol family for socket */
	     int ai_socktype;	     /* socket type */
	     int ai_protocol;	     /* protocol for socket */
	     socklen_t ai_addrlen;   /* length of socket-address */
	     struct sockaddr *ai_addr; /* socket-address for socket */
	     char *ai_canonname;     /* canonical name for service location */
	     struct addrinfo *ai_next; /* pointer to next in list */

     This structure can be used to provide hints concerning the type of socket
     that the caller supports or wishes to use.	 The caller can supply the
     following structure elements in hints:

     ai_family	    The protocol family that should be used.  When ai_family
		    is set to PF_UNSPEC, it means the caller will accept any
		    protocol family supported by the operating system.

     ai_socktype    Denotes the type of socket that is wanted: SOCK_STREAM,
		    SOCK_DGRAM, or SOCK_RAW.  When ai_socktype is zero the
		    caller will accept any socket type.

     ai_protocol    Indicates which transport protocol is desired, IPPROTO_UDP
		    or IPPROTO_TCP.  If ai_protocol is zero the caller will
		    accept any protocol.

     ai_flags	    The ai_flags field to which the hints parameter points
		    shall be set to zero or be the bitwise-inclusive OR of one
		    or more of the values AI_ADDRCONFIG, AI_CANONNAME,

		    AI_ADDRCONFIG   If the AI_ADDRCONFIG bit is set, IPv4
				    addresses shall be returned only if an
				    IPv4 address is configured on the local
				    system, and IPv6 addresses shall be
				    returned only if an IPv6 address is con‐
				    figured on the local system.

		    AI_CANONNAME    If the AI_CANONNAME bit is set, a success‐
				    ful call to getaddrinfo() will return a
				    NUL-terminated string containing the
				    canonical name of the specified hostname
				    in the ai_canonname element of the first
				    addrinfo structure returned.

		    AI_NUMERICHOST  If the AI_NUMERICHOST bit is set, it indi‐
				    cates that hostname should be treated as a
				    numeric string defining an IPv4 or IPv6
				    address and no name resolution should be

		    AI_NUMERICSERV  If the AI_NUMERICSERV bit is set, then a
				    non-null servname string supplied shall be
				    a numeric port string.  Otherwise, an
				    EAI_NONAME error shall be returned.	 This
				    bit shall prevent any type of name resolu‐
				    tion service (for example, NIS+) from
				    being invoked.

		    AI_PASSIVE	    If the AI_PASSIVE bit is set it indicates
				    that the returned socket address structure
				    is intended for use in a call to bind(2).
				    In this case, if the hostname argument is
				    the null pointer, then the IP address por‐
				    tion of the socket address structure will
				    be set to INADDR_ANY for an IPv4 address
				    or IN6ADDR_ANY_INIT for an IPv6 address.

				    If the AI_PASSIVE bit is not set, the
				    returned socket address structure will be
				    ready for use in a call to connect(2) for
				    a connection-oriented protocol or
				    connect(2), sendto(2), or sendmsg(2) if a
				    connectionless protocol was chosen.	 The
				    IP address portion of the socket address
				    structure will be set to the loopback
				    address if hostname is the null pointer
				    and AI_PASSIVE is not set.

     All other elements of the addrinfo structure passed via hints must be
     zero or the null pointer.

     If hints is the null pointer, getaddrinfo() behaves as if the caller pro‐
     vided a struct addrinfo with ai_family set to PF_UNSPEC and all other
     elements set to zero or NULL.

     After a successful call to getaddrinfo(), *res is a pointer to a linked
     list of one or more addrinfo structures.  The list can be traversed by
     following the ai_next pointer in each addrinfo structure until a null
     pointer is encountered.  The three members ai_family, ai_socktype, and
     ai_protocol in each returned addrinfo structure are suitable for a call
     to socket(2).  For each addrinfo structure in the list, the ai_addr mem‐
     ber points to a filled-in socket address structure of length ai_addrlen.

     This implementation of getaddrinfo() allows numeric IPv6 address notation
     with scope identifier, as documented in chapter 11 of draft-ietf-
     ipv6-scoping-arch-02.txt.	By appending the percent character and scope
     identifier to addresses, one can fill the sin6_scope_id field for
     addresses.	 This would make management of scoped addresses easier and
     allows cut-and-paste input of scoped addresses.

     At this moment the code supports only link-local addresses with the for‐
     mat.  The scope identifier is hardcoded to the name of the hardware
     interface associated with the link (such as ne0).	An example is
     “fe80::1%ne0”, which means “fe80::1 on the link associated with the ne0

     The current implementation assumes a one-to-one relationship between the
     interface and link, which is not necessarily true from the specification.

     All of the information returned by getaddrinfo() is dynamically allo‐
     cated: the addrinfo structures themselves as well as the socket address
     structures and the canonical host name strings included in the addrinfo

     Memory allocated for the dynamically allocated structures created by a
     successful call to getaddrinfo() is released by the freeaddrinfo() func‐
     tion.  The ai pointer should be a addrinfo structure created by a call to

     getaddrinfo() returns zero on success or one of the error codes listed in
     gai_strerror(3) if an error occurs.

     The following code tries to connect to “” service “http” via
     a stream socket.  It loops through all the addresses available, regard‐
     less of address family.  If the destination resolves to an IPv4 address,
     it will use an AF_INET socket.  Similarly, if it resolves to IPv6, an
     AF_INET6 socket is used.  Observe that there is no hardcoded reference to
     a particular address family.  The code works even if getaddrinfo()
     returns addresses that are not IPv4/v6.

	   struct addrinfo hints, *res, *res0;
	   int error;
	   int s;
	   const char *cause = NULL;

	   memset(&hints, 0, sizeof(hints));
	   hints.ai_family = PF_UNSPEC;
	   hints.ai_socktype = SOCK_STREAM;
	   error = getaddrinfo("", "http", &hints, &res0);
	   if (error) {
		   errx(1, "%s", gai_strerror(error));
	   s = -1;
	   for (res = res0; res; res = res->ai_next) {
		   s = socket(res->ai_family, res->ai_socktype,
		   if (s < 0) {
			   cause = "socket";

		   if (connect(s, res->ai_addr, res->ai_addrlen) < 0) {
			   cause = "connect";
			   s = -1;

		   break;  /* okay we got one */
	   if (s < 0) {
		   err(1, "%s", cause);

     The following example tries to open a wildcard listening socket onto ser‐
     vice “http”, for all the address families available.

	   struct addrinfo hints, *res, *res0;
	   int error;
	   int s[MAXSOCK];
	   int nsock;
	   const char *cause = NULL;

	   memset(&hints, 0, sizeof(hints));
	   hints.ai_family = PF_UNSPEC;
	   hints.ai_socktype = SOCK_STREAM;
	   hints.ai_flags = AI_PASSIVE;
	   error = getaddrinfo(NULL, "http", &hints, &res0);
	   if (error) {
		   errx(1, "%s", gai_strerror(error));
	   nsock = 0;
	   for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) {
		   s[nsock] = socket(res->ai_family, res->ai_socktype,
		   if (s[nsock] < 0) {
			   cause = "socket";

		   if (bind(s[nsock], res->ai_addr, res->ai_addrlen) < 0) {
			   cause = "bind";
		   (void) listen(s[nsock], 5);

	   if (nsock == 0) {
		   err(1, "%s", cause);

     bind(2), connect(2), send(2), socket(2), gai_strerror(3),
     gethostbyname(3), getnameinfo(3), getservbyname(3), resolver(3),
     hosts(5), resolv.conf(5), services(5), hostname(7), named(8)

     R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic
     Socket Interface Extensions for IPv6, RFC 3493, February 2003.

     S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, IPv6 Scoped
     Address Architecture, internet draft, draft-ietf-ipv6-scoping-
     arch-02.txt, work in progress material.

     Craig Metz, "Protocol Independence Using the Sockets API", Proceedings of
     the freenix track: 2000 USENIX annual technical conference, June 2000.

     The getaddrinfo() function is defined by the IEEE Std 1003.1-2004
     (“POSIX.1”) specification and documented in RFC 3493, “Basic Socket
     Interface Extensions for IPv6”.

BSD				January 6, 2009				   BSD

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