Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
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	    ai_flags is	formed by OR'ing the following values:

		    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_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 re-
				    turned 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 ad-
				    dress 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 el-
     ements 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 ad-
     dresses.  This would make management of scoped addresses easier and al-
     lows 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 in-
     terface 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()	re-
     turns 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 1003.1g-2000
     ("POSIX.1") draft specification and documented in RFC 3493, "Basic	Socket
     Interface Extensions for IPv6".

BSD			       December	20, 2004			   BSD


Want to link to this manual page? Use this URL:

home | help