getaddrinfo,  freeaddrinfo,  gai_strerror - network address and service


   #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():
       Since glibc 2.22: _POSIX_C_SOURCE >= 201112L
       Glibc 2.21 and earlier: _POSIX_C_SOURCE


   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

   The  addrinfo  structure  used  by getaddrinfo() contains the following

       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 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

   ai_flags    This field specifies additional options,  described  below.
               Multiple   flags  are  specified  by  bitwise  OR-ing  them

   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
   connections  on  any  of  the host'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

   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  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
   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  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

   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
   selectively  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
          resolution functions.

          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.

          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.


   getaddrinfo() returns 0 if it succeeds, or one of the following nonzero
   error codes:

          The  specified  network host does not have any network addresses
          in the requested address family.

          The name server returned a temporary  failure  indication.   Try
          again later.

          hints.ai_flags   contains   invalid  flags;  or,  hints.ai_flags
          included AI_CANONNAME and name was NULL.

          The name server returned a permanent failure indication.

          The requested address family is not supported.

          Out of memory.

          The specified network host exists, but does not have any network
          addresses defined.

          The  node  or service is not known; or both node and service are
          NULL; or AI_NUMERICSERV  was  specified  in  hints.ai_flags  and
          service was not a numeric port-number string.

          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
          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).

          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).

          Other system error, check errno for details.

   The gai_strerror() function translates these error  codes  to  a  human
   readable string, suitable for error reporting.




   For   an   explanation   of   the  terms  used  in  this  section,  see

   Interface        Attribute      Value              
   getaddrinfo()    Thread safety  MT-Safe env locale 
   freeaddrinfo(),  Thread safety  MT-Safe            


   POSIX.1-2001,  POSIX.1-2008.   The getaddrinfo() function is documented
   in RFC 2553.


   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, 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.


   The   following   programs   demonstrate   the  use  of  getaddrinfo(),
   gai_strerror(), 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

   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]);

       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));

       /* 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,
           if (sfd == -1)

           if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
               break;                  /* Success */


       if (rp == NULL) {               /* No address succeeded */
           fprintf(stderr, "Could not bind\n");

       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 %zd bytes from %s:%s\n",
                       nread, host, service);
               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

   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]);

       /* 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));

       /* 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,
           if (sfd == -1)

           if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1)
               break;                  /* Success */


       if (rp == NULL) {               /* No address succeeded */
           fprintf(stderr, "Could not connect\n");

       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) {
                       "Ignoring long message in argument %d\n", j);

           if (write(sfd, argv[j], len) != len) {
               fprintf(stderr, "partial/failed write\n");

           nread = read(sfd, buf, BUF_SIZE);
           if (nread == -1) {

           printf("Received %zd bytes: %s\n", nread, buf);



   getaddrinfo_a(3),     gethostbyname(3),     getnameinfo(3),    inet(3),
   gai.conf(5), hostname(7), ip(7)


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   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at


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