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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 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 multihomed, accessi-
       ble  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 res 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
       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 speci-
       fied.

       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 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_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, respectively).

       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.

       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_str-
       error(),  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

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

COLOPHON
       This  page  is  part of release 3.25 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                               2010-06-17                    GETADDRINFO(3)
 

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