NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

GETADDRINFO(3)            Linux Programmer's Manual           GETADDRINFO(3)

NAME         top

       getaddrinfo, freeaddrinfo, gai_strerror - network address and service
       translation

SYNOPSIS         top

       #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         top

       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 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 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, 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 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,
       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 specified.

       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.

       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         top

       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
              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 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).

       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         top

       /etc/gai.conf

CONFORMING TO         top

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

NOTES         top

       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         top

       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

       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 %zd bytes from %s:%s\n",
                           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 %zd bytes: %s\n", nread, buf);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.70 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

GNU                              2014-04-06                   GETADDRINFO(3)