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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():
           Since glibc 2.22: _POSIX_C_SOURCE >= 200112L
           Glibc 2.21 and earlier: _POSIX_C_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 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  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 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

       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

       /etc/gai.conf

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌────────────────┬───────────────┬────────────────────┐
       │InterfaceAttributeValue              │
       ├────────────────┼───────────────┼────────────────────┤
       │getaddrinfo()   │ Thread safety │ MT-Safe env locale │
       ├────────────────┼───────────────┼────────────────────┤
       │freeaddrinfo(), │ Thread safety │ MT-Safe            │
       │gai_strerror()  │               │                    │
       └────────────────┴───────────────┴────────────────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.  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, 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.

EXAMPLES

       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(hints));
           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);
           }

           freeaddrinfo(result);           /* No longer needed */

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

           /* Read datagrams and echo them back to sender */

           for (;;) {
               peer_addr_len = sizeof(peer_addr);
               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;
           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(hints));
           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);
           }

           freeaddrinfo(result);           /* No longer needed */

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

           /* Send remaining command-line arguments as separate
              datagrams, and read responses from server */

           for (int j = 3; j < argc; j++) {
               len = strlen(argv[j]) + 1;
                       /* +1 for terminating null byte */

               if (len > 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

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

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.