PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

FREEADDRINFO(3P)          POSIX Programmer's Manual         FREEADDRINFO(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       freeaddrinfo, getaddrinfo — get address information

SYNOPSIS         top

       #include <sys/socket.h>
       #include <netdb.h>

       void freeaddrinfo(struct addrinfo *ai);
       int getaddrinfo(const char *restrict nodename,
           const char *restrict servname,
           const struct addrinfo *restrict hints,
           struct addrinfo **restrict res);

DESCRIPTION         top

       The freeaddrinfo() function shall free one or more addrinfo
       structures returned by getaddrinfo(), along with any additional
       storage associated with those structures. If the ai_next field of the
       structure is not null, the entire list of structures shall be freed.
       The freeaddrinfo() function shall support the freeing of arbitrary
       sublists of an addrinfo list originally returned by getaddrinfo().

       The getaddrinfo() function shall translate the name of a service
       location (for example, a host name) and/or a service name and shall
       return a set of socket addresses and associated information to be
       used in creating a socket with which to address the specified
       service.

       Note:     In many cases it is implemented by the Domain Name System,
                 as documented in RFC 1034, RFC 1035, and RFC 1886.

       The freeaddrinfo() and getaddrinfo() functions shall be thread-safe.

       The nodename and servname arguments are either null pointers or
       pointers to null-terminated strings. One or both of these two
       arguments shall be supplied by the application as a non-null pointer.

       The format of a valid name depends on the address family or families.
       If a specific family is not given and the name could be interpreted
       as valid within multiple supported families, the implementation shall
       attempt to resolve the name in all supported families and, in absence
       of errors, one or more results shall be returned.

       If the nodename argument is not null, it can be a descriptive name or
       can be an address string.  If the specified address family is
       AF_INET, AF_INET6, or AF_UNSPEC, valid descriptive names include host
       names. If the specified address family is AF_INET or AF_UNSPEC,
       address strings using Internet standard dot notation as specified in
       inet_addr(3p) are valid.

       If the specified address family is AF_INET6 or AF_UNSPEC, standard
       IPv6 text forms described in inet_ntop(3p) are valid.

       If nodename is not null, the requested service location is named by
       nodename; otherwise, the requested service location is local to the
       caller.

       If servname is null, the call shall return network-level addresses
       for the specified nodename.  If servname is not null, it is a null-
       terminated character string identifying the requested service. This
       can be either a descriptive name or a numeric representation suitable
       for use with the address family or families.  If the specified
       address family is AF_INET, AF_INET6, or AF_UNSPEC, the service can be
       specified as a string specifying a decimal port number.

       If the hints argument is not null, it refers to a structure
       containing input values that directs the operation by providing
       options and by limiting the returned information to a specific socket
       type, address family, and/or protocol, as described below. In this
       hints structure every member other than ai_flags, ai_family,
       ai_socktype, and ai_protocol shall be set to zero or a null pointer.
       A value of AF_UNSPEC for ai_family means that the caller shall accept
       any address family. A value of zero for ai_socktype means that the
       caller shall accept any socket type. A value of zero for ai_protocol
       means that the caller shall accept any protocol. If hints is a null
       pointer, the behavior shall be as if it referred to a structure
       containing the value zero for the ai_flags, ai_socktype, and
       ai_protocol fields, and AF_UNSPEC for the ai_family field.

       The ai_flags field to which the hints parameter points shall be set
       to zero or be the bitwise-inclusive OR of one or more of the values
       AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST, AI_NUMERICSERV,
       AI_V4MAPPED, AI_ALL, and AI_ADDRCONFIG.

       If the AI_PASSIVE flag is specified, the returned address information
       shall be suitable for use in binding a socket for accepting incoming
       connections for the specified service. In this case, if the nodename
       argument is null, then the IP address portion of the socket address
       structure shall be set to INADDR_ANY for an IPv4 address or
       IN6ADDR_ANY_INIT for an IPv6 address. If the AI_PASSIVE flag is not
       specified, the returned address information shall be suitable for a
       call to connect() (for a connection-mode protocol) or for a call to
       connect(), sendto(), or sendmsg() (for a connectionless protocol). In
       this case, if the nodename argument is null, then the IP address
       portion of the socket address structure shall be set to the loopback
       address. The AI_PASSIVE flag shall be ignored if the nodename
       argument is not null.

       If the AI_CANONNAME flag is specified and the nodename argument is
       not null, the function shall attempt to determine the canonical name
       corresponding to nodename (for example, if nodename is an alias or
       shorthand notation for a complete name).

       Note:     Since different implementations use different conceptual
                 models, the terms ``canonical name'' and ``alias'' cannot
                 be precisely defined for the general case. However, Domain
                 Name System implementations are expected to interpret them
                 as they are used in RFC 1034.

                 A numeric host address string is not a ``name'', and thus
                 does not have a ``canonical name'' form; no address to host
                 name translation is performed. See below for handling of
                 the case where a canonical name cannot be obtained.

       If the AI_NUMERICHOST flag is specified, then a non-null nodename
       string supplied shall be a numeric host address string. Otherwise, an
       [EAI_NONAME] error is returned. This flag shall prevent any type of
       name resolution service (for example, the DNS) from being invoked.

       If the AI_NUMERICSERV flag is specified, then a non-null servname
       string supplied shall be a numeric port string. Otherwise, an
       [EAI_NONAME] error shall be returned. This flag shall prevent any
       type of name resolution service (for example, NIS+) from being
       invoked.

       If the AI_V4MAPPED flag is specified along with an ai_family of
       AF_INET6, then getaddrinfo() shall return IPv4-mapped IPv6 addresses
       on finding no matching IPv6 addresses (ai_addrlen shall be 16). The
       AI_V4MAPPED flag shall be ignored unless ai_family equals AF_INET6.
       If the AI_ALL flag is used with the AI_V4MAPPED flag, then
       getaddrinfo() shall return all matching IPv6 and IPv4 addresses. The
       AI_ALL flag without the AI_V4MAPPED flag is ignored.

       If the AI_ADDRCONFIG flag is specified, IPv4 addresses shall be
       returned only if an IPv4 address is configured on the local system,
       and IPv6 addresses shall be returned only if an IPv6 address is
       configured on the local system.

       The ai_socktype field to which argument hints points specifies the
       socket type for the service, as defined in socket(3p).  If a specific
       socket type is not given (for example, a value of zero) and the
       service name could be interpreted as valid with multiple supported
       socket types, the implementation shall attempt to resolve the service
       name for all supported socket types and, in the absence of errors,
       all possible results shall be returned. A non-zero socket type value
       shall limit the returned information to values with the specified
       socket type.

       If the ai_family field to which hints points has the value AF_UNSPEC,
       addresses shall be returned for use with any address family that can
       be used with the specified nodename and/or servname.  Otherwise,
       addresses shall be returned for use only with the specified address
       family. If ai_family is not AF_UNSPEC and ai_protocol is not zero,
       then addresses shall be returned for use only with the specified
       address family and protocol; the value of ai_protocol shall be
       interpreted as in a call to the socket() function with the
       corresponding values of ai_family and ai_protocol.

RETURN VALUE         top

       A zero return value for getaddrinfo() indicates successful
       completion; a non-zero return value indicates failure. The possible
       values for the failures are listed in the ERRORS section.

       Upon successful return of getaddrinfo(), the location to which res
       points shall refer to a linked list of addrinfo structures, each of
       which shall specify a socket address and information for use in
       creating a socket with which to use that socket address. The list
       shall include at least one addrinfo structure. The ai_next field of
       each structure contains a pointer to the next structure on the list,
       or a null pointer if it is the last structure on the list. Each
       structure on the list shall include values for use with a call to the
       socket() function, and a socket address for use with the connect()
       function or, if the AI_PASSIVE flag was specified, for use with the
       bind() function. The fields ai_family, ai_socktype, and ai_protocol
       shall be usable as the arguments to the socket() function to create a
       socket suitable for use with the returned address. The fields ai_addr
       and ai_addrlen are usable as the arguments to the connect() or bind()
       functions with such a socket, according to the AI_PASSIVE flag.

       If nodename is not null, and if requested by the AI_CANONNAME flag,
       the ai_canonname field of the first returned addrinfo structure shall
       point to a null-terminated string containing the canonical name
       corresponding to the input nodename; if the canonical name is not
       available, then ai_canonname shall refer to the nodename argument or
       a string with the same contents. The contents of the ai_flags field
       of the returned structures are undefined.

       All fields in socket address structures returned by getaddrinfo()
       that are not filled in through an explicit argument (for example,
       sin6_flowinfo) shall be set to zero.

       Note:     This makes it easier to compare socket address structures.

ERRORS         top

       The getaddrinfo() function shall fail and return the corresponding
       error value if:

       [EAI_AGAIN] The name could not be resolved at this time. Future
                   attempts may succeed.

       [EAI_BADFLAGS]
                   The flags parameter had an invalid value.

       [EAI_FAIL]  A non-recoverable error occurred when attempting to
                   resolve the name.

       [EAI_FAMILY]
                   The address family was not recognized.

       [EAI_MEMORY]
                   There was a memory allocation failure when trying to
                   allocate storage for the return value.

       [EAI_NONAME]
                   The name does not resolve for the supplied parameters.

                   Neither nodename nor servname were supplied. At least one
                   of these shall be supplied.

       [EAI_SERVICE]
                   The service passed was not recognized for the specified
                   socket type.

       [EAI_SOCKTYPE]
                   The intended socket type was not recognized.

       [EAI_SYSTEM]
                   A system error occurred; the error code can be found in
                   errno.

       The following sections are informative.

EXAMPLES         top

       The following (incomplete) program demonstrates the use of
       getaddrinfo() to obtain the socket address structure(s) for the
       service named in the program's command-line argument. The program
       then loops through each of the address structures attempting to
       create and bind a socket to the address, until it performs a
       successful bind().

           #include <stdio.h>
           #include <stdlib.h>
           #include <unistd.h>
           #include <string.h>
           #include <sys/socket.h>
           #include <netdb.h>

           int
           main(int argc, char *argv[])
           {
               struct addrinfo *result, *rp;
               int sfd, s;

               if (argc != 2) {
                   fprintf(stderr, "Usage: %s port\n", argv[0]);
                   exit(EXIT_FAILURE);
               }

               struct addrinfo hints = {};
               hints.ai_family = AF_UNSPEC;
               hints.ai_socktype = SOCK_DGRAM;
               hints.ai_flags = AI_PASSIVE;
               hints.ai_protocol = 0;

               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 a successful bind().
                  If socket(2) (or bind(2)) fails, 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 */

                        /* ... use socket bound to sfd ... */
           }

APPLICATION USAGE         top

       If the caller handles only TCP and not UDP, for example, then the
       ai_protocol member of the hints structure should be set to
       IPPROTO_TCP when getaddrinfo() is called.

       If the caller handles only IPv4 and not IPv6, then the ai_family
       member of the hints structure should be set to AF_INET when
       getaddrinfo() is called.

       The term ``canonical name'' is misleading; it is taken from the
       Domain Name System (RFC 2181). It should be noted that the canonical
       name is a result of alias processing, and not necessarily a unique
       attribute of a host, address, or set of addresses. See RFC 2181 for
       more discussion of this in the Domain Name System context.

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       connect(3p), endservent(3p), gai_strerror(3p), getnameinfo(3p),
       socket(3p)

       The Base Definitions volume of POSIX.1‐2008, netdb.h(0p),
       sys_socket.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                    FREEADDRINFO(3P)

Pages that refer to this page: netdb.h(0p)gai_strerror(3p)getaddrinfo(3p)getnameinfo(3p)