FREEADDRINFO(P) POSIX Programmer's Manual FREEADDRINFO(P)
freeaddrinfo, getaddrinfo - get address information
void freeaddrinfo(struct addrinfo *ai);
int getaddrinfo(const char *restrict nodename,
const char *restrict servname,
const struct addrinfo *restrict hints,
struct addrinfo **restrict res);
The freeaddrinfo() function shall free one or more addrinfo structures returned by getad-
drinfo(), 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-termi-
nated 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 fam-
ily is not given and the name could be interpreted as valid within multiple supported fam-
ilies, 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() are valid.
If the specified address family is AF_INET6 or AF_UNSPEC, standard IPv6 text forms
described in inet_ntop() 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 node-
name. 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 suit-
able 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
may direct the operation by providing options and by limiting the returned information to
a specific socket type, address family, and/or protocol. 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 struc-
ture 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 bit-
wise-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 ``canon-
ical name'' and ``alias'' cannot be precisely defined for the general case. How-
ever, 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
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 getad-
drinfo() 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
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() . 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 sup-
ported 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 spec-
ified 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
are 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 corre-
sponding values of ai_family and ai_protocol.
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
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 con-
tains 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 contain-
ing 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 unde-
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.
The getaddrinfo() function shall fail and return the corresponding value if:
The name could not be resolved at this time. Future attempts may succeed.
The flags parameter had an invalid value.
A non-recoverable error occurred when attempting to resolve the name.
The address family was not recognized.
There was a memory allocation failure when trying to allocate storage for the
The name does not resolve for the supplied parameters.
Neither nodename nor servname were supplied. At least one of these shall be supplied.
The service passed was not recognized for the specified socket type.
The intended socket type was not recognized.
A system error occurred; the error code can be found in errno.
An argument buffer overflowed.
The following sections are informative.
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.
connect() , gai_strerror() , gethostbyaddr() , getnameinfo() , getservbyname() , socket()
, the Base Definitions volume of IEEE Std 1003.1-2001, <netdb.h>, <sys/socket.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 FREEADDRINFO(P)