freeaddrinfo, getaddrinfo - get address information
cc [ flag ... ] file ... -lxnet [ library ... ]
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 frees one or more addrinfo structures
returned by getaddrinfo(), along with any additional storage associated
with those structures. If the ai_next member of the structure is not
null, the entire list of structures is freed. The freeaddrinfo() func‐
tion supports the freeing of arbitrary sublists of an addrinfo list
originally returned by getaddrinfo().
The getaddrinfo() function translates the name of a service location
(for example, a host name) and/or a service name and returns a set of
socket addresses and associated information to be used in creating a
socket with which to address the specified service.
The nodename and servname arguments are either null pointers or point‐
ers to null-terminated strings. One or both of these two arguments are
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 attempts
to resolve the name in all supported families and, in absence of
errors, one or more results are 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(3XNET)
If the specified address family is AF_INET6 or AF_UNSPEC, standard IPv6
text forms described in inet_ntop(3XNET) are valid.
If nodename is not null, the requested service location is named by
nodename; otherwise, the requested service location is local to the
If servname is null, the call returns network-level addresses for the
specified nodename. If servname is not null, it is a null-terminated
character string identifying the requested service. This string 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 can 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 is set to 0 or a
null pointer. A value of AF_UNSPEC for ai_family means that the caller
accepts any address family. A value of 0 for ai_socktype means that the
caller accepts any socket type. A value of 0 for ai_protocol means that
the caller accepts any protocol. If hints is a null pointer, the behav‐
ior is as if it referred to a structure containing the value 0 for the
ai_flags, ai_socktype, and ai_protocol members, and AF_UNSPEC for the
The ai_flags member to which the hints parameter points is set to 0 or
be the bitwise-inclusive OR of one or more of the values AI_PASSIVE,
AI_CANONNAME, AI_NUMERICHOST, and AI_NUMERICSERV.
If the AI_PASSIVE flag is specified, the returned address information
is suitable for use in binding a socket for accepting incoming connec‐
tions for the specified service. In this case, if the nodename argument
is null, then the IP address portion of the socket address structure is
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 is suitable for a call to connect(3XNET) (for a connection-
mode protocol) or for a call to connect(), sendto(3XNET), or
sendmsg(3XNET) (for a connectionless protocol). In this case, if the
nodename argument is null, then the IP address portion of the socket
address structure is set to the loopback address.
If the AI_CANONNAME flag is specified and the nodename argument is not
null, the function attempts to determine the canonical name correspond‐
ing to nodename (for example, if nodename is an alias or shorthand
notation for a complete name).
If the AI_NUMERICHOST flag is specified, then a non-null nodename
string supplied is a numeric host address string. Otherwise, an EAI_NO‐
NAME error is returned. This flag prevents 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 is a numeric port string. Otherwise, an EAI_NONAME
error is returned. This flag prevents any type of name resolution ser‐
vice (for example, NIS+) from being invoked.
If the AI_V4MAPPED flag is specified along with an ai_family of
AF_INET6, then getaddrinfo() returns IPv4-mapped IPv6 addresses on
finding no matching IPv6 addresses (ai_addrlen is 16). The AI_V4MAPPED
flag is ignored unless ai_family equals AF_INET6. If the AI_ALL flag
is used with the AI_V4MAPPED flag, then getaddrinfo() returns all
matching IPv6 and IPv4 addresses. The AI_ALL flag without the
AI_V4MAPPED flag is ignored.
The ai_socktype member to which argument hints points specifies the
socket type for the service, as defined in socket(3XNET). If a specific
socket type is not given (for example, a value of 0) and the service
name could be interpreted as valid with multiple supported socket
types, the implementation attempts to resolve the service name for all
supported socket types and, in the absence of errors, all possible
results are returned. A non-zero socket type value limits the returned
information to values with the specified socket type.
If the ai_family member to which hints points has the value AF_UNSPEC,
addresses are returned for use with any address family that can be used
with the specified nodename and/or servname. Otherwise, addresses are
returned for use only with the specified address family. If ai_family
is not AF_UNSPEC and ai_protocol is not 0, then addresses are returned
for use only with the specified address family and protocol; the value
of ai_protocol is interpreted as in a call to the socket() function
with the corresponding values of ai_family and ai_protocol.
A 0 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 refers to a linked list of addrinfo structures, each of which
specifies a socket address and information for use in creating a socket
with which to use that socket address. The list includes at least one
addrinfo structure. The ai_next member 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 includes
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 speci‐
fied, for use with the bind(3XNET) function. The ai_family , ai_sock‐
type, and ai_protocol members are usable as the arguments to the
socket() function to create a socket suitable for use with the returned
address. The ai_addr and ai_addrlen members 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 member of the first returned addrinfo structure points to
a null-terminated string containing the canonical name corresponding to
the input nodename. If the canonical name is not available, then
ai_canonname refers to the nodename argument or a string with the same
contents. The contents of the ai_flags member of the returned struc‐
tures are undefined.
All members in socket address structures returned by getaddrinfo() that
are not filled in through an explicit argument (for example, sin6_flow‐
info) are set to 0, making it easier to compare socket address struc‐
The getaddrinfo() function will fail if:
The name could not be resolved at this time. Future
attempts might succeed.
The ai_flags member of the addrinfo structure had an
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 return value.
he name does not resolve for the supplied parameters.
Neither nodename nor servname were supplied. At least
one of these must be supplied.
The service passed was not recognized for the specified
The intended socket type was not recognized.
A system error occurred. The error code can be found in
An argument buffer overflowed.
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
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Standard │
│MT-Level │ MT-Safe │
SEE ALSOconnect(3XNET), gai_strerror(3XNET), gethostbyname(3XNET), getname‐
info(3XNET), getservbyname(3XNET), inet_addr(3XNET), inet_ntop(3XNET),
socket(3XNET), attributes(5), standards(5)
Nov 1, 2003 FREEADDRINFO(3XNET)