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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FREEADDRINFO(3P) POSIX Programmer's Manual FREEADDRINFO(3P)
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.
freeaddrinfo, getaddrinfo — get address information
#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);
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.
The application shall ensure that each of the ai_addrlen, ai_addr,
ai_canonname, and ai_next members, as well as each of the non-
standard additional members, if any, of this hints structure is
initialized. If any of these members has a value other than the
value that would result from default initialization, the behavior
is implementation-defined. 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.
By default, with an ai_family of AF_INET6, getaddrinfo() shall
return only IPv6 addresses. 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. 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 shall be
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.
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.
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.
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 = {0};
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 ... */
}
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.
Although it is common practice to initialize the hints structure
using:
struct addrinfo hints;
memset(&hints, 0, sizeof hints);
this method is not portable according to this standard, because
the structure can contain pointer or floating-point members that
are not required to have an all-bits-zero representation after
default initialization. Portable methods make use of default
initialization; for example:
struct addrinfo hints = { 0 };
or:
static struct addrinfo hints_init;
struct addrinfo hints = hints_init;
A future version of this standard may require that a pointer
object with an all-bits-zero representation is a null pointer, and
that addrinfo does not have any floating-point members if a
floating-point object with an all-bits-zero representation does
not have the value 0.0.
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.
None.
None.
connect(3p), endservent(3p), gai_strerror(3p), getnameinfo(3p),
socket(3p)
The Base Definitions volume of POSIX.1‐2017, netdb.h(0p),
sys_socket.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 original Standard can be obtained online at
http://www.opengroup.org/unix/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 2017 FREEADDRINFO(3P)
Pages that refer to this page: netdb.h(0p), gai_strerror(3p), getaddrinfo(3p), getnameinfo(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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