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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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getaddrinfo(3) Library Functions Manual getaddrinfo(3)
getaddrinfo, freeaddrinfo, gai_strerror - network address and
service translation
Standard C library (libc, -lc)
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *restrict node,
const char *restrict service,
const struct addrinfo *restrict hints,
struct addrinfo **restrict 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
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 size 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.
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 node 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; errno is set to indicate the error.
The gai_strerror() function translates these error codes to a
human readable string, suitable for error reporting.
/etc/gai.conf
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────┬───────────────┬────────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────┼───────────────┼────────────────────┤
│ getaddrinfo() │ Thread safety │ MT-Safe env locale │
├───────────────────────────┼───────────────┼────────────────────┤
│ freeaddrinfo(), │ Thread safety │ MT-Safe │
│ gai_strerror() │ │ │
└───────────────────────────┴───────────────┴────────────────────┘
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.
POSIX.1-2008.
getaddrinfo()
RFC 2553.
POSIX.1-2001.
AI_ADDRCONFIG
AI_ALL
AI_V4MAPPED
glibc 2.3.3.
AI_NUMERICSERV
glibc 2.3.4.
getaddrinfo() supports the address%scope-id notation for
specifying the IPv6 scope-ID.
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 <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
int sfd, s;
char buf[BUF_SIZE];
ssize_t nread;
socklen_t peer_addrlen;
struct addrinfo hints;
struct addrinfo *result, *rp;
struct sockaddr_storage peer_addr;
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 (;;) {
char host[NI_MAXHOST], service[NI_MAXSERV];
peer_addrlen = sizeof(peer_addr);
nread = recvfrom(sfd, buf, BUF_SIZE, 0,
(struct sockaddr *) &peer_addr, &peer_addrlen);
if (nread == -1)
continue; /* Ignore failed request */
s = getnameinfo((struct sockaddr *) &peer_addr,
peer_addrlen, 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_addrlen) != nread)
{
fprintf(stderr, "Error sending response\n");
}
}
}
Client program
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
int sfd, s;
char buf[BUF_SIZE];
size_t size;
ssize_t nread;
struct addrinfo hints;
struct addrinfo *result, *rp;
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 (size_t j = 3; j < argc; j++) {
size = strlen(argv[j]) + 1;
/* +1 for terminating null byte */
if (size > BUF_SIZE) {
fprintf(stderr,
"Ignoring long message in argument %zu\n", j);
continue;
}
if (write(sfd, argv[j], size) != size) {
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);
}
getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3),
gai.conf(5), hostname(7), ip(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 getaddrinfo(3)
Pages that refer to this page: getent(1), pmdanetcheck(1), bind(2), connect(2), recv(2), send(2), socket(2), getaddrinfo_a(3), gethostbyname(3), getipnodebyname(3), getnameinfo(3), inet(3), inet_pton(3), NULL(3const), resolver(3), sockaddr(3type), gai.conf(5), resolv.conf(5), hostname(7), agetty(8), systemd-machined.service(8), systemd-resolved.service(8)
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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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