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NAME
getaddrinfo, freeaddrinfo, gai_strerror - network address and service
translation
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *node, const char *service,
const struct addrinfo *hints,
struct addrinfo **res);
void freeaddrinfo(struct addrinfo *res);
const char *gai_strerror(int errcode);
DESCRIPTION
The getaddrinfo() function combines the functionality provided by the
getservbyname(3) and getservbyport(3) functions into a single
interface. The thread-safe getaddrinfo() function creates one or more
socket address structures that can be used by the bind(2) and
connect(2) system calls to create a client or a server socket.
The getaddrinfo() function is not limited to creating IPv4 socket
address structures; IPv6 socket address structures can be created if
IPv6 support is available. These socket address structures can be used
directly by bind(2) or connect(2), to prepare a client or a server
socket.
The addrinfo structure used by this function contains the following
members:
struct addrinfo {
int ai_flags;
int ai_family;
int ai_socktype;
int ai_protocol;
size_t ai_addrlen;
struct sockaddr *ai_addr;
char *ai_canonname;
struct addrinfo *ai_next;
};
getaddrinfo() sets res to point to a dynamically allocated linked list
of addrinfo structures, linked by the ai_next member. There are
several reasons why the linked list may have more than one addrinfo
structure, including: if the network host is multi-homed; or if the
same service is available from multiple socket protocols (one
SOCK_STREAM address and another SOCK_DGRAM address, for example).
The members ai_family, ai_socktype, and ai_protocol have the same
meaning as the corresponding parameters in the socket(2) system call.
The getaddrinfo() function returns socket addresses in either IPv4 or
IPv6 address family, (ai_family will be set to either AF_INET or
AF_INET6).
The hints parameter specifies the preferred socket type, or protocol.
A NULL hints specifies that any network address or protocol is
acceptable. If this parameter is not NULL it points to an addrinfo
structure whose ai_family, ai_socktype, and ai_protocol members specify
the preferred socket type. AF_UNSPEC in ai_family specifies any
protocol family (either IPv4 or IPv6, for example). 0 in ai_socktype
or ai_protocol specifies that any socket type or protocol is acceptable
as well. The ai_flags member specifies additional options, defined
below. Multiple flags are specified by logically OR-ing them together.
All the other members in the hints parameter must contain either 0, or
a null pointer.
The node or service parameter, but not both, may be NULL. node
specifies either a numerical network address (dotted-decimal format for
IPv4, hexadecimal format for IPv6) or a network hostname, whose network
addresses are looked up and resolved. If hints.ai_flags contains the
AI_NUMERICHOST flag then the node parameter must be a numerical network
address. The AI_NUMERICHOST flag suppresses any potentially lengthy
network host address lookups.
The getaddrinfo() function creates a linked list of addrinfo
structures, one for each network address subject to any restrictions
imposed by the hints parameter. The ai_canonname field of the first of
these addrinfo structures is set to point to the official name of the
host, if hints.ai_flags includes the AI_CANONNAME flag. ai_family,
ai_socktype, and ai_protocol specify the socket creation parameters. A
pointer to the socket address is placed in the ai_addr member, and the
length of the socket address, in bytes, is placed in the ai_addrlen
member.
If node is NULL, the network address in each socket structure is
initialized according to the AI_PASSIVE flag, which is set in
hints.ai_flags. The network address in each socket structure will be
left unspecified if AI_PASSIVE flag is set. This is used by server
applications, which intend to accept client connections on any network
address. The network address will be set to the loopback interface
address if the AI_PASSIVE flag is not set. This is used by client
applications, which intend to connect to a server running on the same
network host.
If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4 addresses
are returned in the list pointed to by result only if the local system
has at least one IPv4 address configured, and IPv6 addresses are only
returned if the local system has at least one IPv6 address configured.
If hint.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
result. If both AI_V4MAPPED and AI_ALL are specified in
hints.ai_family, then return both IPv6 and IPv4-mapped IPv6 addresses
in the list pointed to by result. AI_ALL is ignored if AI_V4MAPPED is
not also specified.
service sets the port number in the network address of each socket
structure. If service is NULL the port number 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.
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 host names 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 host name)
flags respectively to be used in the IDNA handling.
RETURN VALUE
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
ai_flags contains invalid flags.
EAI_FAIL
The name server returned a permanent failure indication.
EAI_FAMILY
The requested address family is not supported at all.
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.
EAI_SOCKTYPE
The requested socket type is not supported at all.
EAI_SYSTEM
Other system error, check errno for details.
The gai_strerror() function translates these error codes to a human
readable string, suitable for error reporting.
CONFORMING TO
POSIX.1-2001. The getaddrinfo() function is documented in RFC 2553.
NOTES
AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc 2.3.3.
AI_NUMERICSERV is available since glibc 2.3.4.
EXAMPLE
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.
This is the server:
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <netdb.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
struct addrinfo hints;
struct addrinfo *result, *rp;
int sfd, s;
struct sockaddr_storage peer_addr;
socklen_t peer_addr_len;
ssize_t nread;
char buf[BUF_SIZE];
if (argc != 2) {
fprintf(stderr, "Usage: %s port\n", argv[0]);
exit(EXIT_FAILURE);
}
memset(&hints, 0, sizeof(struct addrinfo));
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);
}
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not bind\n");
exit(EXIT_FAILURE);
}
freeaddrinfo(result); /* No longer needed */
/* Read datagrams and echo them back to sender */
for (;;) {
peer_addr_len = sizeof(struct sockaddr_storage);
nread = recvfrom(sfd, buf, BUF_SIZE, 0,
(struct sockaddr *) &peer_addr, &peer_addr_len);
if (nread == -1)
continue; /* Ignore failed request */
char host[NI_MAXHOST], service[NI_MAXSERV];
s = getnameinfo((struct sockaddr *) &peer_addr,
peer_addr_len, host, NI_MAXHOST,
service, NI_MAXSERV, NI_NUMERICSERV);
if (s == 0)
printf("Received %ld bytes from %s:%s\n",
(long) nread, host, service);
else
fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s));
if (sendto(sfd, buf, nread, 0,
(struct sockaddr *) &peer_addr,
peer_addr_len) != nread)
fprintf(stderr, "Error sending response\n");
}
}
This is the client:
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
struct addrinfo hints;
struct addrinfo *result, *rp;
int sfd, s, j;
size_t len;
ssize_t nread;
char buf[BUF_SIZE];
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(struct addrinfo));
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);
}
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not connect\n");
exit(EXIT_FAILURE);
}
freeaddrinfo(result); /* No longer needed */
/* Send remaining command-line arguments as separate
datagrams, and read responses from server */
for (j = 3; j < argc; j++) {
len = strlen(argv[j]) + 1;
/* +1 for terminating null byte */
if (len + 1 > BUF_SIZE) {
fprintf(stderr,
"Ignoring long message in argument %d\n", j);
continue;
}
if (write(sfd, argv[j], len) != len) {
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 %ld bytes: %s\n", (long) nread, buf);
}
exit(EXIT_SUCCESS);
}
SEE ALSO
getnameinfo(3)
COLOPHON
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