getaddrinfo, freeaddrinfo, gai_strerror - network address and service translation


 #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);


The getaddrinfo(3) function combines the functionality provided by the getipnodebyname(3), getipnodebyaddr(3), getservbyname(3), and getservbyport(3) functions into a single interface. The thread-safe getaddrinfo(3) 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(3) 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(3) 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(3) 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(3) 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(3) function frees the memory that was allocated for the dynamically allocated linked list res.


getaddrinfo(3) returns 0 if it succeeds, or one of the following non-zero error codes:
The specified network host does not have any network addresses in the requested address family.
The name server returned a temporary failure indication. Try again later.
ai_flags contains invalid flags.
The name server returned a permanent failure indication.
The requested address family is not supported at all.
Out of memory.
The specified network host exists, but does not have any network addresses defined.
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.
The requested service is not available for the requested socket type. It may be available through another socket type.
The requested socket type is not supported at all.
Other system error, check errno for details.

The gai_strerror(3) function translates these error codes to a human readable string, suitable for error reporting.


POSIX.1-2001. The getaddrinfo() function is documented in RFC 2553.


AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc 2.3.3. AI_NUMERICSERV is available since glibc 2.3.4.


getipnodebyaddr(3), getipnodebyname(3)