getaddrinfo - network address and service translation


#include 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 link list of addrinfo structures, linked by the ai_next member. There are several reasons why the link 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 PF_INET or PF_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. PF_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 the ai_flags member in the hints parameter 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 link list of addrinfo structures, one for each network address subject to any restrictions imposed by the hints parameter. ai_canonname is set to point to the official name of the host, if ai_flags in hints 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 the ai_flags member of the hints parameter. 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.

service sets the port number in the network address of each socket structure. If service is NULL the port number will be left uninitialized.

The freeaddrinfo(3) function frees the memory that was allocated for the dynamically allocated link list res.


getaddrinfo(3) returns 0 if it succeeds, or one of the following non-zero error codes:


The requested address family is not supported at all.


The requested socket type is not supported at all.


ai_flags contains invalid flags.


The node or service is not known. This error is also returned if both node and service are NULL.


The requested service is not available for the requested socket type. It may be available through another socket type.


The specified network host does not have any network addresses in the requested address family.


The specified network host exists, but does not have any network addresses defined.


Out of memory.


The name server returned a permanent failure indication.


The name server returned a temporary failure indication. Try again later.


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.


getipnodebyname(3), getipnodebyaddr(3)

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