what following are quoted from http://linux.die.net/man/3/getaddrinfo

 

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Description

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 getservbyname(3) and getservbyport(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;
    size_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 bygetaddrinfo().

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_socktypeand ai_protocol to 0; ai_family to AF_UNSPEC; and ai_flags to (AI_V4MAPPED | AI_ADDRCONFIG).

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 byinet_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 hosts's network addresses. If node is not NULL, then the AI_PASSIVEflag 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(while  the AI_PASSIVE flag is not set ) (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).