- get name information
cc [ flag ... ] file ... -lxnet [ library ... ] #include <sys/socket.h> #include <netdb.h> int getnameinfo(const struct sockaddr *restrict sa, socklen_t salen, char *restrict node, socklen_t nodelen, char *restrict service, socklen_t servicelen, unsigned flags);
The getnameinfo() function translates a socket address to a node name and service location, all of which are defined as in getaddrinfo(3XNET).
The sa argument points to a socket address structure to be translated. If the socket address structure contains an IPv4-mapped IPv6 address or an IPv4-compatible IPv6 address, the implementation extracts the embedded IPv4 address and lookup the node name for that IPv4 address.
If the node argument is non-NULL and the nodelen argument is non-zero, then the node argument points to a buffer able to contain up to nodelen characters that receives the node name as a null-terminated string. If the node argument is NULL or the nodelen argument is zero, the node name is not returned. If the node's name cannot be located, the numeric form of the node's address is returned instead of its name.
If the service argument is non-NULL and the servicelen argument is non-zero, then the service argument points to a buffer able to contain up to servicelen bytes that receives the service name as a null-terminated string. If the service argument is NULL or the servicelen argument is zero, the service name is not returned. If the service's name cannot be located, the numeric form of the service address (for example, its port number) is returned instead of its name.
The flags argument is a flag that changes the default actions of the function. By default the fully-qualified domain name (FQDN) for the host is returned, but:
If the flag bit NI_NOFQDN is set, only the node name portion of the FQDN is returned for local hosts.
If the flag bit NI_NUMERICHOST is set, the numeric form of the host's address is returned instead of its name, under all circumstances.
If the flag bit NI_NAMEREQD is set, an error is returned if the host's name cannot be located.
If the flag bit NI_NUMERICSERV is set, the numeric form of the service address is returned (for example, its port number) instead of its name, under all circumstances.
If the flag bit NI_DGRAM is set, this indicates that the service is a datagram service (SOCK_DGRAM). The default behavior assumes that the service is a stream service (SOCK_STREAM).
A 0 return value for getnameinfo() indicates successful completion; a non-zero return value indicates failure. The possible values for the failures are listed in the ERRORS section.
Upon successful completion, getnameinfo() returns the node and service names, if requested, in the buffers provided. The returned names are always null-terminated strings.
The getnameinfo() function will fail if:
The name could not be resolved at this time. Future attempts might succeed.
The flags argument had an invalid value.
A non-recoverable error occurred.
The address family was not recognized or the address length was invalid for the specified family.
There was a memory allocation failure.
The name does not resolve for the supplied parameters. NI_NAMEREQD is set and the host's name cannot be located, or both nodename and servname were NULL.
A system error occurred. The error code can be found in errno.
If the returned values are to be used as part of any further name resolution (for example, passed to getaddrinfo()), applications should provide buffers large enough to store any result possible on the system.
Given the IPv4-mapped IPv6 address “::ffff:220.127.116.11”, the implementation performs a lookup as if the socket address structure contains the IPv4 address “18.104.22.168”.
See attributes(5) for descriptions of the following attributes:
The IPv6 unspecified address (“::”) and the IPv6 loopback address (“::1”) are not IPv4-compatible addresses. If the address is the IPv6 unspecified address (“::”), a lookup is not performed, and the EAI_NONAME error is returned.
The two NI_NUMERICxxx flags are required to support the -n flag that many commands provide.
The NI_DGRAM flag is required for the few AF_INET and AF_INET6 port numbers (for example, [512,514]) that represent different services for UDP and TCP.