Maps addresses to names in a protocol-independent way.
    Format
      #include  <socket.h>
      #include  <netdb.h>
      int getnameinfo  ( const struct sockaddr *sa, size_t salen,
                       char *node, size_t nodelen, char *service,
                       size_t servicelen, int flags );

1  –  Arguments

 sa
    Points either to a sockaddr_in structure (for IPv4) or to a
    sockaddr_in6 structure (for IPv6) that holds the IP address and
    port number.
 salen
    Specifies the length of either the sockaddr_in structure or the
    sockaddr_in6 structure.
 node
    Points to a buffer in which to receive the null-terminated
    network node name or alias corresponding to the address contained
    in the sa. A NULL pointer instructs getnameinfo() to not return
    a node name. The node argument and service argument must not both
    be zero.
 nodelen
    Specifies the length of the node buffer. A value of zero
    instructs getnameinfo() to not return a node name.
 service
    Points to a buffer in which to receive the null-terminated
    network service name associated with the port number contained
    in sa. A NULL pointer instructs getnameinfo() to not return a
    service name. The node argument and service argument must not
    both be 0.
 servicelen
    Specifies the length of the service buffer. A value of zero
    instructs getnameinfo() to not return a service name.
 flags
    Specifies changes to the default actions of getnameinfo(). By
    default, getnameinfo() searches for the fully-qualified domain
    name of the node in the hosts database and returns it. See Flags
    for a list of flags and their meanings.

2  –  Description

    This function looks up an IP address and port number in a
    sockaddr structure specified by sa and returns node name and
    service name text strings in the buffers pointed to by the node
    and service arguments, respectively.
    If the node name is not found, getnameinfo() returns the numeric
    form of the node address, regardless of the value of the flags
    argument. If the service name is not found, getnameinfo() returns
    the numeric form of the service address (port number) regardless
    of the value of the flags argument.
    The application must provide buffers large enough to hold the
    fully-qualified domain name and the service name, including the
    terminating null characters.
    Flags describes the flag bits and, if set, their meanings. The
    flags are defined in the NETDB.H header file.
    Table 4-2 getnameinfo()  Flags
    Flag Value         Description
    NI_DGRAM           Specifies that the service is a datagram
                       service (SOCK_DGRAM). The default assumes
                       a stream service (SOCK_STREAM). This is
                       required for the few ports (512-514) that
                       have different services for UDP and TCP.
    NI_NAMEREQD        Returns an error if the host name cannot be
                       located in the hosts database.
    NI_NOFQDN          Searches the hosts database and returns the
                       node name portion of the fully-qualified
                       domain name for local hosts.
    NI_NUMERICHOST     Returns the numeric form of the host's address
                       instead of its name. Resolution of the host
                       name is not performed.
    NI_NUMERICSERV     Returns the numeric form (port number) of the
                       service address instead of its name. The host
                       name is not resolved.

3  –  Return Values

    0                  Indicates success.
    x                  Indicates an error occurred. The value of
                       errno indicates the error.

4  –  Errors

    EAI_AGAIN          The name could not be resolved at this time.
                       Future attempts may succeed.
    EAI_BADFLAGS       The flags argument had an invalid value.
    EAI_FAIL           A nonrecoverable error occurred when
                       attempting to resolve the name.
    EAI_FAMILY         The address family was not recognized.
    EAI_MEMORY         There was a memory allocation failure when
                       trying to allocate storage for the return
                       value.
    EAI_NONAME         The name does not resolve for the supplied
                       parameters. Neither the node name nor the
                       service name were supplied. At least one of
                       these must be supplied.
    EAI_SYSTEM         A system error occurred; the error code can be
                       found in errno.