GetNameInfoW function (ws2tcpip.h)

The GetNameInfoW function provides protocol-independent name resolution from an address to a Unicode host name and from a port number to the Unicode service name.

Syntax

INT WSAAPI GetNameInfoW(
  [in]  const SOCKADDR *pSockaddr,
  [in]  socklen_t      SockaddrLength,
  [out] PWCHAR         pNodeBuffer,
  [in]  DWORD          NodeBufferSize,
  [out] PWCHAR         pServiceBuffer,
  [in]  DWORD          ServiceBufferSize,
  [in]  INT            Flags
);

Parameters

[in] pSockaddr

A pointer to a socket address structure containing the IP address and port number of the socket. For IPv4, the pSockaddr parameter points to a sockaddr_in structure. For IPv6, the pSockaddr parameter points to a sockaddr_in6 structure.

[in] SockaddrLength

The length, in bytes, of the structure pointed to by the pSockaddr parameter.

[out] pNodeBuffer

A pointer to a Unicode string to hold the host name. On success, a pointer to the Unicode host name is returned as a Fully Qualified Domain Name (FQDN) by default. If the pNodeBuffer parameter is NULL, this indicates the caller does not want to receive a host name string.

[in] NodeBufferSize

The number of WCHAR characters in the buffer pointed to by the pNodeBuffer parameter. The caller must provide a buffer large enough to hold the Unicode host name, including the terminating NULL character.

[out] pServiceBuffer

A pointer to a Unicode string to hold the service name. On success, a pointer is returned to a Unicode string representing the service name associated with the port number. If the pServiceBuffer parameter is NULL, this indicates the caller does not want to receive a service name string.

[in] ServiceBufferSize

The number of WCHAR characters in the buffer pointed to by the pServiceBuffer parameter. The caller must provide a buffer large enough to hold the Unicode service name, including the terminating NULL character.

[in] Flags

A value used to customize processing of the GetNameInfoW function. See the Remarks section.

Return value

On success, GetNameInfoW returns zero. Any nonzero return value indicates failure and a specific error code can be retrieved by calling WSAGetLastError.

Nonzero error codes returned by the GetNameInfoW function also map to the set of errors outlined by Internet Engineering Task Force (IETF) recommendations. The following table shows these error codes and their WSA equivalents. It is recommended that the WSA error codes be used, as they offer familiar and comprehensive error information for Winsock programmers.

You can use the gai_strerror function to print error messages based on the EAI codes returned by the GetNameInfoW function. The gai_strerror function is provided for compliance with IETF recommendations, but it is not thread safe. Therefore, use of traditional Windows Sockets functions such as WSAGetLastError is recommended.

In addition, the following error codes can be returned.

Remarks

The GetNameInfoW function is the Unicode version of a function that provides protocol-independent name resolution. The GetNameInfoW function is used to translate the contents of a socket address structure to a node name and/or a service name.

For the IPv6 and IPv4 protocols, name resolution can be by the Domain Name System (DNS), a local hosts file, or by other naming mechanisms. This function can be used to determine the host name for an IPv4 or IPv6 address, a reverse DNS lookup, or determine the service name for a port number. The GetNameInfoW function can also be used to convert an IP address or a port number in a SOCKADDR structure to an Unicode string. This function can also be used to determine the IP address for a host name.

The ANSI version of this function is getnameinfo.

Macros in the Winsock header file define a mixed-case function name of GetNameInfo that can be used when the application is targeted for Windows XP with Service Pack 2 (SP2) and later (_WIN32_WINNT >= 0x0502). This GetNameInfo function should be called with the pNodeBuffer and pServiceBuffer parameters of a pointer of type TCHAR. When UNICODE or _UNICODE is defined, GetNameInfo is defined to the Unicode version and GetNameInfoW is called with the host and serv parameters of a pointer of type char. When UNICODE or _UNICODE is not defined, GetNameInfo is defined to the ANSI version and getnameinfo is called with the pNodeBuffer and pServiceBuffer parameters of a pointer of type PWCHAR.

To simplify determining buffer requirements for the pNodeBuffer and pServiceBuffer parameters, the following values for maximum host name length and maximum service name are defined in the Ws2tcpip.h header file:

#include <windows.h>

#define NI_MAXSERV    32
#define NI_MAXHOST  1025

The Flags parameter can be used to customize processing of the GetNameInfoW function. The following flags are available:

• NI_NOFQDN
• NI_NUMERICHOST
• NI_NAMEREQD
• NI_NUMERICSERV
• NI_DGRAM

When the NI_NAMEREQD flag is set, a host name that cannot be resolved by the DNS results in an error.

Setting the NI_NOFQDN flag results in local hosts having only their Relative Distinguished Name (RDN) returned in the pNodeBuffer parameter.

Setting the NI_NUMERICHOST flag returns the numeric form of the host name instead of its name. The numeric form of the host name is also returned if the host name cannot be resolved by DNS.

Setting the NI_NUMERICSERV flag returns the port number of the service instead of its name. Also, if a host name is not found for an IP address (127.0.0.2, for example), the hostname is returned as the IP address.

On Windows Vista and later, if NI_NUMERICSERV is not specified in the flags parameter, and the port number contained in sockaddr structure pointed to by the sa parameter does not resolve to a well known service, the GetNameInfoW function returns the numeric form of the service address (the port number) as a numeric string. When NI_NUMERICSERV is specified, the port number is returned as a numeric string. This behavior is specified in section 6.2 of RFC 3493. For more information, see www.ietf.org/rfc/rfc3493.txt

On Windows Server 2003 and earlier, if NI_NUMERICSERV is not specified in the flags parameter and the port number contained in sockaddr structure pointed to by the sa parameter does not resolve to a well known service, the GetNameInfoW function fails. When NI_NUMERICSERV is specified, the port number is returned as a numeric string.

Setting the NI_DGRAM flag indicates that the service is a datagram service. This flag is necessary for the few services that provide different port numbers for UDP and TCP service.

Windows Phone 8: This function is supported for Windows Phone Store apps on Windows Phone 8 and later.

Windows 8.1 and Windows Server 2012 R2: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and later.

Example Code

#ifndef UNICODE
#define UNICODE
#endif

#define WIN32_LEAN_AND_MEAN

#include <winsock2.h>
#include <Ws2tcpip.h>
#include <stdio.h>

// Link with ws2_32.lib
#pragma comment(lib, "Ws2_32.lib")

int __cdecl main(int argc, char **argv)
{

    //-----------------------------------------
    // Declare and initialize variables
    WSADATA wsaData;
    int iResult;

    DWORD dwRetval;

    struct sockaddr_in saGNI;
    WCHAR hostname[NI_MAXHOST];
    WCHAR servInfo[NI_MAXSERV];
    u_short port = 27015;

    // Validate the parameters
    if (argc != 2) {
        wprintf(L"usage: %s IPv4 address\n", argv[0]);
        wprintf(L"  to return hostname\n");
        wprintf(L"       %s 127.0.0.1\n", argv[0]);
        return 1;
    }
    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != 0) {
        wprintf(L"WSAStartup failed: %d\n", iResult);
        return 1;
    }
    //-----------------------------------------
    // Set up sockaddr_in structure which is passed
    // to the getnameinfo function
    saGNI.sin_family = AF_INET;
    saGNI.sin_addr.s_addr = inet_addr(argv[1]);
    saGNI.sin_port = htons(port);

    //-----------------------------------------
    // Call GetNameInfoW
    dwRetval = GetNameInfoW((struct sockaddr *) &saGNI,
                           sizeof (struct sockaddr),
                           hostname,
                           NI_MAXHOST, servInfo, NI_MAXSERV, NI_NUMERICSERV);

    if (dwRetval != 0) {
        wprintf(L"GetNameInfoW failed with error # %ld\n", WSAGetLastError());
        return 1;
    } else {
        wprintf(L"GetNameInfoW returned hostname = %ws\n", hostname);
        return 0;
    }
}

Requirements

See also

GetAddrInfoW

WSAGetLastError

Winsock Functions

Winsock Reference

gai_strerror

getaddrinfo

getnameinfo

sockaddr