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gethostbyaddr function

[gethostbyaddr is no longer recommended for use as of Windows Sockets 2. Instead, use getnameinfo.]

The gethostbyaddr function retrieves the host information corresponding to a network address.


struct hostent* FAR gethostbyaddr(
  _In_ const char *addr,
  _In_       int  len,
  _In_       int  type


addr [in]

A pointer to an address in network byte order.

len [in]

The length, in bytes, of the address.

type [in]

The type of the address, such as the AF_INET address family type (used with TCP, UDP, and other associated Internet protocols). Possible values for the address family are defined in the Winsock2.h header file.

On the Windows SDK released for Windows Vista and later, the organization of header files has changed and the possible values for the address family are defined in the Ws2def.h header file. Note that the Ws2def.h header file is automatically included in Winsock2.h, and should never be used directly.

Note that the values for the AF_ address family and PF_ protocol family constants are identical (for example, AF_INET and PF_INET), so either constant can be used.

The table below lists the possible values for address family that are supported.


The Internet Protocol version 4 (IPv4) address family.


The NetBIOS address family. This address family is only supported if a Windows Sockets provider for NetBIOS is installed.


The Internet Protocol version 6 (IPv6) address family.


Return value

If no error occurs, gethostbyaddr returns a pointer to the hostent structure. Otherwise, it returns a null pointer, and a specific error code can be retrieved by calling WSAGetLastError.

Error codeMeaning

A successful WSAStartup call must occur before using this function.


An invalid argument was supplied. This error is returned if AF_INET6 was specified in the type parameter and the len parameter was not set equal to the size of an IPv6 address.


The network subsystem has failed.


Authoritative answer host not found.


Nonauthoritative host not found, or server failed.


A nonrecoverable error occurred.


Valid name, no data record of requested type.


A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.


The type specified is not supported by the Windows Sockets implementation.


The addr parameter is not a valid part of the user address space, or the len parameter is too small.


A blocking Windows Socket 1.1 call was canceled through WSACancelBlockingCall.



The gethostbyaddr function returns a pointer to the hostent structure that contains the name and address corresponding to the given network address.

The memory for the hostent structure returned by the gethostbyaddr function is allocated internally by the Winsock DLL from thread local storage. Only a single hostent structure is allocated and used, no matter how many times the gethostbyaddr or gethostbyname functions are called on the thread. The returned hostent structure must be copied to an application buffer if additional calls are to be made to the gethostbyaddr or gethostbyname functions on the same thread. Otherwise, the return value will be overwritten by subsequent gethostbyaddr or gethostbyname calls on the same thread. The internal memory allocated for the returned hostent structure is released by the Winsock DLL when the thread exits.

An application should not try to release the memory used by the returned hostent structure. The application must never attempt to modify this structure or to free any of its components. Furthermore, only one copy of this structure is allocated per thread, so the application should copy any information it needs before issuing any other function calls to gethostbyaddr or gethostbyname.

Although gethostbyaddr no longer recommended for use as of Windows Sockets 2 and the getnameinfo function should be used, gethostbyaddr is capable of returning a NetBIOS name; getnameinfo is not. Developers requiring NetBIOS name resolution may need to use gethostbyaddr until their applications are completely independent of NetBIOS names.

Note   The capability to perform reverse lookups using the gethostbyaddr function is convenient, but such lookups are considered inherently unreliable, and should be used only as a hint.

Example Code

The following example demonstrates the use of the gethostbyaddr function.

#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>

int main(int argc, char **argv)

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

    DWORD dwError;
    int i = 0;
    int bIpv6 = 0;

    struct hostent *remoteHost;
    char *host_addr;
    struct in_addr addr = { 0 };
    IN6_ADDR addr6;

    char **pAlias;

    // Validate the parameters
    if (argc < 2) {
        printf("usage: %s 4 ipv4address\n", argv[0]);
        printf(" or\n");
        printf("usage: %s 6 ipv6address\n", argv[0]);
        printf("  to return the hostname\n");
        printf("       %s 4\n", argv[0]);
        printf("       %s 6 0::1\n", argv[0]);
        return 1;
    // Validate parameters 
    if (atoi(argv[1]) == 4)
        bIpv6 = 0;
    else if (atoi(argv[1]) == 6)
        bIpv6 = 1;
    else {
        printf("usage: %s 4 ipv4address\n", argv[0]);
        printf(" or\n");
        printf("usage: %s 6 ipv6address\n", argv[0]);
        printf("  to return the hostname\n");
        printf("       %s 4\n", argv[0]);
        printf("       %s 6 0::1\n", argv[0]);
        return 1;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed: %d\n", iResult);
        return 1;

    host_addr = argv[2];

    printf("Calling gethostbyaddr with %s\n", host_addr);
    if (bIpv6 == 1) {
            iResult = inet_pton(AF_INET6, host_addr, &addr6);
            if (iResult == 0) {
                printf("The IPv6 address entered must be a legal address\n");
                return 1;
            } else
                remoteHost = gethostbyaddr((char *) &addr6, 16, AF_INET6);
    } else {
        addr.s_addr = inet_addr(host_addr);
        if (addr.s_addr == INADDR_NONE) {
            printf("The IPv4 address entered must be a legal address\n");
            return 1;
        } else
            remoteHost = gethostbyaddr((char *) &addr, 4, AF_INET);

    if (remoteHost == NULL) {
        dwError = WSAGetLastError();
        if (dwError != 0) {
            if (dwError == WSAHOST_NOT_FOUND) {
                printf("Host not found\n");
                return 1;
            } else if (dwError == WSANO_DATA) {
                printf("No data record found\n");
                return 1;
            } else {
                printf("Function failed with error: %ld\n", dwError);
                return 1;
    } else {
        printf("Function returned:\n");
        printf("\tOfficial name: %s\n", remoteHost->h_name);
        for (pAlias = remoteHost->h_aliases; *pAlias != 0; pAlias++) {
            printf("\tAlternate name #%d: %s\n", ++i, *pAlias);
        printf("\tAddress type: ");
        switch (remoteHost->h_addrtype) {
        case AF_INET:
        case AF_INET6:
        case AF_NETBIOS:
            printf(" %d\n", remoteHost->h_addrtype);
        printf("\tAddress length: %d\n", remoteHost->h_length);

        if (remoteHost->h_addrtype == AF_INET) {
            while (remoteHost->h_addr_list[i] != 0) {
                addr.s_addr = *(u_long *) remoteHost->h_addr_list[i++];
                printf("\tIPv4 Address #%d: %s\n", i, inet_ntoa(addr));
        } else if (remoteHost->h_addrtype == AF_INET6)
            printf("\tRemotehost is an IPv6 address\n");

    return 0;

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.


Minimum supported client

Windows 8.1, Windows Vista [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]

Minimum supported phone

Windows Phone 8







See also

Winsock Reference
Winsock Functions



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