gethostbyaddr macro

[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.

Syntax

void gethostbyaddr(
   a,
   b,
   c
);

Parameters

a

b

c

Return Value

None

Remarks

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 4 127.0.0.1\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 4 127.0.0.1\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:
            printf("AF_INET\n");
            break;
        case AF_INET6:
            printf("AF_INET6\n");
            break;
        case AF_NETBIOS:
            printf("AF_NETBIOS\n");
            break;
        default:
            printf(" %d\n", remoteHost->h_addrtype);
            break;
        }
        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.

Requirements

   
Minimum supported client Windows 8.1, Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header wsipv6ok.h (include Winsock2.h, Winsock.h)
Library Ws2_32.lib
DLL Ws2_32.dll

See Also

GetAddrInfoEx

GetAddrInfoW

WSAAsyncGetHostByAddr

Winsock Functions

Winsock Reference

addrinfo

addrinfoW

getaddrinfo

gethostbyname

hostent