gethostbyname Function (Windows)

gethostbyname Function

gethostbyname Function

The gethostbyname function retrieves host information corresponding to a host name from a host database.

Note The gethostbyname function has been deprecated by the introduction of the getaddrinfo function. Developers creating Windows Sockets 2 applications are urged to use the getaddrinfo function instead of gethostbyname.

Syntax

C++

struct hostent* FAR gethostbyname(
  __in  const char *name
);

Parameters

name [in]: A pointer to the null-terminated name of the host to resolve.

Return Value

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

Error code	Meaning
WSANOTINITIALISED	A successful WSAStartup call must occur before using this function.
WSAENETDOWN	The network subsystem has failed.
WSAHOST_NOT_FOUND	Authoritative answer host not found.
WSATRY_AGAIN	Nonauthoritative host not found, or server failure.
WSANO_RECOVERY	A nonrecoverable error occurred.
WSANO_DATA	Valid name, no data record of requested type.
WSAEINPROGRESS	A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEFAULT	The name parameter is not a valid part of the user address space.
WSAEINTR	A blocking Windows Socket 1.1 call was canceled through WSACancelBlockingCall.

Remarks

The gethostbyname function returns a pointer to a hostent structure—a structure allocated by Windows Sockets. The hostent structure contains the results of a successful search for the host specified in the name parameter.

The memory for the hostent structure returned by the gethostbyname 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 gethostbyname or gethostbyaddr functions on the same thread. Otherwise, the return value will be overwritten by subsequent gethostbyname or gethostbyaddr 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 gethostbyname or gethostbyaddr .

The gethostbyname function cannot resolve IP address strings passed to it. Such a request is treated exactly as if an unknown host name were passed. Use inet_addr to convert an IP address string to an actual IP address, then use another function, gethostbyaddr, to obtain the contents of the hostent structure.

If the name parameter points to an empty string or name is NULL, the returned string is the same as the string returned by a successful gethostname function call (the standard host name for the local computer).

Note The gethostbyname function does not check the size of the name parameter before passing the buffer. With an improperly sized name parameter, heap corruption can occur.

Example Code

The following examples demonstrates the use of the gethostbyname function.

#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>
#include <windows.h>
#pragma comment(lib, "wininet.lib")

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

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

    DWORD dwError;
    int i = 0;

    struct hostent *remoteHost;
    char *host_name;
    struct in_addr addr;

    char **pAlias;

    // Validate the parameters
    if (argc != 2) {
        printf("usage: %s ipv4address\n", argv[0]);
        printf(" or\n");
        printf("       %s hostname\n", argv[0]);
        printf("  to return the host\n");
        printf("       %s 127.0.0.1\n", argv[0]);
        printf("  to return the IP addresses for a host\n");
        printf("       %s www.contoso.com\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_name = argv[1];

// If the user input is an alpha name for the host, use gethostbyname()
// If not, get host by addr (assume IPv4)
    if (isalpha(host_name[0])) {        /* host address is a name */
        printf("Calling gethostbyname with %s\n", host_name);
        remoteHost = gethostbyname(host_name);
    } else {
        printf("Calling gethostbyaddr with %s\n", host_name);
        addr.s_addr = inet_addr(host_name);
        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);

        i = 0;
        while (remoteHost->h_addr_list[i] != 0) {
            addr.s_addr = *(u_long *) remoteHost->h_addr_list[i++];
            printf("\tIP Address #%d: %s\n", i, inet_ntoa(addr));
        }
    }

    return 0;
}

Requirements