WSAAsyncGetHostByAddr function (winsock.h)

The WSAAsyncGetHostByAddr function asynchronously retrieves host information that corresponds to an address.

Note  The WSAAsyncGetHostByAddr function is not designed to provide parallel resolution of several addresses. Therefore, applications that issue several requests should not expect them to be executed concurrently. Alternatively, applications can start another thread and use the getnameinfo function to resolve addresses in an IP-version agnostic manner. Developers creating Windows Sockets 2 applications are urged to use the getnameinfo function to enable smooth transition to IPv6 compatibility.


HANDLE WSAAsyncGetHostByAddr(
  HWND       hWnd,
  u_int      wMsg,
  const char *addr,
  int        len,
  int        type,
  char       *buf,
  int        buflen
















Return value

The return value specifies whether or not the asynchronous operation was successfully initiated. It does not imply success or failure of the operation itself.

If no error occurs, WSAAsyncGetHostByAddr returns a nonzero value of type HANDLE that is the asynchronous task handle (not to be confused with a Windows HTASK) for the request. This value can be used in two ways. It can be used to cancel the operation using WSACancelAsyncRequest, or it can be used to match up asynchronous operations and completion messages by examining the wParam message parameter.

If the asynchronous operation could not be initiated, WSAAsyncGetHostByAddr returns a zero value, and a specific error number can be retrieved by calling WSAGetLastError.

The following error codes can be set when an application window receives a message. As described above, they can be extracted from the lParam in the reply message using the WSAGETASYNCERROR macro.

Error code Meaning
The network subsystem has failed.
Insufficient buffer space is available.
The addr or buf parameter is not in a valid part of the process address space.
Authoritative answer host not found.
Nonauthoritative host not found, or SERVERFAIL.
Nonrecoverable errors: FORMERR, REFUSED, NOTIMP.
Valid name, no data record of requested type.

The following errors can occur at the time of the function call, and indicate that the asynchronous operation could not be initiated.

Error Code Meaning
WSANOTINITIALISED A successful WSAStartup call must occur before using this function.
WSAENETDOWN The network subsystem has failed.
WSAEINPROGRESS A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEWOULDBLOCK The asynchronous operation cannot be scheduled at this time due to resource or other constraints within the Windows Sockets implementation.


The WSAAsyncGetHostByAddr function is an asynchronous version of gethostbyaddr. It is used to retrieve the host name and address information that corresponds to a network address. Windows Sockets initiates the operation and returns to the caller immediately, passing back an opaque, asynchronous task handle that the application can use to identify the operation. When the operation is completed, the results (if any) are copied into the buffer provided by the caller and a message is sent to the application's window.

When the asynchronous operation has completed, the application window indicated by the hWnd parameter receives message in the wMsg parameter. The wParam parameter contains the asynchronous task handle as returned by the original function call. The high 16 bits of lParam contain any error code. The error code can be any error as defined in Winsock2.h. An error code of zero indicates successful completion of the asynchronous operation.

On successful completion, the buffer specified to the original function call contains a hostent structure. To access the members of this structure, the original buffer address is cast to a hostent structure pointer and accessed as appropriate.

If the error code is WSAENOBUFS, the size of the buffer specified by buflen in the original call was too small to contain all the resulting information. In this case, the low 16 bits of lParam contain the size of buffer required to supply all the requisite information. If the application decides that the partial data is inadequate, it can reissue the WSAAsyncGetHostByAddr function call with a buffer large enough to receive all the desired information (that is, no smaller than the low 16 bits of lParam).

The buffer specified to this function is used by Windows Sockets to construct a structure together with the contents of data areas referenced by members of the same hostent structure. To avoid the WSAENOBUFS error, the application should provide a buffer of at least MAXGETHOSTSTRUCT bytes (as defined in Winsock2.h).

The error code and buffer length should be extracted from the lParam using the macros WSAGETASYNCERROR and WSAGETASYNCBUFLEN, defined in Winsock2.h as:

#include <windows.h>

#define WSAGETASYNCBUFLEN(lParam)           LOWORD(lParam)
#define WSAGETASYNCERROR(lParam)            HIWORD(lParam)

The use of these macros will maximize the portability of the source code for the application.


Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header winsock.h (include Winsock2.h, Winsock.h)
Library Ws2_32.lib
DLL Ws2_32.dll

See also


Winsock Functions

Winsock Reference