WSAAsyncGetHostByName

The Windows Sockets WSAAsyncGetHostByName function asynchronously retrieves host information corresponding to a host name.

HANDLE WSAAsyncGetHostByName (
  HWND hWnd,              
  unsigned int wMsg,      
  const char FAR * name,  
  char FAR * buf,         
  int buflen              
);

Parameters

hWnd: [in] The handle of the window that will receive a message when the asynchronous request completes.
wMsg: [in] The message to be received when the asynchronous request completes.
name: [in] A pointer to the null-terminated name of the host.
buf: [out] A pointer to the data area to receive the HOSTENT data. The data area must be larger than the size of a HOSTENT structure because the supplied data area is used by Windows Sockets to contain a HOSTENT structure and all of the data referenced by members of the HOSTENT structure. A buffer of MAXGETHOSTSTRUCT bytes is recommended.
buflen: [in] The size of data area for the buf parameter.

Remarks

The WSAAsyncGetHostByName function is an asynchronous version of gethostbyname, and is used to retrieve host name and address information corresponding to a host name. Windows Sockets initiates the operation and returns to the caller immediately, passing back an opaque "asynchronous task handle" that whichthe 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 indcated 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 supplied to the original function call contains a HOSTENT structure. To access the elements of this structure, the original buffer address should be 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 supplied to this function is used by Windows Sockets to construct a HOSTENT 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:

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

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

WSAAsyncGetHostByName is guaranteed to resolve the string returned by a successful call to gethostname.

Return Values

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, WSAAsyncGetHostByName 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, WSAAsyncGetHostByName returns a zero value, and a specific error number can be retrieved by calling WSAGetLastError.

Error Codes

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.

WSAENETDOWN	The network subsystem has failed.
WSAENOBUFS	Insufficient buffer space is available.
WSAEFAULT	The name or buf parameter is not in a valid part of the process address space.
WSAHOST_NOT_FOUND	Authoritative Answer Host not found.
WSATRY_AGAIN	Non-Authoritative Host not found, or SERVERFAIL.
WSANO_RECOVERY	Nonrecoverable errors, FORMERR, REFUSED, NOTIMP.
WSANO_DATA	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.

WSANOTINITIALISED	A successful WSAStartup 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.

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