WSAAsyncGetHostByAddr()

Description

Get host information corresponding to an address - asynchronous version.

#include <winsock.h>

HANDLE PASCAL FAR WSAAsyncGetHostByAddr ( HWND hWnd, unsigned int wMsg, const char FAR * addr, int len, int type, char FAR * buf, int buflen);

hWnd

The handle of the window which should receive a message when the asynchronous request completes.
wMsg
The message to be received when the asynchronous request completes.
addr
Apointer to the network address for the host. Host addresses are stored in network byte order.
len
The length of the address, which must be 4 for PF_INET.
type
The type of the address, which must be PF_INET.
buf
A pointer to the data area to receive the hostent data. Note that this must be larger than the size of a hostent structure. This is because the data area supplied is used by the Windows Sockets implementation to contain not only a hostent structure but any and all of the data which is referenced by members of the hostent structure. It is recommended that you supply a buffer of MAXGETHOSTSTRUCT bytes.
buflen
The size of data area buf above.

Remarks

This function is an asynchronous version of gethostbyaddr(), and is used to retrieve host name and address information corresponding to a network address. The Windows Sockets implementation initiates the operation and returns to the caller immediately, passing back an asynchronous task handle which the application may 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 is complete the application's window hWnd receives message wMsg. The wParam argument 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 may be any error as defined in winsock.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.

Note that if the error code is WSAENOBUFS, it indicates that the size of the buffer specified by buflen in the original call was too small to contain all the resultant 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 may reissue the WSAAsyncGetHostByAddr() function call with a buffer large enough to receive all the desired information (i.e. no smaller than the low 16 bits of lParam).

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

Return Value

The return value specifies whether or not the asynchronous operation was successfully initiated. Note that it does not imply success or failure of the operation itself. If the operation was successfully initiated, WSAAsyncGetHostByAddr() returns a nonzero value of type HANDLE which is the asynchronous task handle for the request. This value can be used in two ways. It can be used to cancel the operation using WSACancelAsyncRequest(). It can also be used to match up asynchronous operations and completion messages, by examining the wParam message argument.

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

Comments

The buffer supplied to this function is used by the Windows Sockets implementation 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 noted above, the application should provide a buffer of at least MAXGETHOSTSTRUCT bytes (as defined in winsock.h).

Notes For Windows Sockets Suppliers

It is the responsibility of the Windows Sockets implementation to ensure that messages are successfully posted to the application. If a PostMessage() operation fails, the Windows Sockets implementation must re-post that message as long as the window exists.

Windows Sockets suppliers should use the WSAMAKEASYNCREPLY macro when constructing the lParam in the message.

Error Codes

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

WSAENETDOWN

The Windows Sockets implementation has detected that the network subsystem has failed.
WSAENOBUFS
No/insufficient buffer space is available
WSAHOST_NOT_FOUND
Authoritative Answer Host not found.
WSATRY_AGAIN
Non-Authoritative Host not found, or SERVERFAIL.
WSANO_RECOVERY
Non recoverable errors, FORMERR, REFUSED, NOTIMP.
WSANO_DATA
Valid name, no data record of requested type.

The following errors may 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 API.
WSAENETDOWN
The Windows Sockets implementation has detected that the network subsystem has failed.
WSAEINPROGRESS
A blocking Windows Sockets operation is in progress.
WSAEWOULDBLOCK
The asynchronous operation cannot be scheduled at this time due to resource or other constraints within the Windows Sockets implementation.

See Also

gethostbyaddr(), WSACancelAsyncRequest()

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