WSAAsyncGetHostByName macro (wsipv6ok.h)

Article
08/19/2022

The WSAAsyncGetHostByName function asynchronously retrieves host information that corresponds to a host name.

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

Syntax

void WSAAsyncGetHostByName(
  [in]   a,
  [in]   b,
  [in]   c,
  [out]  d,
  [in]   e
);

Parameters

[in] a

Handle of the window that will receive a message when the asynchronous request completes.

[in] b

Message to be received when the asynchronous request completes.

[in] c

Pointer to the null-terminated name of the host.

[out] d

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 specified 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.

[in] e

Size of data area for the buf parameter, in bytes.

Return value

None

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 which 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 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 WSAAsyncGetHostByName 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 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:

#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.

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

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	wsipv6ok.h (include Winsock2.h, Winsock.h)
Library	Ws2_32.lib
DLL	Ws2_32.dll