The Windows Sockets WSAAsyncGetServByName
function gets service information corresponding to a service name and port
HANDLE WSAAsyncGetServByName (
| HWND hWnd,
| unsigned int wMsg,
| const char FAR * name,
| const char FAR * proto,
| char FAR * buf,
| int buflen
[in] The handle of the window which should receive a message when the
asynchronous request completes.
[in] The message to be received when the asynchronous request completes.
[in] A pointer to a null terminated service name.
[in] A pointer to a protocol name. This can be NULL, in which case WSAAsyncGetServByName
will search for the first service entry for which s_name
or one of the s_aliases
matches the given name
. Otherwise, WSAAsyncGetServByName
matches both name
[out] A pointer to the data area to receive the servent data. Note that this
must be larger than the size of a servent structure. This is because the data
area supplied is used by Windows Sockets to contain not only a servent structure
but any and all of the data which is referenced by members of the servent
structure. It is recommended that you supply a buffer of MAXGETHOSTSTRUCT bytes.
[in] The size of data area buf
This function is an asynchronous version of getservbyname
, and is used to retrieve service information corresponding to a service name.
Windows Sockets initiates the operation and returns to the caller immediately,
passing back an opaque "asynchronous task handle" 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 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 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 servent structure. To access the elements of this structure,
the original buffer address should be cast to a servent 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 can reissue
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 error code and buffer length should be extracted from the lParam
using the macros WSAGETASYNCERROR and WSAGETASYNCBUFLEN, defined in
#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 return value specifies whether or not the asynchronous operation was
successfully initiated. Note that it does not imply success or failure of the
If the operation was successfully initiated, WSAAsyncGetServByName
returns a nonzero value of type HANDLE which is the asynchronous task handle
for the request (not to be confused with a Windows HTASK). 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
If the asynchronous operation could not be initiated, WSAAsyncServByName
returns a zero value, and a specific error number can be retrieved by calling WSAGetLastError
The buffer supplied to this function is used by Windows Sockets to construct a
servent structure together with the contents of data areas referenced by
members of the same servent structure. To avoid the WSAENOBUFS error noted above,
the application should provide a buffer of at least MAXGETHOSTSTRUCT bytes (as
defined in WINSOCK2.H).
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.
||The network subsystem has failed.
||Insufficient buffer space is available.
||buf is not in a valid part of the process address space.
||Authoritative Answer Host not found.
||Non-Authoritative Service not found, or server failure.
||Nonrecoverable errors, the services database is not accessible.
||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.
||A successful WSAStartup must occur before using this function.
||The network subsystem has failed.
||A blocking Windows Sockets 1.1 call is in progress, or the service provider is
still processing a callback function.
||The asynchronous operation cannot be scheduled at this time due to resource or
other constraints within the Windows Sockets implementation.
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