DdeClientTransaction function (ddeml.h)

Begins a data transaction between a client and a server. Only a Dynamic Data Exchange (DDE) client application can call this function, and the application can use it only after establishing a conversation with the server.

Syntax

HDDEDATA DdeClientTransaction(
  [in, optional]  LPBYTE  pData,
  [in]            DWORD   cbData,
  [in]            HCONV   hConv,
  [in, optional]  HSZ     hszItem,
  [in]            UINT    wFmt,
  [in]            UINT    wType,
  [in]            DWORD   dwTimeout,
  [out, optional] LPDWORD pdwResult
);

Parameters

[in, optional] pData

Type: LPBYTE

The beginning of the data the client must pass to the server.

Optionally, an application can specify the data handle (HDDEDATA) to pass to the server and in that case the cbData parameter should be set to -1. This parameter is required only if the wType parameter is XTYP_EXECUTE or XTYP_POKE. Otherwise, this parameter should be NULL.

For the optional usage of this parameter, XTYP_POKE transactions where pData is a data handle, the handle must have been created by a previous call to the DdeCreateDataHandle function, employing the same data format specified in the wFmt parameter.

[in] cbData

Type: DWORD

The length, in bytes, of the data pointed to by the pData parameter, including the terminating NULL, if the data is a string. A value of -1 indicates that pData is a data handle that identifies the data being sent.

[in] hConv

Type: HCONV

A handle to the conversation in which the transaction is to take place.

[in, optional] hszItem

Type: HSZ

A handle to the data item for which data is being exchanged during the transaction. This handle must have been created by a previous call to the DdeCreateStringHandle function. This parameter is ignored (and should be set to 0L) if the wType parameter is XTYP_EXECUTE.

[in] wFmt

Type: UINT

The standard clipboard format in which the data item is being submitted or requested.

If the transaction specified by the wType parameter does not pass data or is XTYP_EXECUTE, this parameter should be zero.

If the transaction specified by the wType parameter references non-execute DDE data ( XTYP_POKE, XTYP_ADVSTART, XTYP_ADVSTOP, XTYP_REQUEST), the wFmt value must be either a valid predefined (CF_) DDE format or a valid registered clipboard format.

[in] wType

Type: UINT

The transaction type. This parameter can be one of the following values.

[in] dwTimeout

Type: DWORD

The maximum amount of time, in milliseconds, that the client will wait for a response from the server application in a synchronous transaction. This parameter should be TIMEOUT_ASYNC for asynchronous transactions.

[out, optional] pdwResult

Type: LPDWORD

A pointer to a variable that receives the result of the transaction. An application that does not check the result can use NULL for this value. For synchronous transactions, the low-order word of this variable contains any applicable DDE_ flags resulting from the transaction. This provides support for applications dependent on DDE_APPSTATUS bits. It is, however, recommended that applications no longer use these bits because they may not be supported in future versions of the Dynamic Data Exchange Management Library (DDEML). For asynchronous transactions, this variable is filled with a unique transaction identifier for use with the DdeAbandonTransaction function and the XTYP_XACT_COMPLETE transaction.

Return value

Type: HDDEDATA

If the function succeeds, the return value is a data handle that identifies the data for successful synchronous transactions in which the client expects data from the server. The return value is nonzero for successful asynchronous transactions and for synchronous transactions in which the client does not expect data. The return value is zero for all unsuccessful transactions.

The DdeGetLastError function can be used to get the error code, which can be one of the following values:

Remarks

When an application has finished using the data handle returned by DdeClientTransaction, the application should free the handle by calling the DdeFreeDataHandle function.

Transactions can be synchronous or asynchronous. During a synchronous transaction, DdeClientTransaction does not return until the transaction either completes successfully or fails. Synchronous transactions cause a client to enter a modal loop while waiting for various asynchronous events. Because of this, a client application can still respond to user input while waiting on a synchronous transaction, but the application cannot begin a second synchronous transaction because of the activity associated with the first. DdeClientTransaction fails if any instance of the same task has a synchronous transaction already in progress.

During an asynchronous transaction, DdeClientTransaction returns after the transaction has begun, passing a transaction identifier for reference. When the server's DDE callback function finishes processing an asynchronous transaction, the system sends an XTYP_XACT_COMPLETE transaction to the client. This transaction provides the client with the results of the asynchronous transaction that it initiated by calling DdeClientTransaction. A client application can choose to abandon an asynchronous transaction by calling the DdeAbandonTransaction function.

Requirements

See also

Conceptual

DdeAbandonTransaction

DdeAccessData

DdeConnect

DdeConnectList

DdeCreateDataHandle

DdeCreateStringHandle

DdeFreeDataHandle

Dynamic Data Exchange Management Library

Reference

XTYP_ADVSTART

XTYP_ADVSTOP

XTYP_EXECUTE

XTYP_POKE

XTYP_REQUEST