lineInitializeExA function (tapi.h)

The lineInitializeEx function initializes the application's use of TAPI for subsequent use of the line abstraction. It registers the application's specified notification mechanism and returns the number of line devices available to the application. A line device is any device that provides an implementation for the line-prefixed functions in the Telephony API.


LONG lineInitializeExA(
  LPHLINEAPP               lphLineApp,
  HINSTANCE                hInstance,
  LINECALLBACK             lpfnCallback,
  LPCSTR                   lpszFriendlyAppName,
  LPDWORD                  lpdwNumDevs,
  LPDWORD                  lpdwAPIVersion,



Pointer to a location that is filled with the application's usage handle for TAPI.


Instance handle of the client application or DLL. The application or DLL can pass NULL for this parameter, in which case TAPI uses the module handle of the root executable of the process (for purposes of identifying call handoff targets and media mode priorities).


Address of a callback function that is invoked to determine status and events on the line device, addresses, or calls, when the application is using the "hidden window" method of event notification (for more information see lineCallbackFunc). This parameter is ignored and should be set to NULL when the application chooses to use the "event handle" or "completion port" event notification mechanisms.


Pointer to a null-terminated text string that contains only displayable characters. If this parameter is not NULL, it contains an application-supplied name for the application. This name is provided in the LINECALLINFO structure to indicate, in a user-friendly way, which application originated, or originally accepted or answered the call. This information can be useful for call-logging purposes. If lpszFriendlyAppName is NULL, the application's module file name is used instead (as returned by the function GetModuleFileName).


Pointer to a DWORD-sized location. Upon successful completion of this request, this location is filled with the number of line devices available to the application.


Pointer to a DWORD-sized location. The application must initialize this DWORD, before calling this function, to the highest API version it is designed to support (for example, the same value it would pass into dwAPIHighVersion parameter of lineNegotiateAPIVersion). Artificially high values must not be used; the value must be accurately set. TAPI translates any newer messages or structures into values or formats supported by the application's version. Upon successful completion of this request, this location is filled with the highest API version supported by TAPI, thereby allowing the application to detect and adapt to having been installed on a system with a different version of TAPI.


Pointer to a structure of type LINEINITIALIZEEXPARAMS containing additional parameters used to establish the association between the application and TAPI (specifically, the application's selected event notification mechanism and associated parameters).

Return value

Returns zero if the request succeeds or a negative error number if an error occurs. Possible return values are:



Applications must select one of three mechanisms by which TAPI notifies the application of telephony events: Hidden Window, Event Handle, or Completion Port.

The Hidden Window mechanism is selected by specifying LINEINITIALIZEEXOPTION_USEHIDDENWINDOW in the dwOptions member in the LINEINITIALIZEEXPARAMS structure. In this mechanism (which is the only mechanism available to TAPI version 1.x applications), TAPI creates a window in the context of the application during the lineInitializeEx or lineInitialize (for TAPI version 1.3 and 1.4 applications) function, and subclasses the window so that all messages posted to it are handled by a WNDPROC in TAPI itself. When TAPI has a message to deliver to the application, TAPI posts a message to the hidden window. When the message is received (which can happen only when the application calls the Windows GetMessage function), Windows switches the process context to that of the application and invokes the WNDPROC in TAPI. TAPI then delivers the message to the application by calling the lineCallbackProc, a pointer to which the application provided as a parameter in its call to lineInitializeEx (or lineInitialize). This mechanism requires the application to have a message queue (which is not desirable for service processes) and to service that queue regularly to avoid delaying processing of telephony events. The hidden window is destroyed by TAPI during the lineShutdown function.

The Event Handle mechanism is selected by specifying LINEINITIALIZEEXOPTION_USEEVENT in the dwOptions member in the LINEINITIALIZEEXPARAMS structure. In this mechanism, TAPI creates an event object on behalf of the application, and returns a handle to the object in the hEvent member in LINEINITIALIZEEXPARAMS. The application must not manipulate this event in any manner (for example, must not call SetEvent, ResetEvent, CloseHandle, and so on) or undefined behavior results; the application can only wait on this event using functions such as WaitForSingleObject or MsgWaitForMultipleObjects. TAPI signals this event whenever a telephony event notification is pending for the application; the application must call lineGetMessage to fetch the contents of the message. The event is reset by TAPI when no events are pending. The event handle is closed and the event object destroyed by TAPI during the lineShutdown function. The application is not required to wait on the event handle that is created; the application could choose instead to call lineGetMessage and have it block waiting for a message to be queued.

The Completion Port mechanism is selected by specifying LINEINITIALIZEEXOPTION_USECOMPLETION PORT in the dwOptions member in the LINEINITIALIZEEXPARAMS structure. In this mechanism, whenever a telephony event needs to be sent to the application, TAPI sends it using PostQueuedCompletionStatus to the completion port that the application specified in the hCompletionPort member in LINEINITIALIZEEXPARAMS, tagged with the completion key that the application specified in the dwCompletionKey member in LINEINITIALIZEEXPARAMS. The application must have previously created the completion port using CreateIoCompletionPort. The application retrieves events using GetQueuedCompletionStatus. Upon return from GetQueuedCompletionStatus, the application has the specified dwCompletionKey written to the DWORD pointed to by the lpCompletionKey parameter, and a pointer to a LINEMESSAGE structure returned to the location pointed to by lpOverlapped. After the application has processed the event, it is the application's responsibility to call LocalFree to release the memory used to contain the LINEMESSAGE structure. Because the application created the completion port (thereby allowing it to be shared for other purposes), the application must close it; the application must not close the completion port until after calling lineShutdown.

When a multithreaded application is using the Event Handle mechanism and more than one thread is waiting on the handle, or the Completion Port notification mechanism and more than one thread is waiting on the port, it is possible for telephony events to be processed out of sequence. This is not due to the sequence of delivery of events from TAPI, but would be caused by the time slicing of threads or the execution of threads on separate processors.

If LINEERR_REINIT is returned and TAPI reinitialization has been requested, for example as a result of adding or removing a telephony service provider, then lineInitializeEx requests are rejected with this error until the last application shuts down its usage of the API (using lineShutdown), at which time the new configuration becomes effective and applications are once again permitted to call lineInitializeEx.

If the LINEERR_INVALPARAM error value is returned, the specified hInstance parameter is invalid.

The application can refer to individual line devices by using line device identifiers that range from zero to dwNumDevs minus one. An application should not assume that these line devices are capable of any particular TAPI function without first querying their device capabilities by lineGetDevCaps and lineGetAddressCaps.


The tapi.h header defines lineInitializeEx as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.


Target Platform Windows
Header tapi.h
Library Tapi32.lib
DLL Tapi32.dll

See also

Basic Telephony Services Reference




TAPI 2.2 Reference Overview