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The WM_CAP_SET_CALLBACK_CAPCONTROL message sets a callback function in the application giving it precise recording control. You can send this message explicitly or by using the capSetCallbackOnCapControl macro.

wParam = (WPARAM) 0; 
lParam = (LPARAM) (LPVOID) (fpProc); 



Pointer to the callback function, of type capControlCallback. Specify NULL for this parameter to disable a previously installed callback function.

Return Value

Returns TRUE if successful or FALSE if a streaming capture or a single-frame capture session is in progress.


A single callback function is used to give the application precise control over the moments that streaming capture begins and completes. The capture window first calls the procedure with nState set to CONTROLCALLBACK_PREROLL after all buffers have been allocated and all other capture preparations have finished. This gives the application the ability to preroll video sources, returning from the callback function at the exact moment recording is to begin. A return value of TRUE from the callback function continues capture, and a return value of FALSE aborts capture. After capture begins, this callback function will be called frequently with nState set to CONTROLCALLBACK_CAPTURING to allow the application to end capture by returning FALSE.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]



See also

Video Capture
Video Capture Messages