Managing MIDI Data Blocks
Applications that use data blocks for passing system-exclusive messages (using the midiOutLongMsg and midiInAddBuffer functions) and stream buffers (using the midiStreamOut function) must continually supply the device driver with data blocks until playback or recording is complete.
Even if a single data block is used, an application must be able to determine when a device driver is finished with the data block so it can free the memory associated with the data block and header structure. Three methods can be used to determine when a device driver is finished with a data block:
- Specify a callback function to receive a message sent by the driver when it is finished with a data block. To get time-stamped MIDI input data, you must use a callback function.
- Use an event callback (for output only).
- Use a window or thread callback to receive a message sent by the driver when it is finished with a data block.
If an application does not get a data block to the device driver when it is needed, an audible gap in playback or a loss of incoming recorded information can occur. At a minimum, an application should use a double-buffering scheme to stay at least one data block ahead of the device driver.
You can write your own callback function to process messages sent by the device driver. To use a callback function, specify the CALLBACK_FUNCTION flag in the dwFlags parameter and the address of the callback function in the dwCallback parameter of the midiInOpen or midiOutOpen function.
Messages sent to a callback function are similar to messages sent to a window, except they have two doubleword parameters instead of an unsigned integer parameter and a doubleword parameter. For more information about these messages, see Sending System-Exclusive Messages and Managing MIDI Recording.
Use one of the following techniques to pass instance data from an application to a callback function:
- Use the dwCallbackInstance parameter of the function that opens the device driver.
- Use the dwUser member of the MIDIHDR structure that identifies a data block being sent to a MIDI device driver.
If you need more than 32 bits of instance data, pass an address of a structure containing the additional information.
An event callback is set by anything that might cause a function callback. Unlike callback functions and window or thread callbacks, event callbacks do not receive specific close, done, or open notifications. Therefore, an application may have to check the status of the process it is waiting for after the event occurs.
For more information about event callbacks, see Using an Event Callback to Manage Buffered Playback.
To use a window callback, specify the CALLBACK_WINDOW flag in the dwFlags parameter and a window handle in the low-order word of the dwCallback parameter of the midiInOpen or midiOutOpen function. Driver messages will be sent to the window procedure function for the window identified by the handle in dwCallback.
Similarly, to use a thread callback, specify the CALLBACK_THREAD flag and a thread identifier in the call to midiInOpen or midiOutOpen. In this case, messages will be posted to the specified thread instead of to a window.