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.
Using a Callback Function to Process Driver Messages
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.
Using an Event Callback to Process Driver Messages
To use an
event callback, use the CreateEvent function to retrieve the handle of an event and
specify CALLBACK_EVENT in the call to the midiOutOpen function.
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.
Using a Window or Thread Callback to Process Driver Messages
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.
Messages sent
to a window or thread callback are specific to the MIDI device used. For more
information about these messages, see Sending System-Exclusive Messages and Managing MIDI Recording.