Audio Data Blocks
The
waveInAddBuffer and
waveOutWrite functions require applications to allocate data blocks to pass to the device
drivers for recording or playback purposes. Both of these functions use the
WAVEHDR structure to describe its data block.
Before using one of these functions to pass a data block to a device driver,
you must allocate memory for the data block and the header structure that
describes the data block. The headers can be prepared and unprepared by using the
following functions.
Before you pass an audio data block to a device driver, you must prepare the
data block by passing it to either
waveInPrepareHeader or
waveOutPrepareHeader. When the device driver is finished with the data block and returns it, you
must clean up this preparation by passing the data block to either
waveInUnprepareHeader or
waveOutUnprepareHeader before any allocated memory can be freed.
Unless the waveform-audio input and output data is small enough to be
contained in a single data block, applications 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 the application can
free the memory associated with the data block and header structure. There are
several ways to determine when a device driver is finished with a data block:
- By specifying a callback function to receive a message sent by the driver when
it is finished with a data block
- By using an event callback
- By specifying a window or thread to receive a message sent by the driver when
it is finished with a data block
- By polling the WHDR_DONE bit in the dwFlags member of the WAVEHDR structure sent with each data block
If an application does not get a data block to the device driver when needed,
there can be an audible gap in playback or a loss of incoming recorded
information. This requires at least a double-buffering scheme
staying at least one data block ahead of the device driver.
The following sections describe ways to determine when a device driver is
finished with a data block:
