Open Sound System
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Some audio (playback) devices have multiple independet engines so it's possible that several applications use them at the same time. In practice OSS will handle this by redirecting the application to the next identical device/engine if the original one was busy. The PCM_CAP_SHADOW capability is related with this one.
The PCM_CAP_MULTI capability tells if the device can do this. However there may not be much use for it (it's mainly used internally by OSS).
Audio device capabilities like PCM_CAP_MULTI can be checked using the SNDCTL_AUDIOINFO or SNDCTL_DSP_GETCAPS ioctl calls.