Open Sound System
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This capability tells that the device supports the Starts audio recording and/or playback in sync ioctl. Applications that depend on this ioctl (for example if they use mmap) shoud check that this capability is present. OTOH all devices will support it so maybe this requirement can be relaxed.
Audio device capabilities like PCM_CAP_TRIGGER can be checked using the SNDCTL_AUDIOINFO or SNDCTL_DSP_GETCAPS ioctl calls.