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This capability is very tricky so be carefull before using it. See the Using simultaneous audio recording and playback (full duplex) section for more info.
However lack of it doesn't mean that the device cannot do full duplex. It may also mean that the application needs to use the two device approach.
This capability will not be reported if the device has been opened with the O_RDONLY or O_WRONLY flags. This means that applications using the single device full duplex method should not check for it (otherwise they will always fail to work.
Audio device capabilities like PCM_CAP_DUPLEX can be checked using the SNDCTL_AUDIOINFO or SNDCTL_DSP_GETCAPS ioctl calls.