CMSIS DSP DCT Type IV or II

I'm trying to use the CMSIS DSP library to compute a DCT, however I don't understand how it works.

Namely what I don't understand is the pointer to the state buffer. What is this state buffer? It's an input parameter, what should be in that buffer? There appears to be no mention of it in the official documentation.

The official documentation also says that the DCT type II can be calculated using a real FFT. How would this be done?

I can't find an example using the CMSIS DSP library to compute a DCT. If anyone knows where to find one, that would also be a great help.

Thanks.

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