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I am guessing that you should allocate the memory for output_p and downsampled_XYZ when you enter the callback.

Also, one more advice, after

pcl::toROSMsg (*output_p, *output);

You should set the frame_id of output to the same value as input

output.header.frame_id = input->header.frame_id;

otherwise if you try to visualize the point cloud with rviz, it will complain because it would not know the frame of reference for output.