It may read in files that are not proper FITS since only a few FITS elements needed by the reader are actually explicitly checked. This may be good or bad depending upon your needs. The main function with some utilities. Code to handle simple images and group data. Find an extension in a FITS file.
Decompression is done using a sub-process and pipes so that no temporary file is created.
In the future I anticipate implementing decompression on other architectures but the process will likely involve temporary files. See description above.