When creating a new output file on magnetic disk with fits_create_file, if the filename is preceded by an exclamation point (!) then if that file already exists it will be deleted prior to creating the new FITS file. Otherwise if there is an existing file with the same name, CFITSIO will not overwrite the existing file and will return an error status code. Note that the exclamation point is a special UNIX character, so if it is used on the command line rather than entered at a task prompt, it must be preceded by a backslash to force the UNIX shell to pass it verbatim to the application program.
If the output disk file name ends with the suffix '.gz', then CFITSIO will compress the file using the gzip compression algorithm before writing it to disk. This can reduce the amount of disk space used by the file. Note that this feature requires that the uncompressed file be constructed in memory before it is compressed and written to disk, so it can fail if there is insufficient available memory.
One can also specify that any images written to the output file should be compressed using the newly developed `tile-compression' algorithm by appending `[compress]' to the name of the disk file (as in `myfile.fits[compress]'). Refer to the CFITSIO User's Reference Guide for more information about this new image compression format.