Philosophy of operation

You have a working image space, on which basic operations are done in memory. For example, commands like the soper, ioper, crop, wavelet commands act on that working space and not on image files.

From the programmer's point of view, this is represented by the fits structure pointed to by com.g. com is an all-purpose structure, instantiated only once and that is basically intended to provide easy access to a set of global variables that may be needed by the different modules of the software; and the working image space is one of those variables.