The default file naming rule in GNAT is that the file name must be derived from the unit name. The exact default rule is as follows:
The -gnatknn switch of the compiler activates a “krunching” circuit that limits file names to nn characters (where nn is a decimal integer). For example, using OpenVMS, where the maximum file name length is 39, the value of nn is usually set to 39, but if you want to generate a set of files that would be usable if ported to a system with some different maximum file length, then a different value can be specified. The default value of 39 for OpenVMS need not be specified.
gnatkr utility can be used to determine the krunched name for
a given file, when krunched to a specified maximum length.