15.3.5.3 DOS Filename Restrictions

The older DOS FAT file systems have severe limitations on file names. These limitations no longer apply to Windows, but they do apply to DOS based systems such as DJGPP.

A file name may consist of no more than 8 characters, followed by an optional extension of no more than 3 characters. This is commonly referred to as an 8.3 file name. Filenames are case insensitive.

There are a couple of filenames which are treated specially. You can not name a file `aux' or `prn'. In some cases, you can not even use an extension, such as `aux.c'. These restrictions apply to DOS and also to at least some versions of Windows.