On 15 May 2017 14:35 +1000, from dave at horsfall.org (Dave Horsfall):
> (Sigh; "cdecl" really needs to print a "> " prompt or something.)

It does print "cdecl> " as a prompt on my system.

   $ cdecl
   Type `help' or `?' for help
   cdecl> explain int*()* f
   syntax error
   cdecl> declare f as ptr to function returning ptr to int
   int *(*f)()
   cdecl> ^D
   $ 

However, running it with -q turns this off:

   $ cdecl -q
   declare f as ptr to function returning ptr to int
   int *(*f)()
   ^D
   $ 

The man page for mine says 2.5, Jan 1996, and the Debian package
version says 2.5-11. 1996 in 2017 is old even for Debian.

Are you sure that you don't have a stray alias somewhere, or something
that might be causing it to misdetect as not running in a terminal?

-- 
Michael Kjörling • https://michael.kjorling.se • michael at kjorling.se
                 “People who think they know everything really annoy
                 those of us who know we don’t.” (Bjarne Stroustrup)