[racket] Detecting named pipe

Hi,

I am trying to obtain the md5sum of a bunch of files, so for all files 
with path 'p' I do : (call-with-input-file p md5)

The problem is that this hangs when I find a named pipe file. 
file-exists? returns #t and then the process hangs. It seems I can't run 
md5 on named pipes, so how can I check if a file is a named pipe?

Cheers,
-- 
PMatos