Glenn Morris wrote:
> I was using ftp://ftp.gnutls.org/gcrypt/gnutls/v3.2/gnutls-3.2.21.tar.xz

Weird, because I was looking at the same source code. It does not build under 
recent Ubuntu or Fedora, so I did not compile it: I just looked at the source 
code. Its implementation of gnutls_cipher_list appears to be straightforward: it 
adds only values found as the 'id' component of an entry in the 'algorithms' 
array in lib/algorithms/ciphers.c. None of these values are 
GNUTLS_CIPHER_UNKNOWN, so gnutls_cipher_list cannot possibly return an array 
containing GNUTLS_CIPHER_UNKNOWN.

For example, on my platform, if I compile and run the attached program via:

gcc gs.c -lgnutls
./a.out

the program silently succeeds. What happens on your platform?