Its a little more complicated than that. The problem arises when linking with 1) a library that links with Unix and 2) a file that depends on Unix. I've found a minimal example that fails on my system (OS X). It involves 4 files: Makefile, mintime.ml, unixprog.ml, minprog.ml (names chosen arbitrarily):

########################
Makefile:
########################

all: minprog

timelib.cma: mintime.ml ocamlfind ocamlc -package unix -linkpkg \ -o $@ -a mintime.ml
minprog: timelib.cma  unixprog.ml  ocamlfind ocamlc $(INCLUDES) -thread -o $@ \ -package netclient -linkpkg $^ minprog.ml


########################
mintime.ml:
#######################
let now () = Unix.time ()

########################
unixprog.ml:#######################

let foo x =  let in_channel, out_channel = Unix.open_process "cat" in  let read_fd = Unix.descr_of_in_channel in_channel in  let write_fd = Unix.descr_of_out_channel out_channel in  ()

########################
minprog.ml:
#######################

let mystring = Http_client.Convenience.http_get "http://www.cnn.com"


Then, 

make minprog
./minprog

results in

Fatal error: exception Unix.Unix_error(38, "connect", "")

Yitzhak

On Apr 2, 2008, at 10:30 PM, Eric Cooper wrote:
On Wed, Apr 02, 2008 at 10:08:55PM -0400, Yitzhak Mandelbaum wrote:
   Sure, here's one:

   let mystring = Http_client.Convenience.http_get "http://www.cnn.com"

   But, whether or not it raises the exception depends what I link it
   with, which is the essential problem.

On my (Debian) system, I can't build that program without linking in
the Unix module, since it's referenced by almost all of the ocamlnet
modules.   So I don't understand the problem you're seeing.

-- 
Eric Cooper             e c c @ c m u . e d u

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Yitzhak Mandelbaum
AT&T Labs - Research