3

I (re-)installed Ocaml on OS X using the following steps:

> brew uninstall ocaml
> brew uninstall opam
> brew install ocaml
> brew install opam
> opam init
> eval `opam config env`
> opam switch 4.02.1
> opam install batteries core

I then tried to compile this program:

open Unix
open Printf

let main () =
    match fork () with
    | 0 -> printf "child\n"
    | pid -> printf "parent\n"

let _ = main ()

I compiled using this command:

ocamlc -o fork fork.ml

But I get an error:

File "fork.ml", line 1:
Error: Error while linking fork.cmo:
Reference to undefined global `Unix'

In fact, I was getting this error before reinstalling; that is why I reinstalled in the first place. But the problem persists and I am not sure how to fix it.

dalastboss
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2 Answers2

10

unix library is not linked by default, so you need to pass some linking flags, to make it work, e.g.,

 ocamlc unix.cma fork.ml -o fork

If you don't want to know anything about cma, you can use ocamlbuild, instead of ocamlc:

 ocamlbuild -lib unix fork.native

Or even more general

 ocamlbuild -pkg unix fork.native

The latter (with pkg option) would be a preferred way, since it will allow you to specify any package installed with opam. E.g., if you would ever try to use lwt, the you can just link with it with 

 ocamlbuild -pkg lwt fork.native
ivg
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6

For those getting this error while using the OCaml interactive toplevel, the command line syntax is similar to that required by ocamlc:

ocaml unix.cma
Throw Away Account
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