Installing ocaml API for Z3 using opam

Question

I want to use Z3 in my OCaml program. Using opam, I did

$ opam install z3
$ eval $(opam env)

then tried compiling with

$ ocamlfind ocamlopt -o main -package z3 -linkpkg main.ml

What I get is a huge dump of thousands of In function foo undefined reference to bar, starting with

/home/andrepd/.opam/4.06.1+flambda/lib/z3/libz3-static.a(api_datatype.o): In function `mk_datatype_decl':
api_datatype.cpp:(.text+0x4bf): undefined reference to `__cxa_allocate_exception'
api_datatype.cpp:(.text+0x522): undefined reference to `__cxa_throw'
api_datatype.cpp:(.text+0x57b): undefined reference to `__cxa_free_exception'
api_datatype.cpp:(.text+0x58f): undefined reference to `__cxa_allocate_exception'
api_datatype.cpp:(.text+0x5f9): undefined reference to `__cxa_throw'
api_datatype.cpp:(.text+0x61f): undefined reference to `__cxa_allocate_exception'
api_datatype.cpp:(.text+0x68b): undefined reference to `__cxa_throw'
...

ending with

binary_heap_upair_queue.cpp:(.text._ZN2lp23binary_heap_upair_queueIjE7dequeueERjS2_[_ZN2lp23binary_heap_upair_queueIjE7dequeueERjS2_]+0x1ab): undefined reference to `__cxa_allocate_exception'
binary_heap_upair_queue.cpp:(.text._ZN2lp23binary_heap_upair_queueIjE7dequeueERjS2_[_ZN2lp23binary_heap_upair_queueIjE7dequeueERjS2_]+0x205): undefined reference to `__cxa_throw'
binary_heap_upair_queue.cpp:(.text._ZN2lp23binary_heap_upair_queueIjE7dequeueERjS2_[_ZN2lp23binary_heap_upair_queueIjE7dequeueERjS2_]+0x226): undefined reference to `__cxa_free_exception'
/home/andrepd/.opam/4.06.1+flambda/lib/z3/libz3-static.a(binary_heap_upair_queue.o): In function `_GLOBAL__sub_I_binary_heap_upair_queue.cpp':
binary_heap_upair_queue.cpp:(.text.startup+0xc): undefined reference to `std::ios_base::Init::Init()'
binary_heap_upair_queue.cpp:(.text.startup+0x13): undefined reference to `std::ios_base::Init::~Init()'
collect2: error: ld returned 1 exit status
File "caml_startup", line 1:
Error: Error during linking
Command exited with code 2.

What am I doing wrong? For the record, I was using ocamlbuild, with

$ ocamlbuild -use-ocamlfind -cflag '-linkpkg' main.native

and true: package(z3) in the _tags file. But calling plain ocamlfind like above gives the same output.

Versions: compiler: 4.06.1 w/ flambda, opam: 2.0.0, z3: 4.8.4.

Answer 1

TL;DR; The package is broken. The fix and a couple of workarounds are below, but in general, such questions should be posted to corresponding issue trackers. So consider opening an issue report or pull request with a fix.

Those linker errors indicate that the symbols from the C++ standard library are missing. Since OCaml is using the C linker to link the final product it is not passing the C++ standard library by default. Of course, a properly made package should do this for us¹, but we can still do it manually via the -cclib -lstdc++ (if you have libstdc++, otherwise use -lc++).

ocamlfind ocamlopt -linkpkg -cclib -lstdc++ -package z3 example.ml -o exe

With ocamlbuild you can use the cclib(x) parametrized tag, e.g.,

 <example.ml>: cclib(-lstdc++)

A note for the dune users

TL;DR; If you're using dune then you still have to add (flags (-cclib -lstdc++)) to your library/executable stanza, since dune is ignoring linkopts (and many other fields of the META files).

Long story. The META file specification is defined and implemented by the findlib library. The dune building system is not using findlib, instead they have re-implemented a small subset of findlib with many features missing, namely fields like linkopts and predicates. That's why you still need to add this field, despite the fact, that META prescribes it. At least as of October 2019.

---

¹ the provided META file contains a bogus

linkopts = "-cclib -L/usr/lib"

which (a) doesn't make sense since -L is not a linker option, but the compiler one, and (b) is useless, as /usr/lib is usually in the search path anyway.

The correct option should be:

linkopts = "-cclib -lstdc++"

You might edit the file directly, it is located at $(ocamlfind query z3)/META.

Please, consider submitting a fix either to OPAM package or (ideally) to z3.