Dune build error on macOS

Ecosystem
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1

May I ask the Dune experts for assistance in tackling a build error on macOS. After a large commit, my project stopped building with the following error:

File "appkit/dune", line 1, characters 0-82:
1 | (library
2 |  (name appkit)
3 |  (public_name camlkit-gui.appkit)
4 |  (libraries foundation))
ar: @/var/folders/8d/j95p9rj549l7f42l5m7qxp500000gn/T/build_415557_dune/camlresp05230a: No such file or directory
File "_none_", line 1:
Error: Error while creating the library appkit/appkit.a

Same on two different Macs: Intel and Arm. But it builds on Linux (with the GNUstep library as a dependency). The build artifact is quite large (48M), so maybe I’m hitting a resource limit on macOS.
Here’s a link to the project: GitHub - dboris/camlkit: OCaml bindings to macOS Cocoa frameworks

2

Heads up to the Dune developers: after trimming down the bindings the project builds again with the library archive size ~25M. Looks like there is a Dune limit I have hit that manifests on macOS.

3

Note that if you are generating stuff it may not be dune but a compiler limit, as it accepts only reasonably generated sources. You should try to dig out the build logs and/or tell us if you are generating unwiedely stuff.

4

You are right. I found in the log a very long line calling ocamlopt.opt. It is 72K when saved as a file. Possibly the compiler has a limit which is different on macOS and Linux.

5

This is a bit cryptic and I’m a bit wild guessing but it looks like the compiler is calling ar with a response file that it failed to generate.

6

I’ve shared the line from the dune log that probably breaks ocamlopt here.

7

You can try to cut an paste that build invocation and add -verbose after ocamlopt.opt it shows the subtools it invokes.

(But probabely there’s too much stuff to ar here).

8

At least we got a ballpark estimate. Up to about 1000 modules compile fine. Above 1K, there is a problem, at least on macOS.

9

I stumbled on this issue by chance which seems related to your issue. I didn’t have the time to look at the resolution but you might dig something out of it.

1 Like
10

Thanks, that seems to be the issue. Pinging @nojb

11

Thanks for the ping. I confirm, that’s the issue. Unfortunately, the fix is only available in OCaml >= 5.1 if memory serves.

Cheers,
Nicolas

12

I was hitting it too with ocaml-terraform bindings (due to large amount of generated code) and the workaround is too use llvm-ar which supports response files.

1 Like
13

@andreypopp How did you set this up in dune?

14

just put a symlink to llvm-ar somewhere on PATH and have it named ar, for example

ln -s /opt/homebrew/Cellar/llvm/17.0.6_1/bin/llvm-ar $OPAM_SWITCH_PREFIX/bin/ar

so it’s not really in dune.

15

Please don’t show these things :–)

We already have enough bug reports in opam with people showing up with weird setups that have a mixture of C compilers installed in their environment which breaks everything.

16

I can confirm that this workaround works. It might be okey during development, but it’s not a sustainable solution if you plan to release your library. I ended up splitting my large library into two small ones instead.