Building dune static using `musl` triggers compilation error

[maiste]

While building the ocaml-dune/binary-distribution, I have encountered a regression when building the static version.

Expected Behavior

It builds without error.

Actual Behavior

> otherlibs/stdune/src/copyfile_stubs.c:91:43: error: unknown type name 'loff_t'; did you mean 'off_t'?
> otherlibs/stdune/src/copyfile_stubs.c:91:57: error: unknown type name 'loff_t'; did you mean 'off_t'?
> otherlibs/stdune/src/copyfile_stubs.c:93:8: error: unknown type name 'copy_file_range_t'
> otherlibs/stdune/src/copyfile_stubs.c:93:47: error: initialization of 'int' from 'void *' makes integer from pointer without a cast [-Wint-conversion]
> otherlibs/stdune/src/copyfile_stubs.c: In function 'dune_copy_file_range':
> otherlibs/stdune/src/copyfile_stubs.c:98:11: error: called object 'copy_file_range_fn' is not a function or function pointer
> otherlibs/stdune/src/copyfile_stubs.c:93:26: note: declared here
> otherlibs/stdune/src/copyfile_stubs.c: In function 'stdune_sendfile':
> otherlibs/stdune/src/copyfile_stubs.c:124:32: error: 'copy_file_range_t' undeclared (first use in this function); did you mean 'copy_file_range_fn'?
> otherlibs/stdune/src/copyfile_stubs.c:124:32: note: each undeclared identifier is reported only once for each function it appears in
> otherlibs/stdune/src/copyfile_stubs.c:124:50: error: expected ')' before 'dlsym'

It can be reduced to this error:

> otherlibs/stdune/src/copyfile_stubs.c:91:43: error: unknown type name 'loff_t'; did you mean 'off_t'?

Reproduction

An example of the error can be found in https://github.com/ocaml-dune/binary-distribution/actions/runs/17226133735/job/48870893140

To reproduce, just run nix build .#dune-static (or nix build .#dune-static-experimental but they have the same root).

Specifications

• Version of dune (output of dune --version): main with commit a88ec65
• Version of ocaml (output of ocamlc --version): 5.3.0
• Operating system (distribution and version): Arch Linux

Problem

The regression can be traced to #12074. Before this PR it works, after it generates the errors above.

From what I found about it, it seems to be related to the fact that musl defines loff_t type as a cpp macro instead of a typedef. Two solutions I have found are:

• Declaring manually loff_t if not on GNU_SOURCE,
• Use off64_t instead (see martijnvanbrummelen/nwipe@d10dca3).

Not having touch C for a while, I'd like to have input from more competent C people 😄

cc @nojb @rgrinberg

[nojb]

Yes, seems to be an portability issue with loff_t which may be specific to the GNU libc. Using off64_t may have the same problem (not portable). We can use any portable signed type which is guaranteed to be 64-bit wide actually I think (eg int64_t).

Am away from my computer at the moment, but I can review the change if someone prepares a patch.

[nojb]

• Use off64_t instead

Actually, since this part of the code is Linux-only, using off64_t should be fine. Let's do that.

[nojb]

(I'll prepare the PR; I was able to access my computer.)

[maiste]

Thanks a lot 🙏🏻

[Alizter]

We should add a CI job for musl so we can catch these in the future.

[maiste]

We could derive the nix one to use the .#dune-static target instead. Also, as we already have the binary distribution building it and testing it, it might be enough to detect the problem without "overloading" the test suite.