Support for cv-qualifiers ("const" etc.)

[yallop]

There's currently no support for const, volatile, etc., so bindings to those functions generate code with incorrect types.

[aantron]

This would be quite useful, especially if Ctypes.string then became a view of const char* rather than char*.

[kit-ty-kate]

This is an issue for 4.10 as String_val now returns const char* instead of char*. This breaks sodium for instance:

#=== ERROR while compiling sodium.0.6.0 =======================================#
# context              2.0.5 | linux/x86_64 | ocaml-variants.4.10.0+trunk | file:///home/opam/opam-repository
# path                 ~/.opam/4.10.0+trunk/.opam-switch/build/sodium.0.6.0
# command              /usr/bin/make
# exit-code            2
# env-file             ~/.opam/log/sodium-6-45397e.env
# output-file          ~/.opam/log/sodium-6-45397e.out
### output ###
# [...]
# lib/sodium_stubs.c:947:17: note: in expansion of macro 'CTYPES_PTR_OF_OCAML_STRING'
#   947 |    char* x801 = CTYPES_PTR_OF_OCAML_STRING(x800);
#       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~
# /home/opam/.opam/4.10.0+trunk/lib/ctypes/ctypes_cstubs_internals.h:20:3: error: initialization discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
#    20 |   (String_val(Field(s, 1)) + Long_val(Field(s, 0)))
#       |   ^
# lib/sodium_stubs.c:950:17: note: in expansion of macro 'CTYPES_PTR_OF_OCAML_STRING'
#   950 |    char* x806 = CTYPES_PTR_OF_OCAML_STRING(x797);
#       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~
# cc1: all warnings being treated as errors
# Command exited with code 2.
# make: *** [Makefile:13: all] Error 10

Granted they use -Werror but this generates 900 lines of warnings when compiled

[yallop]

Thanks for the report! The issue in sodium arises from the use of ocaml_bytes; it's fixed by #622.

[aantron]

There continues to be an issue with warnings on 4.10, with ocaml_string. For example:

c_generated_functions.c: In function ‘luv_stub_96_uv_pipe_connect’:
/mnt/c/Users/antron/code/luv/luv/_opam/lib/ctypes/ctypes_cstubs_internals.h:20:3: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   (String_val(Field(s, 1)) + Long_val(Field(s, 0)))
   ^
c_generated_functions.c:655:17: note: in expansion of macro ‘CTYPES_PTR_OF_OCAML_STRING’    char* x480 = CTYPES_PTR_OF_OCAML_STRING(x475);

The binding is generated from this:

    let connect =
      foreign "uv_pipe_connect"
        (ptr Types.Stream.Connect_request.t @->
         ptr t @->
         ocaml_string @->
         Stream.Connect_request.trampoline @->
          returning void)

The generated code:

value luv_stub_96_uv_pipe_connect(value x477, value x476, value x475,
                                  value x474)
{
   uv_connect_t* x478 = CTYPES_ADDR_OF_FATPTR(x477);
   uv_pipe_t* x479 = CTYPES_ADDR_OF_FATPTR(x476);
   char* x480 = CTYPES_PTR_OF_OCAML_STRING(x475);
   void(* x481)(uv_connect_t*, int) = CTYPES_ADDR_OF_FATPTR(x474);
   uv_pipe_connect(x478, x479, x480, x481);
   return Val_unit;
}

It appears that char* x480 should be const char* x480.

[yallop]

@aantron: #646 will eliminate these warnings.

[aantron]

Great, thanks!

[tjammer]

With gcc14, -Wincompatible-pointer-types is now an error instead of a warning, see https://gcc.gnu.org/gcc-14/porting_to.html#warnings-as-errors.

[yallop]

#780 makes it possible to generate const-correct code, which should help to address issues with gcc14.

[yallop]

This would be quite useful, especially if Ctypes.string then became a view of const char* rather than char*.

That's a good idea, implemented in #782.

[yallop]

const and volatile are available in ctypes 0.23.0.