Some small feedback regading jextract as a user

[Jorn Vernee]

Hi,

On 12/11/2020 21:47, Jerven Tjalling Bolleman wrote:
> Dear panama-devs,
>
> I hope this feedback is in the useful category. My feedback is 
> regarding the jextract tool and ffi. Most of it is trivial but I hope 
> it is still useful.
>
> No --version or -v option to quickly get the exact version of jextract 
> (would make future feedback easier to pinpoint).
>
> Assume the jextract user has minimal or no knowledge (like me) of how 
> the library they want to wrap was built. For example the first error I 
> ran into was “fatal error: 'cstdarg' file not found” which required 
> “-C -x -C c++” to be added to the command. This was not intuitive and 
> certainly not the first thing I tried (I started with trying different 
> -I combinations. Specifically, I am trying to wrap a rust library with 
> a header generated by rust cbindgen. So thinking of needing to add 
> clang options to jextract was not high on my lists of things to do.
The problem here seems that cbindgen produces a C++ header file, not a C 
file, and then exports a bunch of functions with C linkage (i.e. extern 
"C"). Note that jextract currently only handles C well, so parsing C++ 
will likely fail (e.g. with an NPE).

It looks like cbindgen supports generating C headers as well [1]. And, 
after some changes to your build.rs [2], jextract works with the 
following command:

jextract -l rs_handlegraph_ffi -t rs.handlegraph -d out 
libhandlegraph-ffi-c.h

(Note that -l takes a library name, not a path). Producing the following 
files:

out
└───rs
     └───handlegraph
             C.class
             libhandlegraph_ffi_c_h$atexit$x0.class
             libhandlegraph_ffi_c_h$at_quick_exit$x0.class
libhandlegraph_ffi_c_h$bsearch$_CompareFunction.class
libhandlegraph_ffi_c_h$bsearch_s$_CompareFunction.class
             libhandlegraph_ffi_c_h$constants$0.class
             libhandlegraph_ffi_c_h$div_t.class
             libhandlegraph_ffi_c_h$EdgeHolder.class
             libhandlegraph_ffi_c_h$ldiv_t.class
libhandlegraph_ffi_c_h$lfind$_CompareFunction.class
             libhandlegraph_ffi_c_h$lldiv_t.class
libhandlegraph_ffi_c_h$lsearch$_CompareFunction.class
             libhandlegraph_ffi_c_h$mbstate_t.class
             libhandlegraph_ffi_c_h$onexit$_Func.class
libhandlegraph_ffi_c_h$qsort$_CompareFunction.class
libhandlegraph_ffi_c_h$qsort_s$_CompareFunction.class
             libhandlegraph_ffi_c_h$_CRT_DOUBLE.class
             libhandlegraph_ffi_c_h$_CRT_FLOAT.class
             libhandlegraph_ffi_c_h$_div_t.class
             libhandlegraph_ffi_c_h$_LDBL12.class
             libhandlegraph_ffi_c_h$_ldiv_t.class
             libhandlegraph_ffi_c_h$_LDOUBLE.class
libhandlegraph_ffi_c_h$_lfind$_CompareFunction.class
libhandlegraph_ffi_c_h$_lfind_s$_CompareFunction.class
             libhandlegraph_ffi_c_h$_lldiv_t.class
             libhandlegraph_ffi_c_h$_LONGDOUBLE.class
libhandlegraph_ffi_c_h$_lsearch$_CompareFunction.class
libhandlegraph_ffi_c_h$_lsearch_s$_CompareFunction.class
             libhandlegraph_ffi_c_h$_Mbstatet.class
             libhandlegraph_ffi_c_h$_onexit$_Func.class
libhandlegraph_ffi_c_h$_set_invalid_parameter_handler$_Handler.class
libhandlegraph_ffi_c_h$_set_purecall_handler$_Handler.class
libhandlegraph_ffi_c_h$_set_thread_local_invalid_parameter_handler$_Handler.class
libhandlegraph_ffi_c_h$__crt_locale_data_public.class
libhandlegraph_ffi_c_h$__crt_locale_pointers.class
             libhandlegraph_ffi_c_h.class
             RuntimeHelper$VarargsInvoker.class
             RuntimeHelper.class

HTH,
Jorn

>
> Having fixed that I ran into a null pointer exception. i.e.
> WARNING: Using incubator modules: jdk.incubator.jextract, 
> jdk.incubator.foreign
> java.lang.NullPointerException
>
> While we might dislike stack traces. I think a bug report (to myself 
> regarding the C code) or to you would be easier to work on when you 
> have a stack trace.
>
> When generating the helloworld example I needed to compile with -fPIC. 
> Which the document at
> https://github.com/openjdk/panama-foreign/blob/foreign-jextract/doc/panama_jextract.md#jextract-a-jar-file-for-helloworldh 
> claims generates a jar file but actually generates a directory with 
> class files. The example worked fine with clang 10.0.1 but my fedora 
> 10.2.1 gcc needed -fPIC.
>
> Regarding the FFI interface, I would love to see more examples 
> regarding the group layout functionality.
>
> Otherwise with jextract I quite quickly ran into error: unknown type 
> name without much of a suggestion why a certain type was not known. 
> The libraries added to the command line should have had the types. And 
> I am still working on this one.
>
> That said the tools have lots of promise and please keep working on them!
>
> Regards,
> Jerven

[1] :  https://github.com/eqrion/cbindgen#quick-start
[2] :

diff --git a/build.rs b/build.rs
index 2dd7493..02b36e6 100644
--- a/build.rs
+++ b/build.rs
@@ -7,6 +7,7 @@ fn main() {

      cbindgen::Builder::new()
        .with_crate(crate_dir)
+      .with_language(cbindgen::Language::C)
        .generate()
        .expect("Unable to generate bindings")
        .write_to_file("libhandlegraph-ffi-c.h");