I want to use a C++ library in Kotlin Native so I created so kotlinlang #kotlin-native

I want to use a C++ library in Kotlin/Native, so I...

Adam S

05/25/2024, 9:20 AM

I want to use a C++ library in Kotlin/Native, so I created some C wrappers for it. Cinterop correctly generates bindings, and some of them work, but some of them crash instantly. For example, I've got this C++ function that creates a new Table, and wraps it in a struct:

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/*** types.h ***/

struct JPC_Table { void *obj; };
typedef struct JPC_Table JPC_Table_t;


/*** table.cpp ***/
#ifdef __cplusplus
extern "C" {
#endif

JPC_Table_t * JPC_Table_new(unsigned long layers) {
    printf("constructor JPC_Table...\n");
    JPC_Table_t * cInstance = new JPC_Table_t();
    Table * cppInstance = new Table(layers);
    cInstance->obj = cppInstance;
    return cInstance;
};

Now, there's something wrong, but what I'd really appreciate some help with is figuring how to figure it out! I don't get any error messages, there's no stacktrace, and the

printf("...")

doesn't print anything.

Adam S

05/25/2024, 9:22 AM

the KN docs say "use the -g option on the command line", but I'm not using the command line, I'm using Gradle

loke

05/25/2024, 10:07 AM

-g

means enable debugging, you get that automatically when building the debug executable.

loke

05/25/2024, 10:08 AM

Can you show your

.def

file? And the Kotlin code that calls it?

Adam S

05/25/2024, 10:14 AM

sure! It's a little messy so it might not make sense, so I'll try to show the highlights Here's the KN main function:

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import com.jrouwe.jolt.cinterop.internal.*
import com.jrouwe.jolt.cinterop.internal.JPC_EMotionType.JPC_EMotionType_Static
import kotlinx.cinterop.*

fun main(): Unit = memScoped {
  println("starting")
  createFloor()

  println("finished")
}

@OptIn(UnsafeNumber::class)
fun MemScope.createFloor(size: Int = 50) {
  println("Creating floor, size:$size")
  val joltSettings = JPC_JoltSettings_new(null)
  JPC_JoltSettings_mMaxBodies_Set(joltSettings, 1u, null)
  println("Created settings $joltSettings")
  println("JPC_JoltSettings_mMaxBodies_Get: " + JPC_JoltSettings_mMaxBodies_Get(joltSettings, null).toString())

  val errOut = allocPointerTo<ByteVar>()

  val objectFilter = try {
    JPC_ObjectLayerPairFilterTable_new(2u, errOut.ptr)
  } finally {
    val str = errOut.value?.toKString()
    println("JPC_ObjectLayerPairFilterTable_new error $str")
  }

  println("Created object filter $objectFilter")
  // ...
}

This prints

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> Task :kayray-modules:interop-jolt2:runDebugExecutableMacosArm64 FAILED
starting
Creating floor, size:50
Created settings CPointer(raw=0x600001b040b0)
JPC_JoltSettings_mMaxBodies_Get: 1


FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':kayray-modules:interop-jolt2:runDebugExecutableMacosArm64'.
> Process 'command '/Users/dev/projects/adam/kotlin/kayray/kayray-modules/interop-jolt2/build/bin/macosArm64/debugExecutable/interop-jolt2.kexe'' finished with non-zero exit value 139

So it's this line that's failing

JPC_ObjectLayerPairFilterTable_new(2u, errOut.ptr)

Adam S

05/25/2024, 10:16 AM

And here's the function implementation

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#ifdef __cplusplus
extern "C" {
#endif

//region constructors

JPC_ObjectLayerPairFilterTable_t * JPC_ObjectLayerPairFilterTable_new(
  unsigned long inNumObjectLayers,
  char** outErrMsg
) {
  try {
    printf("constructor JPC_ObjectLayerPairFilterTable...\n");
    JPC_ObjectLayerPairFilterTable_t * cInstance = new JPC_ObjectLayerPairFilterTable_t();
    ObjectLayerPairFilterTable * cppInstance = new ObjectLayerPairFilterTable(
      inNumObjectLayers
    );
    cInstance->obj = cppInstance;
    return cInstance;
  }
  catch (exception& e) {
    if (outErrMsg) {
      *outErrMsg = strdup(e.what());
      throw e;
    }
  };
};

I really don't know why the

printf(...)

isn't working... (it's also not working in other C++ functions that otherwise succeed). I tried catching and logging any exceptions, but that doesn't seem to work either.

Adam S

05/25/2024, 10:17 AM

And the C structs that wrap the C++ classes all look like this:

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struct JPC_ObjectLayerPairFilterTable {
  void *obj;
  struct JPC_ObjectLayerPairFilter *superObj;
};
typedef struct JPC_ObjectLayerPairFilterTable JPC_ObjectLayerPairFilterTable_t;

Adam S

05/25/2024, 10:20 AM

and here's the .def file

jolt.def.cpp

loke

05/25/2024, 10:43 AM

This is OSX, and I'm not too familiar with that. But I think you could see such behaviour if there is a runtime linking error, so that it's never able to call the function in the first place.

loke

05/25/2024, 10:43 AM

On Linux I'd load it up into

gdb

and see what it says.

Adam S

05/25/2024, 11:12 AM

thanks for the tip, I've found that lldb is the Mac equivalent, so I'm giving it a go...

Adam S

05/25/2024, 11:13 AM

I don't think there's anything useful though...

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(lldb) r
Process 3190 launched: '[...]/build/bin/macosArm64/debugExecutable/interop-jolt2.kexe' (arm64)
starting
Creating floor, size:50
constructor JoltC_JoltSettings...
Created settings CPointer(raw=0x600000034090)
JoltC_JoltSettings_mMaxBodies_Get: 1
Process 3190 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x00000001000033b8 interop-jolt2.kexe`kfun:dev.adamko.jolt2#createFloor__at__kotlinx.cinterop.MemScope(_this=0x0000000100218020, size=50){} at main.kt:23:16
   20  	  println("Created settings $joltSettings")
   21  	  println("JoltC_JoltSettings_mMaxBodies_Get: " + JoltC_JoltSettings_mMaxBodies_Get(joltSettings, null).toString())
   22  	
-> 23  	  val errOut = allocPointerTo<ByteVar>()
   24  	
   25  	  val objectFilter = try {
   26  	    JoltC_ObjectLayerPairFilterTable_new(2u, errOut.ptr)
Target 1: (interop-jolt2.kexe) stopped.
(lldb) thread step-over
Process 3190 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = step over
    frame #0: 0x00000001000033d0 interop-jolt2.kexe`kfun:dev.adamko.jolt2#createFloor__at__kotlinx.cinterop.MemScope(_this=0x0000000100218020, size=50){} at main.kt:26:46
   23  	  val errOut = allocPointerTo<ByteVar>()
   24  	
   25  	  val objectFilter = try {
-> 26  	    JoltC_ObjectLayerPairFilterTable_new(2u, errOut.ptr)
   27  	  } finally {
   28  	    val str = errOut.value?.toKString()
   29  	    println("JoltC_ObjectLayerPairFilterTable_new error $str")
Target 1: (interop-jolt2.kexe) stopped.
(lldb) thread step-in
Process 3190 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = step in
    frame #0: 0x000000010000d344 interop-jolt2.kexe`kfun:kotlinx.cinterop#<get-ptr>__at__0:0(_this=0x0000600000030008){0§<kotlinx.cinterop.CPointed>}kotlinx.cinterop.CPointer<0:0> at Types.kt:174:5
Target 1: (

loke

05/25/2024, 11:38 AM

What if you just let it run (in gdb the command is just

run

). It should give you a strack trace when it crashes.

Adam S

05/25/2024, 11:39 AM

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(lldb) run
Process 16693 launched: '/[...]/build/bin/macosArm64/debugExecutable/interop-jolt2.kexe' (arm64)
starting
Creating floor, size:50
constructor JoltC_JoltSettings...
Created settings CPointer(raw=0x600002be80a0)
JoltC_JoltSettings_mMaxBodies_Get: 1
constructor JoltC_ObjectLayerPairFilterTable...
Process 16693 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x0000000000000000
error: memory read failed for 0x0
Target 2: (interop-jolt2.kexe) stopped.

loke

05/25/2024, 12:05 PM

That's a null pointer dereferencing

loke

05/25/2024, 12:05 PM

Can you show the stack trace? (in gdb that would be there

where

command.

loke

05/25/2024, 12:06 PM

Apparently it's

thread backtrace

in lldb

Adam S

05/25/2024, 12:09 PM

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(lldb) run
There is a running process, kill it and restart?: [Y/n] y
Process 27133 exited with status = 9 (0x00000009) killed
Process 27150 launched: '/[...]/build/bin/macosArm64/debugExecutable/interop-jolt2.kexe' (arm64)
starting
Creating floor, size:50
constructor JoltC_JoltSettings...
Created settings CPointer(raw=0x600001dbc090)
JoltC_JoltSettings_mMaxBodies_Get: 1
constructor JoltC_ObjectLayerPairFilterTable...
Process 27150 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x0000000000000000
error: memory read failed for 0x0
Target 2: (interop-jolt2.kexe) stopped.
(lldb) thread list
Process 27150 stopped
* thread #1: tid = 0x1fce80c, 0x0000000000000000, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  thread #2: tid = 0x1fce820, 0x000000018ecd99ec libsystem_kernel.dylib`__psynch_cvwait + 8, name = 'GC Timer thread'
  thread #3: tid = 0x1fce821, 0x000000018ecd99ec libsystem_kernel.dylib`__psynch_cvwait + 8, name = 'Main GC thread'
(lldb) thread backtrace
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  * frame #0: 0x0000000000000000
    frame #1: 0x0000000100009e68 interop-jolt2.kexe`::JoltC_ObjectLayerPairFilterTable_new(unsigned long, char **) [inlined] JPH::ObjectLayerPairFilterTable::operator new(inCount=40) at ObjectLayerPairFilterTable.h:32:2
    frame #2: 0x0000000100009e54 interop-jolt2.kexe`JoltC_ObjectLayerPairFilterTable_new(inNumObjectLayers=2, outErrMsg=0x0000600001dbc0a8) at JoltC_ObjectLayerPairFilterTable.cpp:18:48
    frame #3: 0x00000001000033f4 interop-jolt2.kexe`kfun:dev.adamko.jolt2#createFloor__at__kotlinx.cinterop.MemScope(_this=0x0000000100218020, size=50){} at main.kt:26:5
    frame #4: 0x0000000100003814 interop-jolt2.kexe`kfun:dev.adamko.jolt2#createFloor$default__at__kotlinx.cinterop.MemScope(_this=0x0000000100218020, size=0, $mask0=1){} at main.kt:16:1
    frame #5: 0x0000000100002cb8 interop-jolt2.kexe`kfun:dev.adamko.jolt2#main(){} [inlined] <inlined-out:<anonymous>> at main.kt:9:3
    frame #6: 0x0000000100002c94 interop-jolt2.kexe`kfun:dev.adamko.jolt2#main(){} [inlined] kfun:kotlinx.cinterop#memScoped(kotlin.Function1<kotlinx.cinterop.MemScope,0:0>){0§<kotlin.Any?>}0:0 at Utils.kt:703:25
    frame #7: 0x0000000100002c94 interop-jolt2.kexe`kfun:dev.adamko.jolt2#main(){} at main.kt:7:20
    frame #8: 0x0000000100003898 interop-jolt2.kexe`Konan_start(args=0x00000001001d8020) at main.kt:7:1
    frame #9: 0x0000000100005148 interop-jolt2.kexe`Init_and_run_start + 108
    frame #10: 0x000000018e98e0e0 dyld`start + 2360
(lldb)

Adam S

05/25/2024, 12:17 PM

hmmmm maybe I'm not setting the correct target when compiling with clang, and it doesn't match the target that the KN compiler uses?

Adam S

05/25/2024, 12:18 PM

I can see

constructor JoltC_ObjectLayerPairFilterTable...

is printed... it's weird that the

printf()

lines are working when using lldb but not when run via Gradle

loke

05/25/2024, 12:21 PM

It sure seems to call your code at least.

Adam S

05/25/2024, 12:23 PM

oh cool, lldb has a terminal ui

Adam S

05/25/2024, 12:23 PM

so I guess it's failing because of JPH_OVERRIDE_NEW_DELETE...

Adam S

05/25/2024, 12:27 PM

hmmmmmmmmmmmmmmmm

Because operator
new
for
PhysicsSystem
is overridden, you need to do:

```> auto ptr = malloc(sizeof(JPH::PhysicsSystem));

> auto phys = ::new (ptr) JPH::PhysicsSystem();

> ```

Adam S

05/25/2024, 2:15 PM

thanks so much for your help Elias! I hadn't manually debugged something like this before, so it was really useful.

Adam S

06/09/2024, 8:32 AM

Niiiiiiiice got it working! Jolt defines has some custom C++ allocators - I had to disable them by setting

JPH_DISABLE_CUSTOM_ALLOCATOR

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