Mapping a structure inside a union in JNA

I am attempting to map the kstat library in Solaris 11.3 to Java using JNA. While I've managed to get most of the structures working, I've spent the last 24 hours fighting with a particularly difficult union-within-a-structure-within-a-union.

I am successfully retrieving a pointer to a kstat_named structure I need using kstat_data_lookup(). My code properly retrieves most of the data (name, data_type, and non-struct members of the union) in this C structure:

typedef struct kstat_named {
   char    name[KSTAT_STRLEN];    /* name of counter */
   uchar_t data_type;             /* data type */
   union {
            charc[16];            /* enough for 128-bit ints */
            struct {
               union {
                   char *ptr;    /* NULL-terminated string */
               } addr;
               uint32_t len;     /* length of string */
            } str;
            int32_t   i32;
            uint32_t  ui32;
            int64_t   i64;
            uint64_t  ui64;

  /* These structure members are obsolete */

            int32_t   l;
            uint32_t  ul;
            int64_t   ll;
            uint64_t  ull;
         } value;                /* value of counter */
} kstat_named_t;

I have mapped this in JNA as follows:

class KstatNamed extends Structure {
    public static class UNION extends Union {
        public byte[] charc = new byte[16]; // enough for 128-bit ints
        public Pointer str; // KstatNamedString
        public int i32;
        public int ui32;
        public long i64;
        public long ui64;
    }

    public byte[] name = new byte[KSTAT_STRLEN]; // name of counter
    public byte data_type; // data type
    public UNION value; // value of counter

    public KstatNamed() {
        super();
    }

    public KstatNamed(Pointer p) {
        super();
        this.useMemory(p);
        this.read();
    }

    @Override
    public void read() {
        super.read();
        switch (data_type) {
        case KSTAT_DATA_CHAR:
            value.setType(byte[].class);
            break;
        case KSTAT_DATA_STRING:
            value.setType(Pointer.class);
            break;
        case KSTAT_DATA_INT32:
        case KSTAT_DATA_UINT32:
            value.setType(int.class);
            break;
        case KSTAT_DATA_INT64:
        case KSTAT_DATA_UINT64:
            value.setType(long.class);
            break;
        default:
            break;
        }
        value.read();
    }

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList(new String[] { "name", "data_type", "value" });
    }
}

This code works correctly for int32 types (KSTAT_DATA_INT32). However, when the data type is KSTAT_DATA_STRING, which corresponds to the str structure inside the union, I am not having any success in properly retrieving the data.

I have mapped the nested structure like this:

class KstatNamedString extends Structure {
    public static class UNION extends Union {
        public Pointer ptr; // NULL-terminated string
    }

    public UNION addr;
    public int len; // length of string

    public KstatNamedString() {
        super();
    }

    public KstatNamedString(Pointer p) {
        super();
        this.useMemory(p);
        this.read();
    }

    @Override
    public void read() {
        super.read();
        addr.setType(Pointer.class);
        addr.read();
    }

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList(new String[] { "addr", "len" });
    }
}

Ultimately I'm trying to replicate the behavior of this C macro:

#define KSTAT_NAMED_STR_PTR(knptr) ((knptr)->value.str.addr.ptr)

I've tried multiple different methods of trying to get access to the above structure, but it never seems to read the correct data (the len value is in the millions and attempting to read the string ptr causes segfault). I've tried:

Pointer p = LibKstat.INSTANCE.kstat_data_lookup(ksp, name);
KstatNamed data = new KstatNamed(p);
KstatNamedString str = new KstatNamedString(data.value.str);
return str.addr.ptr.getString(0); // <--- Segfault on C side

I've also tried:

• Specifying KstatNamedString as the type instead of the Pointer type
• Using various combinations of ByReference in both the structures and the unions

I've googled everywhere, including trying what I thought was a promising result here, but nothing seems to work.

I'm sure I'm missing something simple.

Use KstatNamedString instead of Pointer type.

Change your pointer-based constructors like this:

public KstatNamed(Pointer p) {
    super(p);
    this.read();
}

public KstatNamedString(Pointer p) {
    super(p);
    this.read();
}

and change the addr field of the str struct field to be a simple Pointer (no need for the union bits around it).

public Pointer /*UNION*/ addr;

Run your JVM with -Djna.dump_memory=true and print your newly-initialized Structure as a string. That will show you how JNA interprets the memory layout of the struct, and how the native memory is initialized. That should help you determine how to extract the string you're looking for (assuming it's there).

You can also tune your union read() method to initially read only the type field (using Structure.readField("data_type")) before setting the union type.