Porting JNA code to Foreign Function & Memory API (FFM), fails for String[] returned from C library

Question

Porting JNA code to Foreign Function & Memory API (FFM) (Java 19, JEP 424), fails for String[] returned array from C

C API:

    int nc_get_att_string(  
        int          ncid,
        int          varid,
        const char * name,
        char **      value) 

"Note that unlike most other nc_get_att functions, nc_get_att_string() allocates a chunk of memory which is returned to the calling function. This chunk of memory must be specifically deallocated with nc_free_string() to avoid any memory leaks. Also note that you must still preallocate the memory needed for the array of pointers passed to nc_get_att_string()."

So, one passes an array of char* pointers, and the library returns an array of strings. I have this working in JNA in Java:

      String[] valss = new String[len];
      ret = nc4.nc_get_att_string(grpid, varid, attname, valss);
      if (ret != 0)
        throw new IOException(ret + ": " + nc4.nc_strerror(ret));
      return valss;

But trying to do the equivilent in the Foreign Function & Memory API in Kotlin fails:

        val strings_p : MemorySegment = session.allocateArray(ValueLayout.ADDRESS, nelems)
        checkErr("nc_get_att_string", nc_get_att_string(grpid, varid, attname, strings_p))
        val result = mutableListOf<String>()
        for (i in 0 until nelems) {
            val s2 : MemoryAddress = strings_p.getAtIndex(ValueLayout.ADDRESS, i) 
            val s = s2.getUtf8String(0)  // FAILS here
            result.add(s)
        }
        return result

fails with:

Symbol is NULL: MemoryAddress{ offset=0x0 }
java.lang.IllegalArgumentException: Symbol is NULL: MemoryAddress{ offset=0x0 }
    at java.base/jdk.internal.foreign.abi.SharedUtils.checkAddressable(SharedUtils.java:344)
    at java.base/jdk.internal.foreign.abi.SharedUtils.checkAddress(SharedUtils.java:338)
    at java.base/jdk.internal.foreign.MemoryAddressImpl.getUtf8String(MemoryAddressImpl.java:107)
    at sunya.cdm.netcdfClib.NCheader.readAttributeValues(NCheader.kt:365)

Anyone see what Im doing wrong?

Answer 1

The problem is with the installed C library, and the JNA code sees the same thing. Sorry for the misdirection.