Provide complete information about the problem

1. Version of JNA and related jars
   JNA version 5.2.0 - jna-5.2.0.jar and jna-platform-5.2.0.jar

2. Version and vendor of the java virtual machine
   java version "1.8.0_181"
   Java(TM) SE Runtime Environment (build 1.8.0_181-b13)
   Java HotSpot(TM) 64-Bit Server VM (build 25.181-b13, mixed mode)

3. Operating system
   Windows 10

4. System architecture (CPU type, bitness of the JVM)
   Xeon X3430 @ 2,40GHz and 64 bit JVM

5. Complete description of the problem
   The bug was discovered in the getUserLocalGroups method under Netapi32Util.java. When running in Line 369:

LOCALGROUP_USERS_INFO_0[] lgroups = (LOCALGROUP_USERS_INFO_0[]) lgroup.toArray(entriesread.getValue()) where entriesread.getValue() has a zero value. In the other word, it essentially sends 0 to Structure#toArray, which it then throws an ArrayIndexOutOfBoundsException exception, because the array size is zero.

6. Steps to reproduce.

I try to be as simple as possible. Only 2 lines need to be added.

1 - Add if (entriesread.getValue() > 0) before LOCALGROUP_USERS_INFO_0 lgroup = new LOCALGROUP_USERS_INFO_0(bufptr.getValue());

The "if" condition above prevents a zero sized array.

2 - Add return new Group[0]; at the end of the method. This is to safely return an empty array rather than null. See a complete listing with the solution in place below. I apologise for not following the JDK style, but you can maintain your usual style with the solution added. Thanks.

    public static Group[] getUserLocalGroups(String userName, String serverName)
    {
        PointerByReference bufptr = new PointerByReference();
        IntByReference entriesread = new IntByReference();
        IntByReference totalentries = new IntByReference();

        try
        {
            int rc = Netapi32.INSTANCE.NetUserGetLocalGroups(serverName, userName, 0, 0, bufptr, LMCons.MAX_PREFERRED_LENGTH, entriesread, totalentries);

            if (rc != LMErr.NERR_Success)
            {
                throw new Win32Exception(rc);
            }

            if (entriesread.getValue() > 0) <-- Prevents a zero sized array 
            {
                LOCALGROUP_USERS_INFO_0 lgroup = new LOCALGROUP_USERS_INFO_0(bufptr.getValue());
                LOCALGROUP_USERS_INFO_0[] lgroups = (LOCALGROUP_USERS_INFO_0[]) lgroup.toArray(entriesread.getValue());

                ArrayList<Group> result = new ArrayList<Group>();

                for (LOCALGROUP_USERS_INFO_0 lgpi : lgroups)
                {
                    LocalGroup lgp = new LocalGroup();

                    if (lgpi.lgrui0_name != null)
                    {
                        lgp.name = lgpi.lgrui0_name.toString();
                    }

                    result.add(lgp);
                }

                return result.toArray(new Group[0]);
            }
        }

        finally
        {
            if (bufptr.getValue() != Pointer.NULL)
            {
                int rc = Netapi32.INSTANCE.NetApiBufferFree(bufptr.getValue());

                if (rc != LMErr.NERR_Success)
                {
                    throw new Win32Exception(rc);
                }
            }
        }

        return new Group[0];  <-- Safely returns an empty array rather than null
    }

I realised the getUserLocalGroups method is not the only one that is affected. Below is the list of other methods, I believe also need to be corrected to eliminate the bug.

getLocalGroups(String serverName)
getGlobalGroups(String serverName)
getUsers(String serverName)
getUserGroups(String userName, String serverName)
getDomainTrusts(String serverName)

Let me know how it goes. Thanks.