Reading String[] from JNA call fails

Question

I'm attempting to call following function via JNA:

struct group * getgrnam (const char *name)

as described in: http://www.gnu.org/software/libc/manual/html_node/Lookup-Group.html#Lookup-Group

According to the documentation the structure is following:

char *gr_name

The name of the group.

gid_t gr_gid

The group ID of the group.

char **gr_mem

A vector of pointers to the names of users in the group. Each user name is a null-terminated string, and the vector itself is terminated by a null pointer.

I created following simple test classes

import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Platform;

public class Test {

    public interface CLibrary extends Library {
            CLibrary INSTANCE = (CLibrary)  Native.loadLibrary((Platform.isWindows() ? "msvcrt" : "c"), CLibrary.class);

            void printf(String format, Object... args);

            public Group getgrnam(String groupName);
        }


        public static void main(String[] args) {
            CLibrary.INSTANCE.printf("Hello, World\n");

            Group group = CLibrary.INSTANCE.getgrnam(args[0]);

            System.out.println(group.gr_name);
            System.out.println(group.gr_mem);

        }
    }

and the Group class that represents the structure:

import java.util.ArrayList;
import java.util.List;

import com.sun.jna.Structure;

public class Group extends Structure {
    public String gr_name;
    public int gr_gid;
    public String[] gr_mem = new String[128];

    @Override
    protected List<String> getFieldOrder() {
        List<String> fields = new ArrayList<>();
        fields.add("gr_name");
        fields.add("gr_gid");
        fields.add("gr_mem");
        return fields;
    }
}

The marshalling documentation of JNA states that char** is converted to String[].

However when I run this code I get following error:

/tmp # java -cp .:jna-4.5.1.jar Test root Hello, World Exception in thread "main" java.lang.IllegalArgumentException: Reading array of class java.lang.String from memory not supported at com.sun.jna.Pointer.readArray(Pointer.java:538) at com.sun.jna.Pointer.getValue(Pointer.java:459) at com.sun.jna.Structure.readField(Structure.java:720) at com.sun.jna.Structure.read(Structure.java:580) at com.sun.jna.Structure.autoRead(Structure.java:2074) at com.sun.jna.Structure.conditionalAutoRead(Structure.java:550) at com.sun.jna.Function.invoke(Function.java:446) at com.sun.jna.Function.invoke(Function.java:354) at com.sun.jna.Library$Handler.invoke(Library.java:244) at com.sun.proxy.$Proxy0.getgrnam(Unknown Source) at Test.main(Test.java:23)

How can the structure field of char** be properly converted?

Answer 1

Self answering since I figured it out after some digging.

Major breakthrough was this tool: https://github.com/nativelibs4java/JNAerator

It generates Java classes from C structures which helps a lot!

In the end the Java class that correctly mapped the group structure was like this:

import com.sun.jna.Pointer;
import com.sun.jna.Structure;
import com.sun.jna.ptr.PointerByReference;
import java.util.Arrays;
import java.util.List;
/**
 * <i>native declaration : line 2</i><br>
 * This file was autogenerated by <a href="http://jnaerator.googlecode.com/">JNAerator</a>,<br>
 * a tool written by <a href="http://ochafik.com/">Olivier Chafik</a> that <a href="http://code.google.com/p/jnaerator/wiki/CreditsAndLicense">uses a few opensource projects.</a>.<br>
 * For help, please visit <a href="http://nativelibs4java.googlecode.com/">NativeLibs4Java</a> , <a href="http://rococoa.dev.java.net/">Rococoa</a>, or <a href="http://jna.dev.java.net/">JNA</a>.
 */
public class Group extends Structure {
  /**
   * Group name.<br>
   * C type : char*
   */
  public Pointer gr_name;
  /**
   * Password.<br>
   * C type : char*
   */
  public Pointer gr_passwd;
  /**
   * Group ID.<br>
   * C type : __gid_t
   */
  public int gr_gid;
  /**
   * Member list.<br>
   * C type : char**
   */
  public PointerByReference gr_mem;
  public Group() {
    super();
  }
  protected List<String> getFieldOrder() {
    return Arrays.asList("gr_name", "gr_passwd", "gr_gid", "gr_mem");
  }
  /**
   * @param gr_name Group name.<br>
   * C type : char*<br>
   * @param gr_passwd Password.<br>
   * C type : char*<br>
   * @param gr_gid Group ID.<br>
   * C type : __gid_t<br>
   * @param gr_mem Member list.<br>
   * C type : char**
   */
  public Group(Pointer gr_name, Pointer gr_passwd, int gr_gid, PointerByReference gr_mem) {
    super();
    this.gr_name = gr_name;
    this.gr_passwd = gr_passwd;
    this.gr_gid = gr_gid;
    this.gr_mem = gr_mem;
  }
  public Group(Pointer peer) {
    super(peer);
  }
  protected ByReference newByReference() { return new ByReference(); }
  protected ByValue newByValue() { return new ByValue(); }
  protected Group newInstance() { return new Group(); }
//  public static Group[] newArray(int arrayLength) {
//    return Structure.newArray(Group.class, arrayLength);
//  }
  public static class ByReference extends Group implements Structure.ByReference {

  };
  public static class ByValue extends Group implements Structure.ByValue {

  };
}

The usage of PointerByReference for the gr_mem field (char** type in C) was the breakthrough.

Afterwards it can be read like this:

  public static void main(String[] args) {

    Group group = CLibrary.INSTANCE.getgrnam(args[0]);
    System.out.println(group.gr_name.getString(0));

    PointerByReference pbr = group.gr_mem;
    String[] groups = pbr.getPointer().getStringArray(0);

    for (String g : groups) {
      System.out.println(g);
    }