Converting String to Pointer for JNA

Question

I'm trying to use JNA to query the effective permissions for a file in Windows. Eventually, I plan on using the GetEffectiveRightsFromAcl function, but to do so, I need to provide a pointer to a populated TRUSTEE structure. The JNA Platform (platform.jar) doesn't appear define this struct, so I'm trying to define it myself instead. Here's what I have so far:

public static class TRUSTEE extends Structure {
    public TRUSTEE() {
        super();
    }
    public TRUSTEE(Pointer p) {
        super(p);
        read();
    }

    public Pointer pMultipleTrustee;
    public int MultipleTrusteeOperation;
    public int TrusteeForm;
    public int TrusteeType;
    public Pointer ptstrName;
}

I'm trying to populate the structure like this:

private TRUSTEE createTrusteeForCurrentUser() {
    TRUSTEE result = new TRUSTEE();
    result.TrusteeForm = TRUSTEE_FORM.TRUSTEE_IS_NAME;
    result.TrusteeType = TRUSTEE_TYPE.TRUSTEE_IS_USER;

    String strName = "CURRENT_USER";
    // How can I set result.ptstrName using strName?
}

This Google Groups thread recommends using String fields in structures when a char * is called for. However, I don't think this is appropriate in my situation, considering the ptstrName field is allowed to point to different types of things, depending on the value of TrusteeForm. So, I think I somehow need to convert from String to Pointer instead. I found the NativeString class in JNA, which would work, except it's a package-private class.

What's the recommended way to convert a Java String to a native format and obtain a Pointer to it? Am I even using the right data type for the TRUSTEE struct? I'm somewhat new to JNA, so please excuse me if I'm missing something obvious.

Update

I found a solution to my problem, but if anyone has a better solution I'd still like to hear it.

Answer 1

Assuming you want char * on the native side (you may need more memory allocated if the string contains non-ascii characters),

String myString = "CURRENT_USER";
Pointer m = new Memory(myString.length() + 1); // WARNING: assumes ascii-only string
m.setString(0, myString); 

You can then use m wherever you need to reference the "native" string.

For wide strings (wchar_t *),

String myString = "CURRENT_USER";
Pointer m = new Memory(Native.WCHAR_SIZE * (myString.length() + 1));
m.setWideString(0, myString);

Answer 2

I solved the problem by copying the source code for package-private NativeString class and creating a public copy in my project. I had to make one minor alteration due to the use of a package-private method in the constructor.

Update: As @fragorl notes in the comments, the implementation of NativeString shown below is by now quite out-of-date.

---

Usage:

private static TRUSTEE createTrusteeForCurrentUser() {
    TRUSTEE result = new TRUSTEE();
    result.TrusteeForm = TRUSTEE_FORM.TRUSTEE_IS_NAME;
    result.TrusteeType = TRUSTEE_TYPE.TRUSTEE_IS_USER;
    result.ptstrName = new NativeString("CURRENT_USER",true).getPointer();
    result.write();
    return result;
}

NativeString.java:

/** Provides a temporary allocation of an immutable C string 
 * (<code>const char*</code> or <code>const wchar_t*</code>) for use when 
 * converting a Java String into a native memory function argument.  
 *
 * @author  Todd Fast, [email protected]
 * @author [email protected]
 */
public class NativeString implements CharSequence, Comparable {

    private Pointer pointer;
    private boolean wide;

    /** Create a native string (NUL-terminated array of <code>char</code>).<p>
     * If the system property <code>jna.encoding</code> is set, its value will
     * be used to encode the native string.  If not set or if the encoding
     * is unavailable, the default platform encoding will be used. 
     */
    public NativeString(String string) {
        this(string, false);
    }

    /** Create a native string as a NUL-terminated array of <code>wchar_t</code>
     * (if <code>wide</code> is true) or <code>char</code>.<p>
     * If the system property <code>jna.encoding</code> is set, its value will
     * be used to encode the native <code>char</code>string.  
     * If not set or if the encoding is unavailable, the default platform 
     * encoding will be used. 
     * 
     * @param string value to write to native memory
     * @param wide whether to store the String as <code>wchar_t</code>
     */
    public NativeString(String string, boolean wide) {
        if (string == null) {
            throw new NullPointerException("String must not be null");
        }
        // Allocate the memory to hold the string.  Note, we have to
        // make this 1 element longer in order to accommodate the terminating 
        // NUL (which is generated in Pointer.setString()).
        this.wide = wide;
        if (wide) {
            int len = (string.length() + 1 ) * Native.WCHAR_SIZE;
            pointer = new Memory(len);
            pointer.setString(0, string, true);
        }
        else {
            byte[] data = Native.toByteArray(string);
            pointer = new Memory(data.length + 1);
            pointer.write(0, data, 0, data.length);
            pointer.setByte(data.length, (byte)0);
        }
    }

    public int hashCode() {
        return toString().hashCode();
    }

    public boolean equals(Object other) {

        if (other instanceof CharSequence) {
            return compareTo(other) == 0;
        }
        return false;
    }

    public String toString() {
        String s = wide ? "const wchar_t*" : "const char*";
        s += "(" + pointer.getString(0, wide) + ")";
        return s;
    }

    public Pointer getPointer() {
        return pointer;
    }

    public char charAt(int index) {
        return toString().charAt(index);
    }

    public int length() {
        return toString().length();
    }

    public CharSequence subSequence(int start, int end) {
        return CharBuffer.wrap(toString()).subSequence(start, end);
    }

    public int compareTo(Object other) {

        if (other == null)
            return 1;

        return toString().compareTo(other.toString());
    }
}