JNA: pointers to arrays of pointers

I recently had a few drinks too many with my coworkers at the Russian Vodka Room and took the subway home. It’s a thirty minute ride during which I tried to implement EnumerateSecurityPackages in JNA. It was not a success and resulted in three dozen logs from JVM crashes. A good night sleep helped make it actually work.

SECURITY_STATUS SEC_Entry EnumerateSecurityPackages(
  PULONG pcPackages,
  PSecPkgInfo* ppPackageInfo
);

This function wants a pointer to an array of pointer to a SecPkgInfo structure.

typedef struct _SecPkgInfo {
  ULONG    fCapabilities;
  USHORT   wVersion;
  USHORT   wRPCID;
  ULONG    cbMaxToken;
  SEC_CHAR *Name;
  SEC_CHAR *Comment;
} SecPkgInfo, *PSecPkgInfo;

In JNA this one is simple. The Name and Comment fields need to be explicitly declared UNICODE and an inner ByReference child added so that SecPkgInfo can be returned.

public static class SecPkgInfo extends Structure {
    public static class ByReference extends SecPkgInfo implements Structure.ByReference {  }
    public NativeLong fCapabilities;
    public short wVersion;
    public short wRPCID;
    public NativeLong cbMaxToken;
    public WString Name;
    public WString Comment;
    public SecPkgInfo() {
        fCapabilities = new NativeLong(0);
        wVersion = 1;
        wRPCID = 0;
        cbMaxToken = new NativeLong(0);
    }
}

So what’s a pointer to an array of these? It’s another structure that has the first item in the array as a reference within it. The first item can also auto-magically give us the entire array of a given size.

public static class PSecPkgInfo extends Structure {
    public static class ByReference extends PSecPkgInfo implements Structure.ByReference { }
    public SecPkgInfo.ByReference pPkgInfo;
    public PSecPkgInfo() { }
    public SecPkgInfo.ByReference[] toArray(int size) {
        return (SecPkgInfo.ByReference[]) pPkgInfo.toArray(size);
    }
}

What’s a pointer to PSecPkgInfo now? It’s a PSecPkgInfo.ByReference. EnumerateSecurityPackages becomes:

public int EnumerateSecurityPackages(IntByReference pcPackages,
        PSecPkgInfo.ByReference ppPackageInfo);

Fortunately using this pointer to an array of pointers is a no-brainer.

IntByReference pcPackages = new IntByReference();
PSecPkgInfo.ByReference pPackageInfo = new PSecPkgInfo.ByReference();
Secur32.INSTANCE.EnumerateSecurityPackages(pcPackages, pPackageInfo);

Now we can do something useful with this.

public static String[] getSecurityPackages() {
    IntByReference pcPackages = new IntByReference();
    PSecPkgInfo.ByReference pPackageInfo = new PSecPkgInfo.ByReference();
    int rc = Secur32.INSTANCE.EnumerateSecurityPackages(pcPackages, pPackageInfo);
    if(W32Errors.SEC_E_OK != rc) {
        throw new LastErrorException(rc);
    }
    SecPkgInfo.ByReference[] packagesInfo = pPackageInfo.toArray(pcPackages.getValue());
    ArrayList<String> names = new ArrayList<String>(pcPackages.getValue());
    for(SecPkgInfo.ByReference packageInfo : packagesInfo) {
        names.add(packageInfo.Name.toString());
    }
    rc = Secur32.INSTANCE.FreeContextBuffer(pPackageInfo.getPointer());
    if(W32Errors.SEC_E_OK != rc) {
        throw new LastErrorException(rc);
    }
    return names.toArray(new String[0]);
}

Daniel Doubrovkine

JNA: pointers to arrays of pointers jna | java | win32

JNA: pointers to arrays of pointers