How do I get a java JNA call to a DLL to get data returned in parameters?-Collection of common programming errors

You need to declare the parameters as special types that convey the fact that they are passed by reference. So if the Delphi parameters are like this:

procedure Foo(var i: Integer; var d: Double);

void Foo(IntByReference i, DoubleByReference d);

IntByReference iref = new IntByReference();
DoubleByReference dref = new DoubleByReference();
INSTANCE.Foo(iref, dref);
int i = iref.getValue();
double d = dref.getValue():

Using ByReference Arguments

When a function accepts a pointer-to-type argument you can use one of the ByReference types to capture the returned value, or subclass your own. For example:

// Original C declaration
void allocate_buffer(char **bufp, int* lenp);

// Equivalent JNA mapping
void allocate_buffer(PointerByReference bufp, IntByReference lenp);

// Usage
PointerByReference pref = new PointerByReference();
IntByReference iref = new IntByReference();
lib.allocate_buffer(pref, iref);
Pointer p = pref.getValue();
byte[] buffer = p.getByteArray(0, iref.getValue());

Alternatively, you could use a Java array with a single element of the desired type, but the ByReference convention better conveys the intent of the code. The Pointer class provides a number of accessor methods in addition to getByteArray() which effectively function as a typecast onto the memory.

Type-safe pointers may be declared by deriving from the PointerType class.