Implement methods for conversions between Long and CPointer

INTEROP.md

@@ -151,6 +151,16 @@ val intPtr: CPointer<IntVar> = bytePtr.reinterpret()
 As in C, those reinterpret casts are unsafe and could potentially lead to
 subtle memory problems in an application.
 
+Also there are unsafe casts between `CPointer<T>?` and `Long` available,
+provided by `.toLong()` and `.toCPointer<T>()` extension methods:
+```
+val longValue = ptr.toLong()
+val originalPtr = longValue.toCPointer<T>()
+```
+
+Note that if the type of the result is known from the context, the type argument
+can be omitted as usual due to type inference.
+
 ### Memory allocation ###
 
 The native memory can be allocated using `NativePlacement` interface, e.g.

Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Types.kt

@@ -161,6 +161,10 @@ val CPointer<*>?.rawValue: NativePtr
 
 fun <T : CPointed> CPointer<*>.reinterpret(): CPointer<T> = interpretCPointer(this.rawValue)!!
 
+fun <T : CPointed> CPointer<T>?.toLong() = this.rawValue.toLong()
+
+fun <T : CPointed> Long.toCPointer(): CPointer<T>? = interpretCPointer(nativeNullPtr + this)
+
 /**
  * The [CPointed] without any specified interpretation.
  */

Interop/Runtime/src/native/kotlin/kotlinx/cinterop/Varargs.kt

@@ -78,7 +78,7 @@ fun callWithVarargs(codePtr: NativePtr, returnValuePtr: NativePtr, returnTypeKin
     // All supported arguments take at most 8 bytes each:
     val argumentsStorage = argumentsPlacement.allocArray<LongVar>(totalArgumentsNumber)
     val arguments = argumentsPlacement.allocArray<CPointerVar<*>>(totalArgumentsNumber)
-    val types = argumentsPlacement.allocArray<CPointerVar<*>>(totalArgumentsNumber)
+    val types = argumentsPlacement.allocArray<COpaquePointerVar>(totalArgumentsNumber)
 
     var index = 0
 
@@ -87,7 +87,7 @@ fun callWithVarargs(codePtr: NativePtr, returnValuePtr: NativePtr, returnTypeKin
         val typeKind = convertArgument(argument, isVariadic = isVariadic,
                 location = storage, additionalPlacement = argumentsPlacement)
 
-        types[index] = interpretCPointer<COpaque>(nativeNullPtr + typeKind.toLong())
+        types[index] = typeKind.toLong().toCPointer()
         arguments[index] = storage
 
         ++index