Add basic support for calling C function pointers

Fix #739

backend.native/tests/build.gradle

@@ -1994,6 +1994,11 @@ kotlinNativeInterop {
         flavor 'native'
     }
 
+    cfunptr {
+        defFile 'interop/basics/cfunptr.def'
+        flavor 'native'
+    }
+
     if (isMac()) {
         opengl {
             defFile '../../samples/opengl/src/main/c_interop/opengl.def'
@@ -2057,6 +2062,13 @@ task interop_bitfields(type: RunInteropKonanTest) {
     interop = 'bitfields'
 }
 
+task interop_funptr(type: RunInteropKonanTest) {
+    disabled = (project.testTarget == 'wasm32') // No interop for wasm yet.
+    goldValue = "42\n17\n"
+    source = "interop/basics/funptr.kt"
+    interop = 'cfunptr'
+}
+
 task interop_echo_server(type: RunInteropKonanTest) {
     disabled = (project.testTarget == 'wasm32') // No interop for wasm yet.
     if (!isMac()) {

backend.native/tests/interop/basics/cfunptr.def

@@ -0,0 +1,44 @@
+headerFilter = NOTHING
+---
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef int (*atoiPtrType)(const char*);
+
+static atoiPtrType getAtoiPtr() {
+    return &atoi;
+}
+
+static void __printInt(int x) {
+    printf("%d\n", x);
+}
+
+static void* __getPrintIntPtr() {
+    return &__printInt;
+}
+
+typedef void* (*getPrintIntPtrPtrType)(void);
+
+static getPrintIntPtrPtrType getGetPrintIntPtrPtr() {
+    return &__getPrintIntPtr;
+}
+
+static double __add(double x, double y) {
+    return x + y;
+}
+
+typedef double (*addPtrType)(double, double);
+
+static addPtrType getAddPtr() {
+    return &__add;
+}
+
+static int __doubleToInt(double x) {
+    return (int) x;
+}
+
+typedef int (*doubleToIntPtrType)(double);
+
+static doubleToIntPtrType getDoubleToIntPtr() {
+    return &__doubleToInt;
+}

backend.native/tests/interop/basics/funptr.kt

@@ -0,0 +1,21 @@
+import kotlinx.cinterop.*
+import cfunptr.*
+
+fun main(args: Array<String>) {
+    val atoiPtr = getAtoiPtr()!!
+
+    val getPrintIntPtrPtr = getGetPrintIntPtrPtr()!!
+    val printIntPtr = getPrintIntPtrPtr()!!.reinterpret<CFunction<(Int) -> Unit>>()
+
+    val fortyTwo = memScoped {
+        atoiPtr("42".cstr.getPointer(memScope))
+    }
+
+    printIntPtr(fortyTwo)
+
+    printIntPtr(
+            getDoubleToIntPtr()!!(
+                    getAddPtr()!!(5.1, 12.2)
+            )
+    )
+}