examples(allocation): free memory after unmarshalling a result from the guest (#1368)

Signed-off-by: Luca Burgazzoli <lburgazzoli@gmail.com>

examples/allocation/tinygo/testdata/greet.go

@@ -6,6 +6,9 @@ import (
 	"unsafe"
 )
 
+// #include <stdlib.h>
+import "C"
+
 // main is required for TinyGo to compile to Wasm.
 func main() {}
 
@@ -70,10 +73,17 @@ func ptrToString(ptr uint32, size uint32) string {
 }
 
 // stringToPtr returns a pointer and size pair for the given string in a way
-// compatible with WebAssembly numeric types.
+// compatible with WebAssembly numeric types. The pointer is not automatically
+// managed by tinygo but must be freed by the host.
 func stringToPtr(s string) (uint32, uint32) {
-	buf := []byte(s)
-	ptr := &buf[0]
-	unsafePtr := uintptr(unsafe.Pointer(ptr))
-	return uint32(unsafePtr), uint32(len(buf))
+	if len(s) == 0 {
+		return 0, 0
+	}
+
+	size := C.ulong(len(s))
+	ptr := unsafe.Pointer(C.malloc(size))
+
+	copy(unsafe.Slice((*byte)(ptr), size), []byte(s))
+
+	return uint32(uintptr(ptr)), uint32(len(s))
 }