c/ffi: add _demo

c/ffi/_demo/printf/main.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"unsafe"
+
+	"github.com/goplus/llgo/c"
+	"github.com/goplus/llgo/c/ffi"
+)
+
+func main() {
+	sig, err := ffi.NewSignatureVar(ffi.TypeInt32, 1, ffi.TypePointer, ffi.TypeInt32)
+	if err != nil {
+		panic(err)
+	}
+	var ret int32
+	text := c.Str("hello world: %d\n")
+	var n int32 = 100
+	ffi.Call(sig, c.Func(c.Printf), unsafe.Pointer(&ret), unsafe.Pointer(&text), unsafe.Pointer(&n))
+	c.Printf(c.Str("ret: %d\n"), ret)
+}