diff --git a/octa/stream.h b/octa/stream.h
index 677893e..69780fd 100644
--- a/octa/stream.h
+++ b/octa/stream.h
@@ -6,11 +6,44 @@
 #ifndef OCTA_STREAM_H
 #define OCTA_STREAM_H
 
+#include <stdio.h>
+#include <sys/types.h>
+
+#include "octa/types.h"
+#include "octa/range.h"
+
 namespace octa {
-    struct stream {
-        stream() {}
-        virtual ~stream() {}
-    };
+
+/* off_t is POSIX - will also work on windows with mingw/clang, but FIXME */
+using StreamOffset = off_t;
+
+enum class Seek {
+    cur = SEEK_CUR,
+    end = SEEK_END,
+    set = SEEK_SET
+};
+
+struct Stream {
+    using Offset = StreamOffset;
+
+    virtual void close() = 0;
+
+    virtual bool end() = 0;
+
+    virtual Offset size() {
+        Offset p = tell();
+        if ((p < 0) || !seek(0, Seek::end)) return -1;
+        Offset e = tell();
+        return (p == e) || (seek(p, Seek::set) ? e : -1);
+    }
+
+    virtual bool seek(Offset pos, Seek whence = Seek::set) { return false; }
+
+    virtual Offset tell() { return -1; }
+
+    virtual bool flush() { return true; }
+};
+
 }
 
 #endif
\ No newline at end of file