Make file positions and lengths thread-safe

memfile.hpp

@@ -1,12 +1,18 @@
 #ifndef MEMFILE_HPP
 #define MEMFILE_HPP
 
+#include <atomic>
+
 struct memfile {
 	int fd = 0;
 	char *map = NULL;
-	long long len = 0;
+	std::atomic<long long> len;
 	long long off = 0;
 	unsigned long tree = 0;
+
+	memfile()
+	    : len(0) {
+	}
 };
 
 struct memfile *memfile_open(int fd);