diff --git a/src/upb_string.c b/src/upb_string.c
new file mode 100644
index 0000000000..91ab9ae1d0
--- /dev/null
+++ b/src/upb_string.c
@@ -0,0 +1,47 @@
+/*
+ * upb - a minimalist implementation of protocol buffers.
+ *
+ * Copyright (c) 2010 Joshua Haberman.  See LICENSE for details.
+ */
+
+#include "upb_string.h"
+
+#include <stdlib.h>
+
+#define UPB_STRING_UNFINALIZED -1
+
+static uint32_t upb_round_up_pow2(uint32_t v) {
+  // http://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
+  v--;
+  v |= v >> 1;
+  v |= v >> 2;
+  v |= v >> 4;
+  v |= v >> 8;
+  v |= v >> 16;
+  v++;
+  return v;
+}
+
+upb_string *upb_string_new() {
+  upb_string *str = malloc(sizeof(*str));
+  str->ptr = NULL;
+  str->size = 0;
+  str->len = UPB_STRING_UNFINALIZED;
+  upb_atomic_refcount_init(&str->refcount, 1);
+  return str;
+}
+
+void _upb_string_free(upb_string *str) {
+  if(str->ptr) free(str->ptr);
+  free(str);
+}
+
+char *upb_string_getrwbuf(upb_string *str, upb_strlen_t len) {
+  assert(str->len == UPB_STRING_UNFINALIZED);
+  if (str->size < len) {
+    str->size = upb_round_up_pow2(len);
+    str->ptr = realloc(str->ptr, str->size);
+  }
+  str->len = len;
+  return str->ptr;
+}