Index: string.c
===================================================================
--- string.c	(revision 26826)
+++ string.c	(working copy)
@@ -3808,6 +3808,15 @@
     return rb_str_substr(str, 0, 1);
 }
 
+static VALUE
+rb_str_set_buffer(VALUE str, VALUE length)
+{
+    long size = NUM2LONG(length);
+    rb_str_modify(str);
+    RESIZE_CAPA(str, size);
+    return str;
+}
+
 /*
  *  call-seq:
  *     str.getbyte(index)          => 0 .. 255
@@ -7423,6 +7432,7 @@
     rb_define_method(rb_cString, "clear", rb_str_clear, 0);
     rb_define_method(rb_cString, "chr", rb_str_chr, 0);
     rb_define_method(rb_cString, "getbyte", rb_str_getbyte, 1);
+    rb_define_method(rb_cString, "buffer", rb_str_set_buffer, 1);
     rb_define_method(rb_cString, "setbyte", rb_str_setbyte, 2);
 
     rb_define_method(rb_cString, "to_i", rb_str_to_i, -1);