src/Pure/System/isabelle_system.scala

     val path = Path.explode("$ISABELLE_TMP_PREFIX")
     path.file.mkdirs  // low-level mkdirs
     File.platform_file(path)
   }
 
-  def tmp_file(name: String, ext: String = ""): JFile =
+  def tmp_file(name: String, ext: String = "", base_dir: JFile = isabelle_tmp_prefix()): JFile =
   {
     val suffix = if (ext == "") "" else "." + ext
-    val file = Files.createTempFile(isabelle_tmp_prefix().toPath, name, suffix).toFile
+    val file = Files.createTempFile(base_dir.toPath, name, suffix).toFile
     file.deleteOnExit
     file
   }
 
   def with_tmp_file[A](name: String, ext: String = "")(body: Path => A): A =

         }
       )
     }
   }
 
-  def tmp_dir(name: String): JFile =
-  {
-    val dir = Files.createTempDirectory(isabelle_tmp_prefix().toPath, name).toFile
+  def tmp_dir(name: String, base_dir: JFile = isabelle_tmp_prefix()): JFile =
+  {
+    val dir = Files.createTempDirectory(base_dir.toPath, name).toFile
     dir.deleteOnExit
     dir
   }
 
   def with_tmp_dir[A](name: String)(body: Path => A): A =