preparations for proper memmap'ed files

lib/pure/memfiles.nim

@@ -0,0 +1,51 @@
+#
+#
+#            Nimrod's Runtime Library
+#        (c) Copyright 2011 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## This module provides support for `memory mapped files`:idx:
+## (Posix's `mmap`:idx:) on the different operating systems.
+## XXX Currently it is implemented with Nimrod's
+## basic IO facilities and does not use any platform specific code!
+## Oh and currently only ``fmRead`` is supported...
+
+type
+  TMemFile* = object {.pure.}
+    file: TFile
+    buffer: pointer
+    fileLen: int
+  
+proc open*(f: var TMemFile, filename: string, mode: TFileMode = fmRead): bool =
+  ## open a memory mapped file `f`. Returns true for success.
+  assert mode == fmRead
+  result = open(f.file, filename, mode)
+
+  var len = getFileSize(f.file)
+  if len < high(int):
+    f.fileLen = int(len)
+    f.buffer = alloc(f.fileLen)
+    if readBuffer(f.file, f.buffer, f.fileLen) != f.fileLen:
+      raise newException(EIO, "error while reading from file")
+  else:
+    raise newException(EIO, "file too big to fit in memory")
+
+proc close*(f: var TMemFile) =
+  ## closes the memory mapped file `f`. All changes are written back to the
+  ## file system, if `f` was opened with write access.
+  dealloc(f.buffer)
+  close(f.file)
+
+proc mem*(f: var TMemFile): pointer {.inline.} =
+  ## retrives a pointer to the memory mapped file `f`. The pointer can be
+  ## used directly to change the contents of the file, if `f` was opened
+  ## with write access.
+  result = f.buffer
+
+proc size*(f: var TMemFile): int {.inline.} =
+  ## retrives the size of the memory mapped file `f`.
+  result = f.fileLen
+