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diff --git a/src/CGroup/Types.hs b/src/CGroup/Types.hs
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+-- |
+-- Module:      CGroup.Types
+-- Copyright:   (c) 2014 Tomislav Viljetić
+-- License:     BSD3
+-- Maintainer:  Tomislav Viljetić <tomislav@viljetic.de>
+--
+
+module CGroup.Types
+    (
+    -- * CGroup
+      CGroup
+    , cgroup
+    , cgroupPath
+    -- * Other types
+    , ProcessID
+    ) where
+
+import Data.Monoid
+import qualified System.FilePath as FP
+
+
+-- | A 'CGroup' is defined by two 'FilePath's, a mount point and a cgroup
+-- name.  The mount point specifies where the cgroup hierarchy is mounted.
+-- The cgroup name is a directory, relative to the mount point.
+data CGroup = CGroup { mountPoint, cgroupName :: FilePath }
+  deriving Show
+
+
+-- | Smart constructor. Takes a mount point and a cgroup name.
+-- It will return 'Nothing' if the cgroup could point outside the mount point,
+-- i.e. if the cgroup name is an absolute path, or contains @".."@.
+cgroup :: FilePath -> FilePath -> Maybe CGroup
+cgroup mp0 cgn0
+    | ".." `elem` parts = Nothing
+    | FP.isAbsolute cgn = Nothing
+    | otherwise = Just CGroup { mountPoint = mp, cgroupName = cgn }
+  where
+    mp = normaliseMountPoint mp0
+    cgn = normaliseCGroupName cgn0
+    parts = FP.splitDirectories cgn
+    normaliseMountPoint = FP.addTrailingPathSeparator . FP.normalise
+    normaliseCGroupName = FP.dropTrailingPathSeparator . FP.normalise
+
+
+-- | Path of a cgroup's tasks file.
+cgroupPath :: CGroup -> FilePath
+cgroupPath CGroup { mountPoint = mp, cgroupName = cgn } =
+    mp <> cgn
+
+
+type ProcessID = Int