net/disk_cache/file_lock.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 // See net/disk_cache/disk_cache.h for the public interface of the cache.
   6
   7 #ifndef NET_DISK_CACHE_FILE_LOCK_H__
   8 #define NET_DISK_CACHE_FILE_LOCK_H__
   9
  10 #include "net/base/net_export.h"
  11 #include "net/disk_cache/disk_format.h"
  12
  13 namespace disk_cache {
  14
  15 // This class implements a file lock that lives on the header of a memory mapped
  16 // file. This is NOT a thread related lock, it is a lock to detect corruption
  17 // of the file when the process crashes in the middle of an update.
  18 // The lock is acquired on the constructor and released on the destructor.
  19 // The typical use of the class is:
  20 //    {
  21 //      BlockFileHeader* header = GetFileHeader();
  22 //      FileLock lock(header);
  23 //      header->max_entries = num_entries;
  24 //      // At this point the destructor is going to release the lock.
  25 //    }
  26 // It is important to perform Lock() and Unlock() operations in the right order,
  27 // because otherwise the desired effect of the "lock" will not be achieved. If
  28 // the operations are inlined / optimized, the "locked" operations can happen
  29 // outside the lock.
  30 class NET_EXPORT_PRIVATE FileLock {
  31  public:
  32   explicit FileLock(BlockFileHeader* header);
  33   virtual ~FileLock();
  34
  35   // Virtual to make sure the compiler never inlines the calls.
  36   virtual void Lock();
  37   virtual void Unlock();
  38  private:
  39   bool acquired_;
  40   volatile int32* updating_;
  41 };
  42
  43 }  // namespace disk_cache
  44
  45 #endif  // NET_DISK_CACHE_FILE_LOCK_H__