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