Add offset support to BitstreamBuffer.

content/common/gpu/media/shared_memory_region.h

+// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_COMMON_GPU_MEDIA_SHARED_MEMORY_REGION_H_
+#define CONTENT_COMMON_GPU_MEDIA_SHARED_MEMORY_REGION_H_
+
+#include "base/memory/shared_memory.h"
+#include "media/base/bitstream_buffer.h"
+
+namespace content {
+
+// Helper class to access a region of a SharedMemory.
+class SharedMemoryRegion {
+ public:
+  SharedMemoryRegion(const base::SharedMemoryHandle& handle,

[Pawel Osciak, 2015/12/31 02:05:54]

We should document arguments, especially offset and size are not obvious, also
the fact that this is taking ownership of shared memory.

[Owen Lin, 2016/01/04 08:54:17]

Done.

+                     off_t offset,
+                     size_t size,
+                     bool read_only);
+
+  SharedMemoryRegion(const media::BitstreamBuffer& bitstream_buffer,
+                     bool read_only);
+
+  // Maps the shared memory into the caller's address space.
+  // Return true on success, false otherwise.
+  bool Map();
+
+  // Gets a pointer to the mapped region if it has been mapped via Map().
+  // Returns |nullptr| if it is not mapped.
+  void* memory();

[Pawel Osciak, 2015/12/31 02:05:54]

We should document how this relates to offset and size passed to constructor.

[Owen Lin, 2016/01/04 08:54:17]

Done.

+
+  size_t size() const { return size_; }
+
+  off_t offset() const { return offset_; }

[Pawel Osciak, 2015/12/31 02:05:53]

Is this used anywhere?

[Owen Lin, 2016/01/04 08:54:17]

No, let me remove it.

+
+ private:
+  base::SharedMemory shm_;
+  off_t offset_;
+  size_t size_;
+
+  DISALLOW_COPY_AND_ASSIGN(SharedMemoryRegion);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_GPU_MEDIA_SHARED_MEMORY_REGION_H_