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:
+  // Creates a SharedMemoryRegion.
+  // The mapped memory region begins at |offset| bytes from the start of the
+  // shared memory and the length is |size|. SharedMemoryRegion will take the
+  // ownership of the |handle| and release the resource when it was destroyed.
+  // Please make sure |handle| is not |auto_close|.

[Pawel Osciak, 2016/01/05 06:22:51]

Could we instead call SharedMemory::DuplicateHandle() in the constructor and use
that to create SharedMemory to avoid this issue?

[Owen Lin, 2016/01/06 03:45:46]

Oops, I was confused by the document.

https://code.google.com/p/chromium/codesearch#chromium/src/base/memory/shared...

The handle we used is actually FileDescriptor. (On POSIX) 

https://code.google.com/p/chromium/codesearch#chromium/src/base/file_descript...

No matter what's the value of |auto_close| is. FileDescriptor won't close it.
Yes, it sounds different from other SharedMemoryHandle.

I just rephrased the document about this.

+  SharedMemoryRegion(const base::SharedMemoryHandle& handle,
+                     off_t offset,
+                     size_t size,
+                     bool read_only);
+
+  // Creates a SharedMemoryRegion from the given |bistream_buffer|.
+  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. The mapped region begins at
+  // |offset| bytes from the start of the whole shared memory.

[Pawel Osciak, 2016/01/05 06:22:51]

The returned pointer points to the memory at the offset previously passed to the
constructor.

[Owen Lin, 2016/01/06 03:45:46]

Done.

+  void* memory();
+
+  size_t size() const { return size_; }
+
+ private:
+  base::SharedMemory shm_;
+  off_t offset_;
+  size_t size_;
+  size_t extra_size_;

[Pawel Osciak, 2016/01/05 06:22:51]

Nit: s/extra_size_/aligned_size_/

[Owen Lin, 2016/01/06 03:45:46]

Done.

+
+  DISALLOW_COPY_AND_ASSIGN(SharedMemoryRegion);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_GPU_MEDIA_SHARED_MEMORY_REGION_H_