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|. It will take the ownership of
+  // the |handle| and release the resource when being destroyed.
+  SharedMemoryRegion(const base::SharedMemoryHandle& handle,
+                     off_t offset,

[dcheng, 2016/01/22 18:27:39]

Does it make sense to use off_t here? I know that posix uses it, but off_t is
signed, and for our purposes, this can only be an offset from the start of the
buffer. 

[Pawel Osciak, 2016/01/25 08:20:02]

SharedMemory::MapAt(), which we pass this to uses off_t, so this helps avoid
conversions, even if we only use positive values.

+                     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 returned pointer points
+  // to the memory at the offset previously passed to the constructor.
+  void* memory();
+
+  size_t size() const { return size_; }
+
+ private:
+  base::SharedMemory shm_;
+  off_t offset_;
+  size_t size_;
+  size_t alignment_size_;
+
+  DISALLOW_COPY_AND_ASSIGN(SharedMemoryRegion);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_GPU_MEDIA_SHARED_MEMORY_REGION_H_