Add offset support to BitstreamBuffer.

media/base/bitstream_buffer.h

 // Copyright (c) 2011 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 MEDIA_BASE_BITSTREAM_BUFFER_H_
 #define MEDIA_BASE_BITSTREAM_BUFFER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/macros.h"
 #include "base/memory/shared_memory.h"
 #include "base/time/time.h"
 #include "media/base/decrypt_config.h"
 #include "media/base/media_export.h"
 #include "media/base/timestamp_constants.h"
 
 namespace media {
 
 // Class for passing bitstream buffers around.  Does not take ownership of the
 // data.  This is the media-namespace equivalent of PP_VideoBitstreamBuffer_Dev.
 class MEDIA_EXPORT BitstreamBuffer {
  public:
-  BitstreamBuffer(int32_t id, base::SharedMemoryHandle handle, size_t size);
-
+  // Constructs a new BitstreamBuffer. The content of the bitstream is located
+  // at |offset| bytes away from the start of the shared memory and the payload
+  // is |size| bytes. When not provided, the default value for |offset| is 0.
+  // |presentation_timestamp| is when the decoded frame should be displayed.
+  // When not provided, |presentation_timestamp| will be
+  // |media::kNoTimestamp()|.
   BitstreamBuffer(int32_t id,
                   base::SharedMemoryHandle handle,
                   size_t size,
-                  base::TimeDelta presentation_timestamp);
+                  off_t offset = 0,
+                  base::TimeDelta presentation_timestamp = kNoTimestamp());
 
   ~BitstreamBuffer();
 
   void SetDecryptConfig(const DecryptConfig& decrypt_config);
 
   int32_t id() const { return id_; }
   base::SharedMemoryHandle handle() const { return handle_; }
+
+  // The number of bytes of the actual bitstream data. It is the size of the
+  // content instead of the whole shared memory.
   size_t size() const { return size_; }
 
+  // The offset to the start of actual bitstream data in the shared memory.
+  off_t offset() const { return offset_; }

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

Any particular to reason to use off_t? I know mmap (and SharedMemory::MapAt
currently) uses it, but since off_t is signed, this means we need to do checks
in various places. Would it make more sense to just use size_t?

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

MapAt() will fail us if the offset is invalid, e.g. if we pass a negative value
for some reason. So I don't think we have to check for <0 explicitly. Also,
technically only in the context of SharedMemoryRegion (not BitstreamBuffer
itself) we need to check and only due to the way we calculate alignment.
Otherwise we can always depend on the system to fail us on invalid offset
values.

+
   // The timestamp is only valid if it's not equal to |media::kNoTimestamp()|.
   base::TimeDelta presentation_timestamp() const {
     return presentation_timestamp_;
   }
 
   // The following methods come from DecryptConfig.
 
   const std::string& key_id() const { return key_id_; }
   const std::string& iv() const { return iv_; }
   const std::vector<SubsampleEntry>& subsamples() const { return subsamples_; }
 
  private:
   int32_t id_;
   base::SharedMemoryHandle handle_;
   size_t size_;
+  off_t offset_;
 
   // This is only set when necessary. For example, AndroidVideoDecodeAccelerator
   // needs the timestamp because the underlying decoder may require it to
   // determine the output order.
   base::TimeDelta presentation_timestamp_;
 
   // The following fields come from DecryptConfig.
   // TODO(timav): Try to DISALLOW_COPY_AND_ASSIGN and include these params as
   // scoped_ptr<DecryptConfig> or explain why copy & assign is needed.
 
   std::string key_id_;                      // key ID.
   std::string iv_;                          // initialization vector
   std::vector<SubsampleEntry> subsamples_;  // clear/cypher sizes
 
   // Allow compiler-generated copy & assign constructors.
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_BITSTREAM_BUFFER_H_