Reland "Replace Chrome IPC with Mojo IPC for querying BatteryStatus service"
[chromium-blink-merge.git] / content / renderer / shared_memory_seqlock_reader.cc
blob5cf93478db4b757f73d3b914dd285776926f6807
1 // Copyright 2013 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.
5 #include "shared_memory_seqlock_reader.h"
7 namespace content {
8 namespace internal {
10 SharedMemorySeqLockReaderBase::SharedMemorySeqLockReaderBase() { }
12 SharedMemorySeqLockReaderBase::~SharedMemorySeqLockReaderBase() { }
14 void*
15 SharedMemorySeqLockReaderBase::InitializeSharedMemory(
16 base::SharedMemoryHandle shared_memory_handle, size_t buffer_size) {
17 renderer_shared_memory_handle_ = shared_memory_handle;
18 if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
19 return 0;
20 renderer_shared_memory_.reset(new base::SharedMemory(
21 renderer_shared_memory_handle_, true));
23 return (renderer_shared_memory_->Map(buffer_size))
24 ? renderer_shared_memory_->memory()
25 : 0;
28 bool SharedMemorySeqLockReaderBase::FetchFromBuffer(
29 content::OneWriterSeqLock* seqlock, void* final, void* temp, void* from,
30 size_t size) {
32 if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
33 return false;
35 // Only try to read this many times before failing to avoid waiting here
36 // very long in case of contention with the writer.
37 int contention_count = -1;
38 base::subtle::Atomic32 version;
39 do {
40 version = seqlock->ReadBegin();
41 memcpy(temp, from, size);
42 ++contention_count;
43 if (contention_count == kMaximumContentionCount)
44 break;
45 } while (seqlock->ReadRetry(version));
47 if (contention_count >= kMaximumContentionCount) {
48 // We failed to successfully read, presumably because the hardware
49 // thread was taking unusually long. Don't copy the data to the output
50 // buffer, and simply leave what was there before.
51 return false;
54 // New data was read successfully, copy it into the output buffer.
55 memcpy(final, temp, size);
56 return true;
59 } // namespace internal
60 } // namespace content