大家在使用Chrome浏览器的时候，只要在地址栏输入 about:memory，便可以详细的查
看浏览器中各种元素的内存使用情况，现在我们就来研究并实现这个功能。

  在 Chrome的代码中，我们可以看到一个叫memory_watcher的工具，这个便是关键所在
了。

 memory_watcher中定义了MemoryWatcher的类，初始化代码如下：
MemoryWatcher::MemoryWatcher()
  : file_(NULL),
    hooked_(false),
    active_thread_id_(0) {
  MemoryHook::Initialize();
  CallStack::Initialize();
  block_map_ = new CallStackMap();
  // Register last - only after we're ready for notifications!
  Hook();
}  

  从代码中我们可以清楚的发现，监视内存使用使用的是 Hook 技术，我们接着再来看看
MemoryHook是如何实现的。
  在 MemoryHook::Hook中，我们看到他Hook了几乎所有的内存分配函数：

bool MemoryHook::Hook() {
  DCHECK(!hooked_);
  if (!hooked_) {
    DCHECK(global_hook_);
    // Luckily, Patch() doesn't call malloc or windows alloc routines
    // itself -- though it does call new (we can use PatchWithStub to
    // get around that, and will need to if we need to patch new).
    HMODULE hkernel32 = ::GetModuleHandle(L"kernel32");
    CHECK(hkernel32 != NULL);
    HMODULE hntdll = ::GetModuleHandle(L"ntdll");
    CHECK(hntdll != NULL);
    // Now that we've found all the functions, patch them
    INSTALL_PATCH(HeapCreate);
    INSTALL_PATCH(HeapDestroy);
    INSTALL_PATCH(HeapAlloc);
    INSTALL_PATCH(HeapReAlloc);
    INSTALL_PATCH(HeapFree);
    INSTALL_PATCH(VirtualAllocEx);
    INSTALL_PATCH(VirtualFreeEx);
    INSTALL_PATCH(MapViewOfFileEx);
    INSTALL_PATCH(MapViewOfFile);
    INSTALL_PATCH(UnmapViewOfFile);
    INSTALL_NTDLLPATCH(NtUnmapViewOfSection);
    INSTALL_PATCH(GlobalAlloc);
    INSTALL_PATCH(GlobalReAlloc);
    INSTALL_PATCH(GlobalFree);
    INSTALL_PATCH(LocalAlloc);
    INSTALL_PATCH(LocalReAlloc);
    INSTALL_PATCH(LocalFree);
    // We are finally completely hooked.
    hooked_ = true;
  }
  return true;
}

为了简化代码，这些操作都是使用宏来完成的。
有趣的，为了使操作变得简单，Chrome 将分配内存的可修改大小的标志都去掉了，在
像使用 LocalReAlloc 的时候重新分配内存，并将原来的数据拷贝回去，这样就大大的减轻
了内存分配管理的不确定因素。
 static HLOCAL WINAPI Perftools_LocalAlloc(UINT uFlags, SIZE_T dwBytes) {
  // LocalAlloc is built atop HeapAlloc anyway.  So we don't track these.
  // LocalAlloc will internally call into HeapAlloc and we track there.
  // Force all memory to be fixed.
  uFlags &= ~LMEM_MOVEABLE;
  HLOCAL rv = patch_LocalAlloc()(uFlags, dwBytes);
  return rv;
}
static HLOCAL WINAPI Perftools_LocalReAlloc(HLOCAL hMem, SIZE_T dwBytes,
                                            UINT uFlags) {
  // LocalDiscard is a macro which calls LocalReAlloc with size 0.
  if (dwBytes == 0) {
    return patch_LocalReAlloc()(hMem, dwBytes, uFlags);
  }
  HGLOBAL rv = Perftools_LocalAlloc(uFlags, dwBytes);
  if (hMem != 0) {
    size_t size = LocalSize(hMem);
    if (size > dwBytes)
      size = dwBytes;
    // Note: size could be 0; HeapAlloc does allocate 0 length buffers.
    memcpy(rv, hMem, size);
    Perftools_LocalFree(hMem);
  }
  return rv;