Create "persistent memory allocator" for persisting and sharing objects.

base/memory/shared_memory_allocator.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 BASE_MEMORY_SHARED_MEMORY_ALLOCATOR_H_
+#define BASE_MEMORY_SHARED_MEMORY_ALLOCATOR_H_
+
+#include <stdint.h>
+
+#include "base/base_export.h"
+#include "base/macros.h"
+
+namespace base {
+
+// Simple allocator for pieces of a memory block that may be shared across
+// multiple processes.
+//
+// This class provides for thread-safe (even across different processes)
+// allocation of memory within a designated block and also a mechanism by
+// which other threads can learn of the allocations with any additional
+// shared information.
+//
+// There is (currently) no way to release an allocated block of data because
+// doing so would risk invalidating pointers held by other processes and
+// greatly complicate the allocation algorithm.
+class BASE_EXPORT SharedMemoryAllocator {
+ public:
+  struct Iterator {
+    int32_t last;
+  };
+
+  struct MemoryInfo {
+    int32_t total;
+    int32_t free;
+  };
+
+  // The allocator operates on any arbitrary block of memory.  Creation and
+  // sharing of that block with another process is the responsibility of the
+  // caller.  The allocator needs to know only the block's |base| address, the
+  // total |size| of the block, and any internal |page| size (zero if not
+  // paged) across which allocations should not span.
+  //
+  // SharedMemoryAllocator does NOT take ownership of this memory block.  The
+  // caller must manage it and ensure it stays available throughout the lifetime
+  // of this object.
+  SharedMemoryAllocator(void* base, int32_t size, int32_t page);
+  ~SharedMemoryAllocator();
+
+  // Get an object referenced by an |offset|.  For safety reasons, the |type|
+  // code and size-of |unused| are compared to ensure the reference is valid
+  // and cannot return an object outside of the memory segment.  A |type| of
+  // zero will match any though the size is still checked.  NULL is returned
+  // if any problem is detected, such as corrupted storage or incorrect
+  // parameters.  Callers MUST check that the returned value is not-null EVERY
+  // TIME before accessing it or risk crashing!  Once dereferenced, the pointer
+  // is safe to reuse forever.
+  template<typename T> T* GetObject(int32_t offset, int32_t type) {
+    return static_cast<T*>(GetBlockData(offset, type, sizeof(T), false));
+  }
+
+  // Reserve space in the memory segment of the desired |size| and |type|.
+  // A return value of zero indicates the allocation failed, otherwise the
+  // returned offset can be used by any process to get a real pointer via
+  // the GetObject() call.
+  int32_t Allocate(int32_t size, int32_t type);
+
+  // Get the information about the amount of free space in the allocator.  The
+  // amount of free space should be treated as approximate due to extras from
+  // alignment and metadata, but will never return less than could actually
+  // be allocated.
+  void GetMemoryInfo(MemoryInfo* meminfo);
+
+  // Allocated objects can be added to an internal list that can then be
+  // iterated over by other processes.
+  void MakeIterable(int32_t offset);
+
+  // Iterating uses a |state| structure (initialized by GetFirstIterable) and
+  // returns both the offset reference to the object as well as the |type| of
+  // that object.  A zero return value indicates there are currently no more
+  // objects to be found but future attempts can be made without having to
+  // reset the iterator to "first".
+  int32_t GetFirstIterable(Iterator* state, int32_t* type);
+  int32_t GetNextIterable(Iterator* state, int32_t* type);
+
+  // If there is some indication that the shared memory has become corrupted,
+  // calling this will attempt to prevent further damage by indicating to
+  // all processes that something is not as expected.
+  void SetCorrupted();
+  bool IsCorrupted();
+
+  // Flag set if an allocation has failed because memory was full.
+  bool IsFull();
+
+ private:
+  struct SharedMetadata;
+  struct BlockHeader;
+
+  BlockHeader* GetBlock(int32_t offset, int32_t type, int32_t size,
+                        bool special);
+  void* GetBlockData(int32_t offset, int32_t type, int32_t size, bool special);
+
+  SharedMetadata* shared_meta_;
+  char* mem_base_;  // char because sizeof guaranteed 1 -- easy offset calc
+  int32_t mem_size_;
+  int32_t mem_page_;
+  int32_t last_seen_;
+  bool corrupted_;
+
+  DISALLOW_COPY_AND_ASSIGN(SharedMemoryAllocator);
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_SHARED_MEMORY_ALLOCATOR_H_