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** Copyright 2011, The Android Open Source Project
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** See the License for the specific language governing permissions and
** limitations under the License.
#include <stddef.h>
#include <memory>
#include <vector>
namespace android {
// A BlobCache is an in-memory cache for binary key/value pairs. A BlobCache
// does NOT provide any thread-safety guarantees.
// The cache contents can be serialized to an in-memory buffer or mmap'd file
// and then reloaded in a subsequent execution of the program. This
// serialization is non-portable and the data should only be used by the device
// that generated it.
class BlobCache {
// Create an empty blob cache. The blob cache will cache key/value pairs
// with key and value sizes less than or equal to maxKeySize and
// maxValueSize, respectively. The total combined size of ALL cache entries
// (key sizes plus value sizes) will not exceed maxTotalSize.
BlobCache(size_t maxKeySize, size_t maxValueSize, size_t maxTotalSize);
// set inserts a new binary value into the cache and associates it with the
// given binary key. If the key or value are too large for the cache then
// the cache remains unchanged. This includes the case where a different
// value was previously associated with the given key - the old value will
// remain in the cache. If the given key and value are small enough to be
// put in the cache (based on the maxKeySize, maxValueSize, and maxTotalSize
// values specified to the BlobCache constructor), then the key/value pair
// will be in the cache after set returns. Note, however, that a subsequent
// call to set may evict old key/value pairs from the cache.
// Preconditions:
// key != NULL
// 0 < keySize
// value != NULL
// 0 < valueSize
void set(const void* key, size_t keySize, const void* value,
size_t valueSize);
// get retrieves from the cache the binary value associated with a given
// binary key. If the key is present in the cache then the length of the
// binary value associated with that key is returned. If the value argument
// is non-NULL and the size of the cached value is less than valueSize bytes
// then the cached value is copied into the buffer pointed to by the value
// argument. If the key is not present in the cache then 0 is returned and
// the buffer pointed to by the value argument is not modified.
// Note that when calling get multiple times with the same key, the later
// calls may fail, returning 0, even if earlier calls succeeded. The return
// value must be checked for each call.
// Preconditions:
// key != NULL
// 0 < keySize
// 0 <= valueSize
size_t get(const void* key, size_t keySize, void* value, size_t valueSize);
// getFlattenedSize returns the number of bytes needed to store the entire
// serialized cache.
size_t getFlattenedSize() const;
// flatten serializes the current contents of the cache into the memory
// pointed to by 'buffer'. The serialized cache contents can later be
// loaded into a BlobCache object using the unflatten method. The contents
// of the BlobCache object will not be modified.
// Preconditions:
// size >= this.getFlattenedSize()
int flatten(void* buffer, size_t size) const;
// unflatten replaces the contents of the cache with the serialized cache
// contents in the memory pointed to by 'buffer'. The previous contents of
// the BlobCache will be evicted from the cache. If an error occurs while
// unflattening the serialized cache contents then the BlobCache will be
// left in an empty state.
int unflatten(void const* buffer, size_t size);
// clear flushes out all contents of the cache then the BlobCache, leaving
// it in an empty state.
void clear() { mCacheEntries.clear(); }
// mMaxTotalSize is the maximum size that all cache entries can occupy. This
// includes space for both keys and values. When a call to BlobCache::set
// would otherwise cause this limit to be exceeded, either the key/value
// pair passed to BlobCache::set will not be cached or other cache entries
// will be evicted from the cache to make room for the new entry.
const size_t mMaxTotalSize;
// Copying is disallowed.
BlobCache(const BlobCache&);
void operator=(const BlobCache&);
// A random function helper to get around MinGW not having nrand48()
long int blob_random();
// clean evicts a randomly chosen set of entries from the cache such that
// the total size of all remaining entries is less than mMaxTotalSize/2.
void clean();
// isCleanable returns true if the cache is full enough for the clean method
// to have some effect, and false otherwise.
bool isCleanable() const;
// A Blob is an immutable sized unstructured data blob.
class Blob {
Blob(const void* data, size_t size, bool copyData);
bool operator<(const Blob& rhs) const;
const void* getData() const;
size_t getSize() const;
// Copying is not allowed.
Blob(const Blob&);
void operator=(const Blob&);
// mData points to the buffer containing the blob data.
const void* mData;
// mSize is the size of the blob data in bytes.
size_t mSize;
// mOwnsData indicates whether or not this Blob object should free the
// memory pointed to by mData when the Blob gets destructed.
bool mOwnsData;
// A CacheEntry is a single key/value pair in the cache.
class CacheEntry {
CacheEntry(const std::shared_ptr<Blob>& key, const std::shared_ptr<Blob>& value);
CacheEntry(const CacheEntry& ce);
bool operator<(const CacheEntry& rhs) const;
const CacheEntry& operator=(const CacheEntry&);
std::shared_ptr<Blob> getKey() const;
std::shared_ptr<Blob> getValue() const;
void setValue(const std::shared_ptr<Blob>& value);
// mKey is the key that identifies the cache entry.
std::shared_ptr<Blob> mKey;
// mValue is the cached data associated with the key.
std::shared_ptr<Blob> mValue;
// A Header is the header for the entire BlobCache serialization format. No
// need to make this portable, so we simply write the struct out.
struct Header {
// mMagicNumber is the magic number that identifies the data as
// serialized BlobCache contents. It must always contain 'Blb$'.
uint32_t mMagicNumber;
// mBlobCacheVersion is the serialization format version.
uint32_t mBlobCacheVersion;
// mDeviceVersion is the device-specific version of the cache. This can
// be used to invalidate the cache.
uint32_t mDeviceVersion;
// mNumEntries is number of cache entries following the header in the
// data.
size_t mNumEntries;
// mBuildId is the build id of the device when the cache was created.
// When an update to the build happens (via an OTA or other update) this
// is used to invalidate the cache.
int mBuildIdLength;
char mBuildId[];
// An EntryHeader is the header for a serialized cache entry. No need to
// make this portable, so we simply write the struct out. Each EntryHeader
// is followed imediately by the key data and then the value data.
// The beginning of each serialized EntryHeader is 4-byte aligned, so the
// number of bytes that a serialized cache entry will occupy is:
// ((sizeof(EntryHeader) + keySize + valueSize) + 3) & ~3
struct EntryHeader {
// mKeySize is the size of the entry key in bytes.
size_t mKeySize;
// mValueSize is the size of the entry value in bytes.
size_t mValueSize;
// mData contains both the key and value data for the cache entry. The
// key comes first followed immediately by the value.
uint8_t mData[];
// mMaxKeySize is the maximum key size that will be cached. Calls to
// BlobCache::set with a keySize parameter larger than mMaxKeySize will
// simply not add the key/value pair to the cache.
const size_t mMaxKeySize;
// mMaxValueSize is the maximum value size that will be cached. Calls to
// BlobCache::set with a valueSize parameter larger than mMaxValueSize will
// simply not add the key/value pair to the cache.
const size_t mMaxValueSize;
// mTotalSize is the total combined size of all keys and values currently in
// the cache.
size_t mTotalSize;
// mRandState is the pseudo-random number generator state. It is passed to
// nrand48 to generate random numbers when needed.
unsigned short mRandState[3];
// mCacheEntries stores all the cache entries that are resident in memory.
// Cache entries are added to it by the 'set' method.
std::vector<CacheEntry> mCacheEntries;