About layout of the mmaped file

[jianzhongli]

After calling mmkv.putInt("int", 100) several times, I got a file named after mmkv id under the app's data folder. By inspecting the file content with xxd, I am a little bit confused about the layout of the file.

• The first four bytes are value of m_actualSize ，and we have 6700 0000 0x67 = 103 bytes.
• Then we have 0603 696e 7401 64
  • 06 is the size of the map
  • 03 696e 74 represents the length(03 bytes) and content of the key (696e 74, "int")
  • 01 64 represents the length(01 bytes) and the content of the value (0x64 = 100)

Why do we need to write size of the map into the mmaped file anyway? It seems that the size is of no use when decoding.

MMKV/Android/MMKV/mmkv/src/main/cpp/MiniPBCoder.cpp

Lines 253 to 271 in 2a3920a

	unordered_map<string, MMBuffer> MiniPBCoder::decodeOneMap(size_t size) {
	unordered_map<string, MMBuffer> dic;
	if (size == 0) {
	auto length = m_inputData->readInt32();
	}
	while (!m_inputData->isAtEnd()) {
	const auto &key = m_inputData->readString();
	if (key.length() > 0) {
	auto value = m_inputData->readData();
	if (value.length() > 0) {
	dic[key] = move(value);
	} else {
	dic.erase(key);
	}
	}
	}
	return dic;
	}

[lingol]

You're right. The size of of the map is not in used. It's here for historical compatibility reason.

Great skills of analyzing the data structure of the file, by the way.