Description

deserializeMsgPack() parses a MessagePack input and puts the result in a JsonDocument.

This function behaves differently depending on the type of the input:

  • For a read-only input, it duplicates the strings in the input document. This duplication consumes additional space in the JsonDocument.
  • For a writeable input, it stores pointers to the strings in the input buffer. This is the zero-copy mode. In this mode, the JsonDocument can be smaller, but ArduinoJson needs to modify the input buffer to insert null-terminators and replace escaped characters. Because the JsonDocument stores pointers to the input buffer, you must ensure that this buffer remains in memory.

Prefer the first mode when the input comes from a Stream; prefer the second when the input is in a buffer.

If the input is a char*, ensure the input buffer stays in memory until the JsonDocument is destructed.

Signatures

// writable input => zero-copy
DeserializationError deserializeMsgPack(JsonDocument& doc, char* input);
DeserializationError deserializeMsgPack(JsonDocument& doc, char* input, size_t inputSize);

// read-only input => duplication
DeserializationError deserializeMsgPack(JsonDocument& doc, const char* input);
DeserializationError deserializeMsgPack(JsonDocument& doc, const char* input, size_t inputSize);
DeserializationError deserializeMsgPack(JsonDocument& doc, const __FlashStringHelper* input);
DeserializationError deserializeMsgPack(JsonDocument& doc, const __FlashStringHelper* input, size_t inputSize);
DeserializationError deserializeMsgPack(JsonDocument& doc, const String& input); // 💀
DeserializationError deserializeMsgPack(JsonDocument& doc, const std::string& input);
DeserializationError deserializeMsgPack(JsonDocument& doc, Stream& input);
DeserializationError deserializeMsgPack(JsonDocument& doc, std::istream& input);

template<Reader> // custom reader class (see below)
DeserializationError deserializeMsgPack(JsonDocument& doc, Reader& input);

// all overloads also accept an optional parameter of type DeserializationOption::NestingLimit (see below)

Arduino’s String doesn’t allow nulls inside the string; don’t use this class for MessagePack documents.

Arguments

doc: the JsonDocument that will store the memory representation of the MessagePack document.

input: the MessagePack document to parse:

  • const char* is a string in RAM
  • const __FlashStringHelper* is a Flash string, usually created with F()
  • Stream is Arduino’s I/O stream interface, implemented by:

inputSize: the maximum number of bytes to read from input

This function supports an optional argument of type DeserializationOption::NestingLimit to change the maximum number of nesting that the parser will accept. See deserializeJson() for details.

Return value

deserializeMsgPack() returns a DeserializationError.

Performance

When you pass a Stream to deserializeMsgPack(), it consumes bytes one by one, which can be slow depending on the input you use. For example, if you read from a SPIFFS file, you can read twenty times faster by reading chunks of 64 bytes.

To read the stream in chunks, you can use ReadBufferingStream from the StreamUtils library.

Suppose your program is:

deserializeMsgPack(doc, file);

If you want to make deserializeMsgPack() read chunks instead of reading bytes one by one, replace this line with:

ReadBufferingStream bufferingStream(file, 64);
deserializeMsgPack(doc, bufferingStream);

The first line creates a new Stream that reads blocks of 64 bytes from file.

Custom reader

If none of the supported input types is suitable for you, you can implement a custom reader class. This class must implement two member functions, as shown below:

struct CustomReader {
  // Reads one byte, or returns -1
  int read();
  // Reads several bytes, returns the number of bytes read.
  size_t readBytes(char* buffer, size_t length);
};

Then, pass a reference to an instance of this class as the second argument of deserializeMsgPack().

Example

char input[] = "\x81\xA5hello\xA5world";
StaticJsonDocument<200> doc;
deserializeMsgPack(doc, input);
const char* world = doc["hello"];

See also