deserializeMsgPack()

Description

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

Before reading the input, this function resets the document, so you don’t need to call JsonDocument::clear().

This function behaves differently depending on the type of 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);
DeserializationError deserializeMsgPack(JsonDocument& doc, JsonVariantConst input);

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

// all overloads also accept optional parameters (see below)

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

Support for JsonVariantConst as input was added in ArduinoJson 6.15.2

Arguments

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

input: the MessagePack document to parse:

inputSize: the maximum number of bytes to read from input

This function supports two optional parameters:

  • a parameter of type DeserializationOption::NestingLimit to change the maximum number of nesting levels that the parser will accept (see “Nesting Limit” below);
  • a parameter of type DeserializationOption::Filter to filter the input document and keep only the fields you need (see “Filtering” below).

Return value

deserializeMsgPack() returns a DeserializationError.

Configuration

deserializeMsgPack() can be configured with the following settings:

Nesting limit

The ArduinoJson’s parser contains a recursive function that is called each time an object or an array begins. In other words, each object/array nesting level causes a recursive call. This recursive call is a security risk because an attacker could craft a JSON input with many opening brackets to cause a stack overflow.

To protect against this security risk, ArduinoJson limits the number of nesting levels. The macro ARDUINOJSON_DEFAULT_NESTING_LIMIT sets the default value.

If your MessagePack input contains more nesting levels than allowed, you can pass an extra parameter of type DeserializationOption::NestingLimit to deserializeMsgPack().

Filtering

When the input document contains many fields that are not relevant to your application, you can ask deserializeMsgPack() to ignore them and save a lot of space in the JsonDocument.

To use this feature, create an ancillary JsonDocument that contains the value true as a placeholder for every field you want to keep in the final document. For arrays, only create one element in the filter document, it will serve as a filter for all elements of the original array. Wrap this document in a DeserializationOption::Filter before passing it to deserializeMsgPack().

See JsonFilterExample.ino for an example.

When filtering, deserializeMsgPack() needs some extra room in the JsonDocument to store object keys temporarily.
Make sure to increase the capacity a bit, or you may get a NoMemory error.

This feature was added in ArduinoJson 6.17.0

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

Global warming stripes by Professor Ed Hawkins (University of Reading)