Issue with JSON_OBJECT_SIZE on ESP8266

[marvinroger]

Here is a problematic code:

StaticJsonBuffer<JSON_OBJECT_SIZE(2)> json_buffer;
String json = String("{\"test1\":\"test\",\"test2\":\"test\"}");

// 1st attempt
JsonObject& parsed_json = json_buffer.parseObject(json); // parsed_json.success() -> false

// 2nd attempt
JsonObject& parsed_json = json_buffer.parseObject(json.c_str()); // parsed_json.success() -> false

// 3rd attempt
JsonObject& parsed_json = json_buffer.parseObject((char*)json.c_str()); // parsed_json.success() -> true

As explained here:

parseObject() takes a char* as a parameter. Be careful, it's not a const char*, the memory must be writable.

But as stated here, the function can also take a String or const char* but will make a copy of this string.

So all 3 should work, I guess... What's wrong?

[bblanchon]

There is not enough room in json_buffer to store the copy of the string.
Therefore, only the overload that makes no copy can work.

To make them work, you need to increase the size of the StatisJsonBuffer of the size of the string (plus some bytes for the alignment and the terminator).

#define MAX_INPUT_LENGTH 40
StaticJsonBuffer<JSON_OBJECT_SIZE(2)+MAX_INPUT_LENGTH> json_buffer;

[marvinroger]

Oh, okay thanks for the information. This would deserve to be in the Wiki, don't you think?
Feel free to close.

[bblanchon]

I started an API reference section, this will eventually go there.