Parsing JSON from multiple buffer calls.

[dawinaj]

I have an ESP32 with a webserver running. User sends some config in POST as JSON.
Now, these configs may(or may not) get quite long, so I would like to avoid copying the entire thing into a buffer and then parsing.
I guess I should make some kind of stream or iterator, which would feed characters into the parser, and then if internal buffer is empty, call a function to read some length of the request again. However I'm not sure how I can implement that, which std class to inherit from, etc.

The function declaration is int httpd_req_recv(httpd_req_t *r, char *buf, size_t buf_len).
https://github.com/espressif/esp-idf/blob/master/components/esp_http_server/include/esp_http_server.h#L905

[nlohmann]

The parse function (https://json.nlohmann.me/api/basic_json/parse/) can read from a lot of inputs, including iterator ranges.

[dawinaj]

I would do it with two pointers, but I have no access to the internal memory address. If it even exists, because now I think it's being written straight from the socket.
I guess I would have to implement custom istream.
Unless, maybe I can treat socket as a FILE*? But then, there is no way to limit the length probably, and it will read past the POST field and maybe even break the whole API...