I am currently Working on a project, where I save values in a JSON File on an SD Card.

The JSON looks something like this:

{ "ID1":{[[starttime, endtime],
          [starttime, endtime],
  "ID2":{[[starttime, endtime],
          [starttime, endtime],
          [starttime, endtime],

I have to add start and endtimes to IDs, so I have to edit the file. Is there a nicer/easier/better way than the following?:

  1. Create Buffer (e.g. 512 Bytes)
  2. Read 512 Bytes from file to buffer
  3. Edit/Add Values in Buffer
  4. Save Buffer to another File
  5. Repeat 2. to 4. until end of file
  6. Remove old File and rename new File

I am currently using the ESP32 Core SD.h Libary

  • you could seek() in the file and overwrite bytes in the file. but not add or remove bytes.
    – Juraj
    May 10 '20 at 9:17
  • To add data, no, there is no better way. The way you describe is called "stream editing" and is the normal way on low-memory machines. TBH I'd work in lines, not blocks of 512 bytes.
    – Majenko
    May 10 '20 at 9:22
  • 1
    Also TBH I wouldn't use JSON. It's not great (as you see) at editing. Instead a simple CSV format would be better where you can just append data to the end of the file, then collate it later on the computer in excel.
    – Majenko
    May 10 '20 at 9:33
  • Going by line is a good Idea! I will give it a try. I need a JSON File, because I want to use the data in a React App. The reason why I use the SD Card is in case of a failure I want to be able to retrieve the data. Thanks a lot!
    – Markus
    May 10 '20 at 9:44
  • you can use notepad++ text editor to sort a csv file and to add JSON formatting to the columns
    – jsotola
    May 10 '20 at 17:41

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