SdFat 写入浮点数到文件 ESP32

Question

I Need to write float or Strings value into the SDVolume Cache in SDFAT library, I'm using ESP32 with SdCard module.

  uint8_t* pCache = (uint8_t*)sd.vol()->cacheClear();
  memset(pCache, ' ', 512);
  for (uint16_t i = 0; i < 512; i += 4) {
    
    pCache[i + 0] = 'r'; // I Need to write a Float value or String into this cell
    pCache[i + 1] = ',';
    pCache[i + 2] = '0';
    pCache[i + 3] = '\n';
  }

Libray link: https://github.com/greiman/SdFat

Answer 1

First:

 // I Need to write a Float value or String into this cell

makes no sense - that "cell" is a single character, like 'e' . How do you write a full float value into a single character?

You probably just want to fill pCache with a string representation of your float. So do that!

We've got all C++ at our disposal, so let's use a somewhat memory efficient method:

std::to_chars is the C++ way to convert numeric values to strings. Something like

#include <charconv>
…
    std::to_chars(pCache, pCache+512-1, floatvalue);

would be appropriate.

Couple of things:

• You need to zero-terminate strings if your needs to be able to know where they end. So, instead of memset(pCache, ' ', 512) , memset(pCache, 0, 512) ; would be more sensible.
• Are you sure your even using the right functions? vol()->cacheClear() is pretty unambigously marked as "do not use in your application" here ! It also seems to be very unusual to me that you would write your string to some block device cache instead of getting a buffer for a file from the file system, or passing a buffer to the file system to write to a file.