How to modify values inside a file

Question

I am writing a wav file to an SD card on a ESP32 and that requires that every time I append data to the the end of the file I modify the header to set the right size in some fields.

I cannot find how to replace a value inside a file, I tried to write the new values with the seek function but unless I'm missing something looks like this deletes everything after the new value added.

I do not want to create a new file and to delete the old one as I will storing potentially large wav files and looks like wasting resources.

Is there a way to overwrite one value inside a file. I'm using SDlib but I'm open to others too.

Here the code I use to create the file and to update it:

#include "WavFileWriter.h"

void WavFileWriter::createFile(char const* filename, WavFileHeader* wavFileHeader)
{
  byte headerBuffer[44];
  wavFileHeader->getHeaderData(headerBuffer);
  File file = SD.open(filename, FILE_WRITE);
  file.write(headerBuffer, 44);
  file.close();
}

void WavFileWriter::appendData(char const* filename, WavFileHeader* wavFileHeader, byte* data, int dataLength)
{
  File file = SD.open(filename, FILE_WRITE);
  file.write(data, dataLength);

  byte headerBuffer[44];
  wavFileHeader->setFileSize(file.size());
  wavFileHeader->getHeaderData(headerBuffer);

  file.seek(0);
  file.write(headerBuffer, 44);
  file.close();
}

After calling appendData I end up with a file with just the header size (with the right header for a file with data but the data is gone).

Answer 1

The FILE_WRITE in SD library is 'append' (for historical reasons)

#define FILE_WRITE (O_READ | O_WRITE | O_CREAT | O_APPEND)

so use

 File file = SD.open(filename, O_WRITE);

Answer 2

SD.seek() is the API you're looking for. It allow you to position the read/write pointer within a file. In standard C++ the fseek() API is used.