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).

• jsotola: why do you need to store the sound data in wav format?
• imalipusram: can you identify the position of the sector on the SD card where the header is stored, keep a copy of it in RAM, keep this copy updated and write it to SD from time to time?
• jsotola: just store the raw data ... if you need a wav file then use something like Audacity to convert raw to wav
• Juraj: show your code.
• Ignacio Soler Garcia: added the code. I should have done from the beginning, my bad.

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);

• Ignacio Soler Garcia: Looks like the ESP32 uses a different library and the definitions are different: #define FILE_READ "r" #define FILE_WRITE "w" #define FILE_APPEND "a"
• Ignacio Soler Garcia: Should I use SPIFFS instead of SD?

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.

• Juraj: OP is using seek()
• Ignacio Soler Garcia: I added the code so you can see if this is the expected usage.
• Jeff Wahaus: It looks like you're not seek'ing to the end of the file when you attempt to append data. Try: file.seek(file.size()) between the SD.open() and file.write() in the WavFileWriter::appendData() method.