Random access to SD card using SD.h or another library (ESP32)

Question

I'm trying to use a SD card with my ESP32 in order to save some variables in a txt file. Each variable uses 1 byte, so they can be represented by an 8 bit extended ASCII character.

The issue is it seems that the SD.h library has only 3 open modes (Read only, FILE_WRITE, FILE_APPEND), and I cannot figure out how to use random access for writing a specific byte of the file.

Imagine the situation: I have a file called myFile.txt which initially has a size of 5 bytes, and its content is: abcde

The code I'm using is the following:

#include <SPI.h>
#include <SD.h>
#define SD_CS_PIN 5
File myFile;
void setup() {
 // Open serial communications and wait for port to open:
 Serial.begin(115200);
 while (!Serial) {
 ; // wait for serial port to connect. Needed for native USB port only
 }
 Serial.println("Initializing SD card...");
 if (!SD.begin(SD_CS_PIN)) {
 Serial.println("initialization failed!");
 return;
 }
 Serial.println("initialization done.");
 myFile = SD.open("/myFile.txt", FILE_WRITE);
 myFile.seek(3);
 myFile.write('a');
 myFile.close();
}
void loop() {}

But after running the code, the file has an unexpected behaviour, its size changes from 5 bytes to 4 bytes, and its content is: abca instead of the expected abcae

I know it happens because when writing into the file I'm using the open mode FILE_WRITE which uses the FILE_APPEND mode internally, so when doing myFile.seek(3);, I'm truncating the file to 4 bytes since the FILE_APPEND mode needs to point to the end of the file.

So, my question is, how can I open a file in random access mode with SD.h, or another library?

Thanks in advance!

• Juraj: I would try to seek() to the end before close(). note: the SD library in ESP32 boards support package is different than the standard Arduino SD library.
• AlexSp3: @Juraj I tried it out but I got the same result, the first myFile.seek(3); truncates the file before.

Answer 1

Solved!

The solution was to migrate from the SD library to mySD, which seems to be a SdFat wrapper for ESP32.

You can see in the file mySD.h that the FILE_WRITE mode is defined as:

#define FILE_WRITE (F_READ | F_WRITE | F_CREAT)

Which means that it allows random access to the file for writing (F_WRITE instead of FILE_APPEND).

PS: I want to remark @Juraj comment since the forum is mainly for Arduino board questions

the SD library in ESP32 boards support package is different than the standard Arduino SD library

Answer 2

I had the same problem with a nano esp32. I used the esp32 SD.h. The FILE_WRITE is a macro for "w", FILE_READ a macro for "r". By specifying mode as "w+r", SD.open(fname,"w+r"); I was able to randomly access the file.