Read and write a c++ struct to a file. Arduino IDE. ESP32.

structToFile.ino

/*
 * EXAMPLE READING AND WRITING A C++ STRUCT TO A FILE
 */
#include <FS.h>
#include <FFat.h>

const char *fileName = "/somefile.txt";

struct aStruct {
    char someString[11];
    float someFloat;
};

void setup() {
    if (!FFat.begin()) {
        log_e("FFat Mount Failed");
        while (1)
            delay(100);
    }

    aStruct firstStruct = { "1234567890", 1.7 };  // String is 10 chars plus a terminator '\0'

    log_i("struct size: %i", sizeof(aStruct));

    File file = FFat.open(fileName, FILE_WRITE);

    const size_t bytesWritten = file.write((byte *)&firstStruct, sizeof(firstStruct));

    file.close();

    if (bytesWritten < sizeof(aStruct)) {
        log_e("Write error");
    }
}

void loop() {

    aStruct secondStruct = { "----------", 0.0 };

    File file = FFat.open(fileName, FILE_READ);

    const size_t bytesRead = file.read((byte *)&secondStruct, sizeof(secondStruct));

    log_i("file size: %i", file.size());

    if (bytesRead < file.size()) {
        log_e("Read error");
    }

    file.close();

    log_i("Data: %s, %.2f\n", secondStruct.someString, secondStruct.someFloat);

    while (1)
        delay(100);
}

No, not really. You are using Arduino ESP32 Core 2.0.3 or later? Try a lower version number like 1.0.6. It might solve this issue.

@heliophagus: Your code does not check the return value of myFile.read(). The call probably failed, given that myFile has been opened in FILE_WRITE mode.

Cleaned up the code. It is now checking the amount that has been written or read.

How to write further struct contents (records) and (random) read them in this same file?

@logowe By indexing?

Yes. Your code works fine in my app - thanks!
However, I intend to have different setups saved within the same file instead of several files with 1 record/structure each.
I'm looking for a library to prevent an own effort to add this functionality. As for writing, appending would work fine for me.
Do you know such a library or did you even realise indexed/random file access with structures?

Make a struct from your structs? Do this in a separate header file that you add to all units using it.

OK, thats a good idea! I will try it. Thanks.