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.