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);
}

thanks

THANKS

thanks

Thanks a lot!

The code doesn't work as posted, at least for me. No errors, but the read operation does not work. I tested this by changing the values in the struct after writing the struct to the file; after reading from the file, the original values written to the file are not present. Any ideas on why?

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

typedef struct {
float EEE = 0.0;
int8_t mark = -1;
} SPIFFdata;

SPIFFdata sd;

void setup() {
// put your setup code here, to run once:
Serial.begin(115200);

if (!SPIFFS.begin()) {
Serial.println("Card Mount Failed");
return;
}

sd.EEE = 123.456;
sd.mark = 65;

// strncpy( myStruct.someString, "testtesttest", sizeof(myStruct.someString) );
File myFile = SPIFFS.open("/SPIFFtest1.bin", FILE_WRITE);
myFile.write((byte *)&sd, sizeof(sd));
myFile.close();
}

void loop() {
// put your main code here, to run repeatedly:
// File myFile = SPIFFS.open("/SPIFFtest1.bin", FILE_WRITE);

// overwrite the values in the struct that were written to the file in setup()
sd.EEE = 0.0;
sd.mark = -1;

File myFile = SPIFFS.open("/SPIFFtest1.txt", FILE_WRITE);
myFile.read((byte *)&sd, sizeof(sd));
myFile.close();
Serial.println("File contains:");
Serial.println(sd.EEE, 3);
Serial.println(sd.mark);

//Serial.printf( "read: %s, %.2f\n", myStruct.someString, myStruct.someFloat );
delay(1000);
}
`

In the serial monitor window, I just get:

File contains:
0.000
-1

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.