Arduino writes file to SD but not content to file

Question

I'm using an Arduino UNO, the Extended Database Library (EDB), and an microSD Card to read/write 2 database tables. I've managed to create both ".db" files on the SD (they show up in Finder when I load the microSD directly onto my Mac).

I can also populate them with placeholder data, but this data does not seem to be actually written to the file (and thus doesn't survive reboot).

Also, trying dbFile = SD.open(db_name1, FILE_WRITE); returns false, even though the files do exist on the drive?

Here's my create code:

#include <SPI.h>
#include <SD.h>
#define SD_PIN 10 //SD read/write pin
#define TABLE_SIZE 8192 //256*32
#include "Arduino.h"
#include <EDB.h>
char* db_name1 = "/DB/SENSOR1.db";
char* db_name2 = "/DB/SENSOR2.db";
File dbFile;
struct sensorLog1 {
 unsigned int id;
 float input;
 unsigned int timestamp; 
}
sensorLog1;
struct sensorLog2 {
 unsigned int id;
 float input;
 unsigned int timestamp; 
}
sensorLog2;
// Setting up EDB object
inline void writer (unsigned long address, const byte* data, unsigned int recsize) {
 dbFile.seek(address);
 dbFile.write(data,recsize);
 dbFile.flush();
}
inline void reader (unsigned long address, byte* data, unsigned int recsize) {
 dbFile.seek(address);
 dbFile.read(data,recsize);
}
// Create an EDB object with the appropriate write and read handlers
EDB db(&writer, &reader);
void setup() {
 Serial.begin(9600);
 createDB(db_name1, "sensorLog1"); 
 createDB(db_name2, "sensorLog2");
 dbFile = SD.open(db_name1, FILE_WRITE); // Doesn't work!
 if (!dbFile) { Serial.println("File does not exist"); }
}
void loop() {} 
void createDB(char* db_name, char* table_name) {
 // create file
 dbFile = SD.open(db_name, FILE_WRITE);
 db.create(0, TABLE_SIZE, (unsigned int)sizeof(table_name));
 // populate with placeholder data
 for (unsigned int i = 1; i <= 20; i++) {
 table_name.id = i;
 table_name.input = 0;
 table_name.timestamp = 0;
 EDB_Status result = db.appendRec(EDB_REC table_name);
 }
 // check data
 Serial.println(db.count()); // returns '20'
 // close file to write
 dbFile.close();
}

• esoterik: does the sample work? the sample uses different code for creating and opening an existing database.
• user1092247: @esoterik, yes, the code works if I replace all instances of table_name in the for loop with one of the table names (e.g. sensorLog1), but then obviously the function only works for that particular table...
• esoterik: since your structs are identical except for the name, you should be able to do this with only one struct. sizeof doesn't do reflection, it won't do what your trying to do.
• Juraj: what for inline? how can you have a reference to inline function?
• user1092247: @Juraj, I'm not sure what you're trying to say -- that part comes straight from the EDB code (github.com/jwhiddon/EDB/blob/master/examples/…)

Answer 1

db.create(0, TABLE_SIZE, (unsigned int)sizeof(table_name));

The third parameter is the record size! sizeof doesn't work on data, it works on types, so your getting the size of a char* instead of the size of your sensorlogN struct.

Your code as written shouldn't compile! a char* doesn't have a .id member.

table_name.id = i;
table_name.input = 0;
table_name.timestamp = 0;

• user1092247: Hmm I clearly don't have a perfect understanding of the different variable types yet... How could I make this for loop work, when the two structs are not the same size?
• esoterik: @user1092247 you would have to pass in the size, and a factory function pointer that would create the the correct objects for you, but I don't think that will actually help you accomplish whatever it is your trying to do.
• Juraj: @esoterik, add solution to the answer