How to read SD card file using an array of filenames

[D4VE777]

I am using the Feather Adalogger 32u4. I need to store a list of filenames in flash, then based on the user push-button response, open and read a certain file on the SD card.

I am able to read files from the SD card using the direct approach:
File myFile;
myFile = SD.open("BOOKNAM1.TXT", FILE_READ);

And the indirect approach also works:
File myFile;
char* BookFilename = "BOOKNAM1.TXT";
myFile = SD.open(BookFilename, FILE_READ);

But I cannot read a file from the SD card using an array of filenames that are stored in flash memory (for example):
const char BookFilename[3][13] PROGMEM = {"BOOKNAM1.TXT","BOOKNAM2.TXT","BOOKNAM3.TXT"};
File myFile;
myFile = SD.open(BookFilename[0], FILE_READ); // Doesn't work!

If I move the array to RAM, it takes up more RAM (4.5K) than the processor has (32u4 = 2.5K).
I have searched, but haven't found a similar issue. Thank you.

[dastels]

"Doesn't work" ... that doesn't help much. What actually happens?

Dave

[D4VE777]

Hi dastels, the program just hangs up there. I have a print statement after that, and it never gets there.

[dastels]

OK. This works fine:

Code: Select all

const char BookFilename[3][13] PROGMEM = {"BOOKNAM1.TXT", "BOOKNAM2.TXT", "BOOKNAM3.TXT"};

void setup() {
  Serial.begin(115200);
  while (!Serial);
  Serial.println("Starting");
  Serial.println(BookFilename[0]);
  Serial.println(BookFilename[1]);
  Serial.println(BookFilename[2]);
  Serial.println("Done");
}

Outputing:

Code: Select all

Starting
BOOKNAM1.TXT
BOOKNAM2.TXT
BOOKNAM3.TXT
Done

So it isn't the array.

I would see if an exception is being thrown from SD.open.

Dave

[D4VE777]

Thanks for your help so far, dastels. Where can I go to learn how to check for SD.open exceptions?

[dastels]

Ah. https://www.arduino.cc/en/Reference/SDopen it returns false if there's a problem. Have a look at the example for reading: https://www.arduino.cc/en/Tutorial/Libr ... s/DumpFile.

Dave

[D4VE777]

Thanks dastels, that was good to know. I figured out why I was able to use the array information stored in RAM to read files from the SD card, but NOT able to read the array stored in flash. It is because of PROGMEM. After an array is declared to be in PROGMEM, you have to use a completely different set of instructions to retrieve the data from the array.