External file ressource on embedded system (C language with FAT)

Question

My application/device is running on an ARM Cortex M3 (STM32), without OS but with a FatFs) and needs to access many resources files (audio, image, etc..)

• The code runs from internal flash (ROM, 256Kb).
• The resources files are stored on external flash (SD card, 4Gb).
• There is not much RAM (32Kb), so malloc a complete file from package is not an option.

As the user has access to the resources folder for atomic update, I would like to package all theses resources files in a single (.dat, .rom, .whatever) So the user doesn't mishandle theses data.

Can someone point me to a nice solution to do so?

I don't mind remapping fopen, fread, fseek and fclose in my application, but I would not like starting from scratch (coding the serializer, table of content, parser, etc...). My system is quite limited (no malloc, no framework, just stdlib and FatFs)

Thanks for any input you can give me.

note: I'm not looking for a solution where the resources are embedded IN the code (ROM) as obviously they are way too big for that.

Answer 1

It should be possible to use fatfs recursively.

Drive 0 would be your real device, and drive 1 would be a file on drive 0. You can implement the disk_* functions like this

#define BLOCKSIZE 512
FIL imagefile;
DSTATUS disk_initialize(BYTE drv) {
  UINT r;
  if(drv == 0)
    return SD_initialize();
  else if(drv == 1) {
    r = f_open(&image, "0:/RESOURCE.DAT", FA_READ);
    if(r == FR_OK)
      return 0;
  }
  return STA_NOINIT;
}
DRESULT disk_read(BYTE drv, BYTE *buff, DWORD sector, DWORD count) {
  UINT br, r;
  if(drv == 0)
    return SD_read_blocks(buff, sector, count);
  else if(drv == 1) {
    r = f_seek(&imagefile, sector*BLOCKSIZE);
    if(r != FR_OK)
      return RES_ERROR;
    r = f_read(&imagefile, buff, count*BLOCKSIZE, &br);
    if((r == FR_OK) && (br == count*BLOCKSIZE))
      return RES_OK;
  }
  return RES_ERROR;
}

To create the filesystem image on Linux or other similar systems you'd need mkfs.msdos and the mtools package. See this SO post on how to do it. Might work on Windows with Cygwin, too.