Data overwritten when opening file using FatFs

Question

If I close a file and then reopen it, I cannot write more data to it after reopening it, but if i keep it open i can write as many lines as i want then close it when i am finish writing. See the example below. Thanks.

if (f_mount(&FatFs, "", 1) == FR_OK) {
      f_mkdir ("TEST");

      count = 0;
      while(count < 200){

          if(f_open(&fil, "TEST/test.txt", FA_OPEN_ALWAYS | FA_WRITE) != FR_OK){
              break;
          }
          else{
              sprintf(array,"This is file entry number: %d\r\n",count);
              f_puts(array, &fil);
              if(f_close(&fil) != FR_OK){
                  break;
              }
          }
          count++;
      }
      f_mount(0, "", 1);
}

It will count to the max value but it will only write the last entry which is 199.

rjp · Accepted Answer · 2016-09-20T18:19:43.053

You need to set your open mode so that it appends to the file rather than writing at the start:

From f_open

FA_OPEN_APPEND Same as FA_OPEN_ALWAYS except read/write pointer is set end of the file.

When you open the file with this:

f_open(&fil, "TEST/test.txt", FA_OPEN_ALWAYS | FA_WRITE);

you are opening the file for writing with the write pointer at the start of the file, so when you go to write to the file with:

f_puts(array, &fil);

you overwrite the previous data in the file.

If you change your open to:

f_open(&fil, "TEST/test.txt", FA_OPEN_APPEND | FA_WRITE);

then you should get the behavior you desire. There is an exception, though, and that's that each time running this, you will continue appending to the file. If that isn't desired, you may need to delete the file first or open it initially with FA_OPEN_ALWAYS and then re-open each pass with FA_OPEN_APPEND.

Depending on what you are trying to do, you should take a look at f_sync, which will perform all clean up and writes that an f_close would perform, but keeps the file open. From the documentation:

This is suitable for the applications that open files for a long time in write mode, such as data logger. Performing f_sync function of periodic or immediataly after f_write function can minimize the risk of data loss due to a sudden blackout or an unintentional media removal.

This would cover nearly every case I can think of for why you might be repeatedly opening and closing a file to append data, so this may be a better solution to your problem.

thanks, but the version i am using doesn't have that FA_OPEN_APPEND flag. i was able to achieve the same thing with f_seek(). Thank you for your help. — o_tech, Sep 23 '16 at 01:52
@o_tech, it seems odd to me that it doesn't include `FA_OPEN_APPEND`, since that's a somewhat common mode for opening files. I'm glad you were able to work around the limitation, though. — rjp, Sep 23 '16 at 02:09
these are the flags it has: #define FA_READ 0x01 #define FA_WRITE 0x02 #define FA_OPEN_EXISTING 0x00 #define FA_CREATE_NEW 0x04 #define FA_CREATE_ALWAYS 0x08 #define FA_OPEN_ALWAYS 0x10 #define _FA_MODIFIED 0x20 #define _FA_DIRTY 0x40 I tried to use the _FA_MODIFY but it didnt work — o_tech, Sep 23 '16 at 02:34
i recently downloaded the newer version of the FATfs and it works better. — o_tech, Sep 23 '16 at 02:36

Data overwritten when opening file using FatFs

1 Answers1