stm32在sd卡上保存十六进制数

Question

I use this code to save data on sd card. it works good for character but when I want to save hex numbers it's output is not desirable.

FATFS myfatfs;
FIL myfile;
UINT my_biytes;
char my_data[1];
// or uint_8
char myfilename;

/* USER CODE BEGIN 2 */

f_mount(&myfatfs, SDPath ,1)     
     
/* USER CODE END 2 */

  while (1)
  {
    /* USER CODE END WHILE */
    /* USER CODE BEGIN 3 */

          f_open (&myfile , "test2.txt" , FA_WRITE| FA_OPEN_ALWAYS  );
          data= 0x03;

          sprintf(my_data, "%x",data);
          // or  sprintf(my_data, "%c",data);

          f_lseek(&myfile, f_size(&myfile));
          f_write(&myfile, my_data, sizeof(my_data), &my_biytes);
          f_close(&myfile);

      }

when I use %x it saves "3" but i need "03".

when I use %c it saves "ETX" that is ascii form of "03"

how can I save hex numbers correctly in a txt file on sd card!!

Answer 1

when data is 0x03 I want to have 03 in my_data; so I should write %02x then two numbers (03) will come in my_data variable. and I should define my_data and sprintf like this:

uint8_t my_data[2]

sprintf(my_data, "%02x",data);

Answer 2

First of all, your "my_data" array that you write into the file if of length "1". Which means, you're writing strictly one character. "0x03" is 4 characters.

Second of all, character and value is not the same. Meaning, number 3 is not the same as character "3". Number 3 is 0x03 or 0x3 in hex (or just 3 in hex, as long as you know it's hexadecimal) or 0b00000011 in binary. So when you write 0x03 into file, you get ascii character represented by 0x03. If some kind of software on the receiving side can read that character and simply cast it to uint8_t, it will interpret everything correctly. You don't need to do anything, there is no problem at all. It's not a human-readable 0x03, because in the file that 0x03 is treated and displayed as if it's a char, but the receving program will easily recognize it as integer. 0x03 value is there, it's just a matter of interpretation. Treat that char as uint, and you get your value.

Alternatively, if you actually want to write the whole string "0x03" into the file (unless there is a good reason for specifically this format, it's pretty much a waste of processing power, casting incoming data on the receiver side is easier), you will have to manually assemble this string (array of chars). You will have to convert number 3 into char "3", manually append "0x0" to "3" and then write an array of 4 chars. And the receiver will still see it as an arrach of chars, that you will have to cast to hex number. Much headache on the transmitter side, much headache on the receiver side.

Just write "3" like you did initially and interpret (cast) it as uint8_t on the receiver side if it's possible.