stm32f4 fatfs f_write whitemarks - stm32

Question

Im struggling with fatfs on stm32f4. With no problem i can mount, create file and write on it by : char my_data[]="hello world" and in windows file shows normally but when i try use code as loger :
float bmp180Pressure=1000.1;
char presur_1[6];//bufor znakow do konwersji
sprintf(presur_1,"%0.1f",bmp180Pressure);
char new_line[]="\n\r";
if(f_mount(&myFat, SDPath, 1)== FR_OK)
{
f_open(&myFile, "dane.txt", FA_READ|FA_WRITE);
f_lseek(&myFile, f_size(&myFile));//sets end of data
f_write(&myFile, presur_1, 6, &byteCount);
f_write(&myFile, new_line,4, &byteCount);
f_close(&myFile);
HAL_GPIO_TogglePin(GPIOD, GPIO_PIN_15);
}
When i was read from computer i have :top : notepad ++ buttom :windows notepad

Answer 1

There are at least two problems in your code:
The string for the number is too short. C strings are terminated with a null byte. So presur_1 needs to be at least 7 bytes long (6 for the number and 1 for the null byte). Since it's only 6 bytes long, sprintf will write beyond the allocated length and destroy some other data.
The string for the newline is initialized with a string of 2 characters (plus the null byte). However, you write 4 characters to the file. So in addition to the newline, a NUL character and a garbage byte will end up in the file.
The fixed code looks like this:
float bmp180Pressure = 1000.1;
char presur_1[20];//bufor znakow do konwersji
int presur_1_len = sprintf(presur_1,"%0.1f\n\r",bmp180Pressure);
if(f_mount(&myFat, SDPath, 1)== FR_OK)
{
f_open(&myFile, "dane.txt", FA_READ|FA_WRITE);
f_lseek(&myFile, f_size(&myFile));//sets end of data
f_write(&myFile, presur_1, presur_1_len, &byteCount);
f_close(&myFile);
HAL_GPIO_TogglePin(GPIOD, GPIO_PIN_15);
}